Fredric M. Serchuk, Marvin D. Grosslein, R. Gregory Lough, David G. Mountain, and Loretta ...... spawning in March and April (Berrien and Sibunka,. 1993; Fig.
II. Diagnosis of the causes of trends and fluctuations in cod stocks ICES mar. Sei. Symp., 198: 77-109. 1994
Fishery and environmental factors affecting trends and fluctuations in the Georges Bank and Gulf of Maine Atlantic cod stocks: an overview Fredric M. Serchuk, Marvin D. Grosslein, R. Gregory Lough, David G. M ountain, and Loretta O 'B rien
Serchuk, F. M., Grosslein, M. D., Lough. R. G .. M ountain, D. G ., and O 'B rien. L. 1994. Fishery and environmental factors affecting trends and fluctuations in the Georges Bank and Gulf of Maine Atlantic cod stocks: an overview. - ICES mar. Sei. Symp., 198: 77-109. The Georges Bank and Gulf of Maine Atlantic cod stocks have been commercially exploited for centuries, and continue today to support important commercial and recreational fisheries in both the United States and Canada. Prior to 1960, the two stocks were fished exclusively by the United States. During the 1960s, marked increases in exploitation occurred due to the development of Canadian and distantwater fisheries on Georges Bank. Total landings from the two stocks increased from 14 400 1 in 1960 to 57 500 1 in 1966, but subsequently declined to 30 0001 in 1976. U nder extended fisheries jurisdiction - enacted in 1977 by both the USA and Canada landings and stock sizes initially increased and a record-high catch (71 000 t) was attained in 1982. However, during the late 1980s and early 1990s, fishing effort and fishing mortality also increased to record-high levels, resulting in marked reductions in spawning-stock size. Research vessel indices, commercial c.p.u.e. indices, and VPA results all indicate that biomass levels in both stocks are now at record-lows. Although it is clear that fishery-induced perturbations have had a major impact on the Georges Bank and Gulf of Maine stocks, environmental and biotic influences may also have affected the stocks. These influences are not easily discernible, given the magnitude of the fishery-induced changes. Biophysical processes appear to be extremely important during the first year of life of cod and affect growth, mortality, and year-class strength. These processes are reviewed and discussed in relation to their impacts on various life history stages of cod. Current and future research studies aimed at differentiating environmental from fishery-related factors affecting recruitment and abundance of cod in the Georges B ank-G ulf of Maine region are discussed. Fredric M. Serchuk, Marvin D. Grosslein, R. Gregory Lough, David G. Mountain, and Loretta O Brien: National Oceanic and Atm ospheric Administration, National Marine Fisheries Service, Northeast Fisheries Science Center, Woods Hole, M A 02543, USA.
Introduction O n 14 M ay 1602, the sailing ship " C o n c o rd ” - in search o f sassafras trees to o btain quantities o f sassafras extract for treating syphilis in E u ro p e - com p leted a 50-day trans-A tlantic voyage from F alm o u th , E n gland to “a mighty h e ad lan d ” in the New W orld. N e a r this p ro m o n tory, the " C o n co rd ” cam e to anchor in 15 fathom s [27 m] w here such a "g reat store o f codfish” was tak en th a t the area was n am ed C ap e C od (Jensen, 1972). Sassafras trees w ere also found in a b u n d an ce, b u t it was cod that
was to becom e the source of original w ealth for b o th the early New E ngland colonists and for the nation tha t was to becom e the U n ite d States (Serchuk and Wigley, 1992) T he earliest U S A fisheries for A tlantic cod ( G adus m orhua) began in the 1600s in nearshore w aters off the states of M aine and Massachusetts. By the early 1700s, how ever, vessels had started to fish m uch farth er off shore, with the G eorges B ank fishery developing b e tw een 1720 and 1750. By the end of the nineteenth century, the offshore fisheries supported a thriving d o m estic industry; in 1880, g reater than 12 000 t of cod were
78
F. M. Serchuk et al.
ICES mar. Sei. Sym p.. 198 (1994)
lan d ed from G eo rg es B an k by th e 163 schooners taking p art in th e fishery. C on tin u o u s com mercial landings statistics for cod exist from 1893 o n w ard (T able 1). T o ta l landings of cod from the G u lf of M aine an d G eo rg es B a n k regions [I C N A F /N A F O S ubareas 5 and 6: Fig. 1] exceeded 60 000 t in both 1895 and 1906, but declined to less than 20 000 t in 1915-1916 (Fig. 2). B etw een 1920 and 1940, cod was generally ta k e n as a by-catch in the offshore fishery for h ad d o ck ( M elanogram m us aeglefinus) which had rapidly e x p an d ed in th e early 1920s owing to
developm ent of a packaged trade in quick-frozen h a d dock fillets. D uring this 20-year p eriod, annual landings o f cod w ere relatively stable, fluctuating betw een 24 000 and 41000 t and averaging 33 000 t annually. C od catches declined in the 1940s and early 1950s, reaching a record-low of 11 000 t in 1953. In the 1960s, C an ad ian and distant-w ater fisheries developed on G eorges B an k , resulting in a rapid in crease in fishing effort for cod (T able 2). C o d landings increased from 14000 t in 1960 to 58 000 t in 1966, but afterw ard declined sharply to 30 000 t in 1976. In 1977,
Table 1. U S A commercial landings (tonnes, live) of Atlantic cod from the Georges Bank (N A FO Division 5Z) and Gulf of Maine (N A FO Division 5Y) cod stocks (1893-1959). Prior to 1932, landings can only be assigned to Subarea 5.
Year
N A FO Subarea 5
1893 1894
47 425 55 916
1895 1896 1897 1898 1899
62 095 48 001 45 928 44150 48 290
1930 1931 1932 1933 1934
1900 1901 1902 1903 1904
33 999 45 238 41 957 39 024 30 396
1905 1906 1907 1908 1909
Year
N A FO Subarea 5
N A FO Division 5Z
N A FO Division 5Y
Totals Subarea 5
25 122 25 155 15 976
5858 7025 11 619
41 341 33 338 30 980 32180 27 595
1935 1936 1937 1938 1939
21 162 23 349 32 282 24 891 22 048
9679 7442 7432 7547 5504
30 841 30 791 39 714 32 438 27 552
36 316 61 667 56 516 40 397 39 672
1940 1941 1942 1943 1944
18418 25 453 18 333 17 341 17 632
5836 6124 6679 9397 10516
24 254 31 577 25 012 26 738 28148
1910 1911 1912 1913 1914
34 796 26 427 27 961 24 101 32 818
1945 1946 1947 1948 1949
14 251 20 875 16 582 17 640 17 681
14 532 9248 6916 7462 7033
28 783 30123 23 498 25 102 24 714
1915 1916 1917 1918 1919
18 459 18 786 21 330 29 799 29 953
1950 1951 1952 1953 1954
15 389 14 791 10 904 8105 8826
5062 3567 3011 3121 3411
20451 18 358 13 915 11226 12 237
1920 1921 1922 1923 1924
27 345 32 044 30 847 30 498 32 639
1955 1956 1957 1958 1959
9286 10 535 10 598 11582 12 423
3171 2693 2562 4670 3795
12 457 13 228 13 160 16 252 16 218
1925 1926 1927 1928 1929
34 320 40 102 39 335 34 743 37 021
41341 33 338
-
-
-
-
Georges B a n k an d G u lf o f M aine A tlantic co d stocks
ICES mar. Sei. Symp.. 198 (1994)
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Figure 1. (A) Map depicting N A FO Subareas 3-6, and (B) statistical unit areas in the Gulf of Maine and Georges Bank (from Serchuk and Wiglcy, 1992).
80
F. M . Serchuk et al.
lC E S m a r. Sei. Sym p.. 198(1994)
OOOs OF TONNES (LIVE WEIGHT)
CANADA ^
1900
1910
1920
1930
1940
FOREIGN
1950
HU
1960
1970
1980
1990
Figure 2. Total reported commercial landings of cod from the Georges Bank and Gulf of Maine stocks (1893-1992).
the U S A an d C an a d a in de pendently enacted extended fisheries jurisdiction which p rohibited distant-w ater fleets from fishing cod (an d o th e r species) in the U S A an d C an ad ian exclusive econom ic zones. By this tim e, th e G eo rg es B an k stocks o f h ad d o ck , herring ( Clupea harengus), A tlan tic m ackerel (S com b er sc o m b ru s). and silver h ak e (M erluccius bilinearis) h ad already collapsed fro m overfishing ascribed largely to the distant-w ater fleets (M ayo et a l., 1992). C o d - which had always b e e n a significant c o m p o n e n t of th e groundfish com plex in the G u lf of M aine an d G eo rg es B a n k regions - had now b ecom e the most d o m in an t co m p o n en t. Total cod land ings (solely U S A and C a n a d ian ) d o u b le d b e tw e e n 1977 and 1982 (30 000 to 71 0 0 0 1, a record-high), declined to 36 000 t in 1986, b u t increased to 58 000 t in 1990. In 1992, how ever, landings fell to 39 000 t. Since 1977, cod has annually a ccounted for m o re catch (by weight) than any o th e r species in the U S A A tlantic coast groundfish fishery. In recent years, cod has been so d o m in an t th a t U S A com mercial landings o f cod have typically exceeded those fro m h a d d o c k , redfish (Sebastes fa scia tu s), pollock (P ollachius virens), and six m ajor species of flatfish co m bined (US D e p a rtm e n t of
C o m m erce, 1992). In addition, b etw een 3000 and 8 0 0 0 1 of cod are ta k en annually in recreational fisheries (S er chuk and Wigley, 1992). G iven the im portance of cod off the n o rth e a st coast of the U n ite d States, in this p a p er we review the changes that have occurred over the past 30 years to the G eorges B ank and G ulf of M aine cod stocks, and evaluate the influence of fishery and environm ental factors in affect ing stock size fluctuations, recru itm en t, grow th, and m atu ration.
Stock definitions In w aters west of Nova Scotia and south o f the F undian C hannel (i.e., N A F O S ubareas 5 and 6), cod have been distinguished into three or possibly four m a jor groups: (1) G eorges B an k ; (2) G ulf o f M aine; and (3) one or tw o groups in the S o u th ern New E n g la n d -M id d le A tlantic a re a (Wise, 1963; Serchuk and Wigley, 1992). Little interchange of cod occurs b etw een the G ulf of M aine and G eorges B ank groups, as inferred from tagging studies (Smith, 1902; Schroeder, 1930; N orth A m erican
G eorges B a n k a nd G u lf o f M aine A d a n tic co d stocks
ICES mar. Sei. Symp., 198 (1994)
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i c e s mar. s d . sy m p .. 1 98 ( 1994)
G eorges B a n k and G u lf o f M aine A tlantic co d stocks 89
90
F. M. Serchuk ei al.
ICES mar. Sei. Sym p.. 198 (1994)
during the late 1960s, declined during the early 1970s, sharply in creased in 1974, an d th e n stabilized at a relatively high level during 1975-1983 (Fig. 8). Sub sequently, c .p .u .e . indices declined, reaching recordlow levels in 1987. B etw een 1988 and 1991, the c .p .u .e. indices again increased, but in 1992 b o th the total and directed indices declined to n ear-reco rd lows. T ak en at face value, the time series o f c .p .u .e . values for b o th stocks suggest th a t exploitable biom ass p e aked during the late 1970s a n d early 1980s, but has since declined - in each stock - to historic low levels. O n a relative basis, the p re se n t stock levels are probably even lower than indicated by the c .p .u .e . d ata because the c .p .u .e. indices have not b een adjusted for increases in fleet efficiency in recent years. U S A o tte r traw l effort increased m a rk ed ly after 1976 (Fig. 9). In th e G eo rg e s B an k fishery, U S A trawling effort for cod (nom inal days fished) rose by 71% b e tw een 1977 and 1988(11 700 to 19 800 days fished), while in the G u lf o f M aine fishery trawling effort nearly doubled (7400 to 13 000 days fished). D uring the past few years, effort in b o th fisheries has b e e n at o r n ear historically high levels.
T ren d s in research vessel survey indices Stratified-random b o tto m trawl surveys o f the G eorges Bank and G u lf o f M aine regions have b een c onducted by
the N o rth east Fisheries Science C e n te r (N E F S C ) each au tu m n since 1963 and each spring since 1968 using the research vessels “A lbatross IV " and “D e law are 11” (A zarovitz, 1981). T h e d ata from these surveys have been used to derive standardized indices of relative abundance and biom ass of cod (stratified m e an catch per tow in n u m b e r and weight, respectively) for b oth the G eorges B ank and G ulf of M aine stocks. F o r G eorges B an k cod. the spring and autum n N E F S C survey indices show similar trends (Fig. 10). Survey biom ass indices w ere relatively low and stable during 1963-1971, fluctuated at a generally higher level betw een 1972 and 1981, but have since declined to record-low values. E levated survey biom ass indices in 1969-1970, 1972-1973, 1978, 1981, and 1984-1985 re flect above-average recru itm en t of the 1966, 1971,1975, 1980, and 1983 year classes. F o r the G ulf of M aine stock, the time series of survey indices suggests tha t stock biom ass was high in (1) the early 1960s; (2) during the late 1960s-early 1970s; and (3) and in the late 1970s-early 1980s. A fter 1985, how ever, both the spring and au tu m n survey biom ass indices steadily declined, reaching record-low levels during 1989-1991 (Fig. 10). In 1992 [as in 1973], the spring and au tu m n indices w ere discrepant; the spring index m a rk edly increased (due to a large contribution of fish from the strong 1987 y ear class) while the au tu m n index declined. R ecru itm en t indices for the G u lf of M aine
NOMINAL DAYS FISHED 1000s) 24
24 22
GEORGES BANK
22
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20
20
18
18
6
16
14
14
2 10
8 6 4
2
0 63
65
67
69
73
75
77
79
81
83
85
87
89
91
93
Figure 9. USA commercial otter trawl effort (nominal days fished) for trips landing cod from the Georges Bank and Gulf of Maine stocks (1965-1992).
GEORGES
BANK COD
GULF OF MAINE COD USA RESEARCH VESSEL BOTTOM-TRAWL SU RVEYS STRATIFIED MEAN CATCH I KOI PER TOW
ES ES
Figure 10. Standardized stratified mean catch per tow indices (top) and survey recruitment indices (bottom) for the Georges Bank and Gulf of Maine cod stocks from NEFSC spring and autumn bottom-trawl surveys (1963-1992).
USA RESEARCH VESSEL BOTTOM-TRAWL SURVEYS STRATTFTED MEAN CATCH IKO] PER TOW
ICES mar. Sei. Symp., 198 (1994)
Georges B a n k and G u lf o f M aine A tlantic co d stocks 91
92
F. M. Serchuk et al.
stock (survey nu m b er-p er-to w indices at age 2) indicate that th e 1971, 1973, 1978, 1980, 1986, and 1987 year classes w ere all above average.
ICES m ar. S d . Sym p., 198 (1994)
stocks but these influences are n ot easily discernible given the magnitude of the fishery-induced perturbations.
T rends in stock size, recruitm ent, and fishing mortality
Environm ental and biotic processes affecting cod life history
E stim ates o f stock size, recru itm en t, and fishing m o r tality for the G eo rg es B a n k an d G u lf o f M aine cod stocks are available from virtual p o p u la tio n analyses (V P A ). F o r G eo rg es B a n k cod, the V P A covers the 1978-1992 p eriod (Serchuk et al., 1993); for the G ulf of M ain e stock, the V P A e xtends from 1982-1992 (M ayo et al., 1993). D u rin g the past 10-15 years, m a rk e d reductions in the ab u n d an ce o f b o th cod stocks have occurred coincident with increases in fishing m ortality rates. In the G eorges B an k stock, spawning-stock biom ass (SSB) declined by over 40% b etw een 1980 and 1985 (93 000 to 55 000 t) while fishing mortality rates increased by 50% (Fig. 11). SSB increased in 1987 to 66 000 t and stabilized at a bout 70 000 t during 1988-1990 due to strong recru itm ent from the 1983 and 1985 cohorts. Fishing mortality, how ever, co n tin u ed to rise, a n d increased fro m F = 0.63 in 1989 to a record-high F = 1.07 in 1991 (a 70% increase). SSB subse quently declined sharply in 1991 and fell to a record-low o f 41 000 t in 1992. F u rther declines in SSB are e x p ected in 1993 a n d 1994 w hen the 1989-1991 year classes, all below average in abundance (i.e., below 21 million fish at age 1), recruit to the spawning stock. In the G u lf of M aine stock, fishing m ortality increased from F = 0.62 in 1982 to above F = 1.0 d u r i n g 1985-1987 (Fig. 11). In this sam e time period (1982-1987), SSB declined from 25 700 t to 13 900 t. B ecause of aboveaverage recru itm en t o f th e 1986 year class an d the exceptionally strong 1987 year class, SSB increased sharply in 1989 and reached a record-high level of 27 5 0 0 1 in 1990. H o w ev er, p o o r re cru itm en t o f th e 1988-1990 y ear classes, coupled with fishing m ortality rates above F = 1.0 in b o th 1990 and 1991, resulted in SSB declining in 1992 to a record-low of 13 600 t. SSB o f G u lf o f Maine cod is expected to decline below 12 000 t in 1993 as the 1987 co h o rt (which a cco u n ted for o v e r 60% of th e catch an d SSB in 1992) is fu rth e r fished down. T h e striking declines in biom ass in both the G eorges B an k an d G ulf of M aine cod stocks have occurred during a p erio d w hen landings, fishing effort, and fishing m ortality have increased to record-high levels. F or both stocks, recent fishing m ortality rates have far exceeded the levels generally associated with stock rep lacem ent (F med o r F 20%) o r grow th overfishing (F max). O n an annual basis, th e fisheries o n the two stocks currently rem ove b etw een 40 an d 60% o f the exploitable biomass. Facto rs o th e r th a n fishing m ay also have affected the
D espite the long history of exploitation of cod in the G eorges B a n k - G u lf o f M aine region, it has only been ra th e r recently that sufficient inform ation has b een o b tained to begin sorting out the environm ental factors from the fishery events affecting cod ab undance. This inform ation has com e largely from a tim e series of research vessel surveys and accom panying biological studies, which indicate that environm ental effects are most significant during the first year o f life. A tte m p ts to correlate recruitm ent and egg/larval abu n d an ce of G eorges B an k cod have failed so far (Sm ith et a l. , 1981; F ogarty et al., 1987), b u t significant correlations have been observed betw een recruitm ent and trawl survey catches o f 0-group and 1-year cod o n G eo rg es B ank (Serchuk et al., 1993). This supports the inference tha t factors controlling cod year-class success o p e ra te chiefly during the first y e a r of life. A pproxim ate m ortality estim ates in late-larval and postlarval stages of cod (> 9 9 .9 % ) are typically as high o r higher than m ortality in the egg and early larval stages (Sissenwine et al., 1984); thus a small change in natural mortality at these late larval o r juvenile stages could m a k e the difference b etw een a strong or w eak y ear class. Given the high m ortality rates in b oth the early and late 0-group stages, Sissenwine (1984) concluded th a t re cruitm ent is likely to be a function o f highly variable processes occurring thro u g h o u t the first y ear of life. While d ensity-dependent m ortality in late-larval and juvenile stages is im p o rtan t in regulating year-class size, a general view is em erging that the m ajor variability in recruitm ent ( “b u m p e r” vs. “b u st” year classes) is largely due to density-independent m ortality during the egg and early larval stages. B ased on a c om prehensive review of egg and larval p red atio n m ortality, Bailey and H o u d e (1989) concluded tha t year-class strength is established very early (i.e., at th e egg and larval stages), but th a t pred a tio n on later stages can have m o d e ra te to large effects on recruitm ent (i.e., altering year-class strength by an o rd e r of m agnitude o r less). Myers and C adigan (1993a) exam ined juvenile mortality d ata from six cod stocks and found strong evidence of density-dependent m ortality within cohorts, as well as a negative au to c o rre lation b etw een adjoining cohorts. T he G eorges B ank and G ulf o f M aine stocks were included in these analyses and generally fit the p a ttern s seen in the o th e r stocks, except th a t d ensity-dependent m ortality was n ot evident for G eorges B an k cod, possibly due to the very low abundance level of this stock. Little in terannual vari-
GEORGES
BANK COD
W4 CA TCH AND FISHING MORTAUTY. 1977 ■ 1992
- 1 99 2
GULF OF MAINE COD
i
TRENDS IN CA TCH AND FISHING M ORTAUTY. 1980
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G eorges B a n k a nd G u lf o f M aine A tlantic cod stocks
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9 9 .9 % , respectively. T he level o f postlarval m ortality ( > 9 9 .9 % ) d uring 1979-1984 is similar to that estim ated by Sissenwine et al. (1984) for the 1974-1980 period. Larval and postlarval m ortality rates can also be derived by com paring direct estim ates o f postlarval (> 2 0 m m ) cod a b u n d a n c e for th e 1986 and 1987 G eorges B an k coh orts [from Ju n e surveys using a 10 n r M O C N ESS sam pler] w ith estim ates o f eggs spaw ned an d age 1 recruits for these y ear classes (unpubl. d a ta , P e te r B e r rien and G rego ry L ough). T hese d ata indicate that total egg and larval m ortality rates in 1986 an d 1987 w ere M t = 6.8 (9 9.9 % ) and M, = 5.6 (9 9.6% ), respectively. Postlarval m ortality (from ab ou t m id-June until age 1) was M, = 6.2 (99.8% ) in 1986 and M t = 6.8 (99.9% ) in 1987. T hese values are consistent with the 1974-1980 and 1977-1987 averaged estim ates, and again d e m o n strate th a t postlarval m ortality in cod is co m p arable to that ex perienced in the egg an d larval stages. P r e y a n d p r e d a t o r s o f 0 - g r o u p s ta g e s In th e G eo rges B a n k -G u lf of M aine region, the prey of larval cod consists o f zo o p lan kton . Y olk sac and firstfeeding larvae prey prim arily o n small p la nk to n such as co pep od nauplii, p h y to p lan k to n , and lam ellibranch lar vae. W ith th e ex ception o f the yolk-sac stage, cod larvae feed on the sam e prey species th ro u g h o u t their early life, selecting the m ost num erically d o m in an t prey (K an e, 1984; Buckley and L ough, 1987; A u d ito re et al., 1994). Prey size plays an im p ortan t role; as cod grow larger, they te n d to select larger size prey items. O n G eorg es B an k , cod larvae fe ed on c o p ep o d nauplii, copep od ites, an d adults o f C alanusfinm archicus, P seudocahinus s p p ., O ithona similis, and Centropages typicus. A s pelagic and recently settled juveniles, cod shift to ep ibenthic prey and sw arm ing species tha t - like cod - also u n derg o diel vertical m igrations. T hese food item s include mysids (N eom ysis am ericana), am ph ip od s (G am m aru s annulatus, Them isto gaudichau dii, T. com pressa), chaetognaths (Sagitta elegans), and euphausiids (M eganyctiphanes norvegica) (A u d ito re et al., 1988; Perry and Neilson, 1988; L ough et al., 1989).
I C E S m a r . Sei. S y m p ., 198 (1994)
M uch less is know n of the p red atio n o n young cod in the G eorg es B a n k -G u lf o f M aine region tha n o f th eir prey and feeding. C od larvae and pelagic juveniles are them selves likely p re d a to rs o f sm aller cod larvae (L a u r ence e ta l., 1981). H o w ev er, to date, th e re is little direct evidence o f cannibalism . M ack erel are confirm ed fish p red ato rs of cod and h ad do ck larvae, and herring very likely also p rey on small cod larvae (M ichaels, 1991). D ogfish, silver h ake, squid, larger cod, and m any o th e r piscivores p rey on juvenile cod (E dw ards an d B ow m an , 1979). D esp ite limited direct evidence, the im p ortan ce of p re d a tio n as a d o m in an t source of p re -recruit m ortality in m any fish populatio ns is no w a widely accepted assum ption (H u n te r, 1981; Sissenwine, 1984; H o u d e , 1987, 1989; Bailey and H o u d e, 1989). H o w e v e r, there are insufficient d ata from the studies o n G eo rges B ank and in the G u lf of M aine to reliably quantify egg and larval p re d a tio n m ortality, let alone p artition these m o r talities am on g th e various p red ato rs. N ew research in itiatives currently underw ay on G e o rg es B ank are designed to address these questions (see concluding section on F u tu re R esearch).
Post-juvenile growth and m aturation G eo rg es B ank cod grow m ore rapidly than G u lf of M aine cod. P enttila and G ifford (1976) fitted von B ertalanffy grow th curves for cod collected from research vessel surveys in the early 1970s and fou nd tha t at age 2 (Jan uary 1) G eorg es B an k cod averaged 39.9 cm in length while G ulf of M aine cod w ere only 26.4 cm long. T h e v o n B ertalanffy p a ram eters for the two stocks are as follows:
G eorg es B ank G ulf o f M aine
Ljnf
K
to
148.1 146.5
0.120 0.116
- 0 .6 1 6 0.285
B ased on m e a n w eights-at-age in com m ercial catches, the grow th rates of G eo rges B ank and G u lf of M aine cod a p p e a r to be in term ed iate com p a red with those in o th e r N orth A tlantic cod stocks (Fig. 15). G eorges B an k cod m a tu re earlier than G u lf o f M aine cod, and fish in both stocks a p p ea r to m a tu re earlier and at a sm aller size - tha n in m ost o th e r N orth A tlantic stocks (Table 3). D uring th e early 1970s, G eorg es B an k cod reach ed 50% m aturity at ab o ut 2.8 years old as co m p ared to 3.5 years for cod in th e G ulf o f M aine (Livingstone and D ery , 1976). R ecently, how ever, th e A 50 values have declined in b oth stocks an d are now 1.8 a nd 2.2 years, respectively ( O ’B rien et al., 1993; T able 3). T h ese A 5(> values are p robably the lowest in any N orth A tlantic cod stock, including th e N orth Sea stock, w here cod display even faster som atic grow th th an on G eorg es B an k (T able 3, Fig. 15).
i c e s mar.
sd. Symp.,
G eorges B a n k a nd G u lf o f M aine A tlantic cod stocks
198 ( 1994)
99
WEIGHT (KG) 18
North Sea
Georges Bank
Iceland
2J3KL
Gulf of Maine
16
14
14
10
0
2
1
3
4
5
6
7
8
9
0 10
AGE
Figure 15. Mean catch weight-at-age values for five North Atlantic cod stocks. Values depicted are those used in the most recent stock assessments.
O f the few studies condu cted on cod m aturatio n in the G eorg es B a n k -G u lf o f M aine area, only the m ost recently com pleted study ( O ’B rien, 1990) is tem porally and spatially com prehensive. This study, based on d ata from N E F S C spring research surveys collected during
1970-1990, indicates that b o th length and age at m aturation have declined in b o th stocks over the past two decades. O n G eorges B an k , th e estim ated m e dian age (A 50) and m edian length (L 50) at m aturity for fem ale cod declined from 2.7 y ear and 53.4 cm for the 1970 year
Table 3. Median age at maturity ( A 5()) and median length at maturity (L50) of Atlantic cod in various North Atlantic stocks. Data are provided for females (F) and males (M), or sexes combined (C). A.,,, (years)
L50 (cm)
Time period
F
C
M
F
C
M
1972 1985-1990
2.9 1.7
_
2.6 1.9
51.5 38.8
_
-
44.0 41.0
Livingstone and Dery (1976) O 'Brien et al. (1993)
1878 1972 1985-1990
4.0 3.5 2.1
—
3.0 3.5 2.3
_
_
_
-
32.1
-
36.0
Earll (1880) Livingstone and Dery (1976) O'Brien et al. (1993)
Nova Scotia to Cape Hatteras
1977
-
-
-
49.6
-
53.7
Morse (1979)
Scotian Shelf
1963 1978
5.0 2.0
_
-
3.9 2.0
52.0 35.0
-
51.0 38.0
Beacham (1983) Beacham (1983)
N A FO Area 2J3KL
-
-
6.0
-
-
49.7
-
Xu et al. (1991)
Iceland
-
-
3.4
-
-
-
-
ICES (1993a)
North Sea
-
-
3.7
-
-
-
-
ICES (1993b)
Area Georges Bank
Gulf of Maine
-
Sourcc of data
F. M . S erchu k et al.
Georges B a n k a nd G u lf o f M aine A tlantic cod stocks
I C E S m a r . Sei. S y m p . , 1 9 8 (1 9 9 4 )
class to 0.8 years and 30.5 cm, respectively, for the 1985 y ear class (Fig. 16). Similar trend s have occurred in G ulf o f M aine cod for the sam e y ear classes; A 50 and L5() values for fem ales declined from 4.3 years and 58.3 cm (1970 year class) to 2.0 years and 33.4 cm (1985 year class). In b oth stocks, A 50 and L 50 for m ales have declined in ta n d em w ith those for fem ales (Fig. 16). T h e accelerated m a tu ratio n rates in b oth stocks have occurred over a time p eriod during which m arked re ductions in stock ab un dan ce have occurred. C hanges in m a tu ratio n show a significant correlation with changes in ab un dan ce o f the two stocks, b u t n o t with annual changes in w ater te m p eratu re s (O 'B rie n , 1990).
Fo od and feeding of juvenile and adult cod E xtensive studies o f food habits o f fishes on G eorges B ank and in the G u lf o f M aine have b een co n du cted in conjunction with the N E F S C research vessel surveys in these areas (G rosslein et al., 1980). T hese studies show th a t the diet o f juvenile and adult cod in b o th areas is basically similar to th a t found in o th e r N o rth A tlantic cod stocks; fish rep re se n t the m ajor prey item (in term s o f p ercentage weight o f stom ach co ntents), followed by crustaceans and m ollusks (E dw ard s and B ow m an, 1979; Fig. 17). O n G eo rges B an k , fish com prised 62% o f the diet of cod during 1969-1972, with about 15% each for crustacea and mollusks (L angton and B ow m an, 1980).
101
D uring the sam e tim e perio d, the diet of G u lf o f M aine cod was com prised of 70% fish, 26% crustaceans, and 1% mollusks. T h e m ajor fish prey o f cod in b o th areas was clupeids (chiefly C lupea harengus), b ut flounders and o th e r gadids were also consum ed on G eorg es B ank, while redfish, m ackerel, and o th e r gadids w ere ta ken as prey in the G ulf of M aine (L angton and B ow m an, 1980). T h e principal crustaceans con sum ed by cod in both areas are d ecap od crabs an d shrimp. O f th e mollusks e aten , squid constitute the m ajor prey o f G u lf o f M aine cod, while the discarded rem ains o f sea scallops (from scallop vessels shucking at sea) are o ften an im p ortan t molluscan com p on ent o f the diet o f G eorg es B ank cod (L angton and B ow m an, 1980; B ow m an and M ichaels, 1984). Juvenile cod (6-10 cm) consum e som e fish but d ep en d m ost heavily on am phipods and small decapod shrim p. L arger crustacea and m ore fish are e aten by cod as they grow. By the time cod attain a length o f 50-60 cm , their overall die t is ab o ut half fish an d half in v e rte b rates; th e re after, the p ro p o rtio n o f fish in th e diet continues to increase (B ow m an an d M ichaels, 1984). D u rin g the mid-1960s an d early 1970s, large declines in fish biom ass occurred o n G eo rges B a n k d ue to in te n sive fisheries exploitation (M ayo et al., 1992). C o m p a ri sons of cod diets before and after this p eriod indicated a change in diet com position; th e percent frequency occurrence o f fish (all species) in c o d stom achs d ro p p ed from 37% in 1963-1966 to 29% in 1973-1976 (G rosslein
— —/
/
Other Fish
Crustacea
Mollusks
■/
Herring
Mackerel
69-72 Figure 17.
73-76
77-80
81-84
85-86
Diet composition (% by weight of prey) of Atlantic cod off the northeast coast of the United States (1969-1986).
102
F. M . Serch.uk et al.
et al., 1980). T h e red uced incidence o f fish in the diet of cod was due to decreased consu m ption of gadids, since the incidence o f clupeids rem ain e d u n chan ged (G rosslein et a l., 1980). A ltho ug h the stom ach sam pling p ro to cols w ere quite different b etw een the 1963-1966 and 1973-1976 perio ds (and hence the d ata n o t strictly co m p arable), the red u ce d incidence o f gadids in the post-1972 diet ap p ears to be a real reflection o f the dim inished a b u n d a n c e and availability o f th e G eorg es B ank gadid stocks. O n the o th e r h an d, sea herring ab un d a n c e w as considerably g reater on G eo rges B ank during 1963-1966 than in 1973-1976, yet the frequency of occurrence of clupeidae in cod stom achs was virtually the sam e in b o th tim e periods (G rosslein e ta l., 1980). In m ore recen t years (1977-1986), cod diets in b o th the G ulf o f M aine a n d o n G eorg es B an k have show n a lower fraction o f fish balanced by an increased p ro p o rtio n of in vertebrate prey (Fig. 17). This a p p a re n t shift could be a reflection of re du ced availability o f p refe rred fish prey and partial com p ensation (substitution) of in v erteb rate prey for fish (IC E S , 1992). E stim ates o f daily ration for cod in the G eorg es B a n k G ulf o f M aine region range from a b o ut 1-2% body w eight p e r day (E d w ard s and B ow m an, 1979; G rosslein et al., 1980; C o h en and G rosslein, 1981; D urb in et al., 1983). T h ese estim ates are som ew h at higher than the daily ration estim ates of 0 .8 -1 .0 % body weight p e r day for similar-sized cod (50-60 cm) in the N orth Sea (D aan , 1973) - despite the fact th a t m ean w eight o f gut c o ntents is m o re th an twice as great in N o rth S ea cod (U rsin et al. , 1985) and b o th stocks have similar grow th rates (U rsin, 1984). H o w ever, the m e an stom ach co n ten t weights of cod o n th e Scotian Shelf an d off N ew fo un dlan d are also m uch larger than those observ ed on G eo rges B an k , but grow th rates o f cod in th ese areas are m uch slower (U rsin, 1984; U rsin et al., 1985). R ecent analyses o f cod food habits d ata by th e IC E S M ultispecies A ssessm ent W orking G ro u p indicate th a t for most N orth A tlantic cod stocks stom ach con ten t w eight is inversely co rre lated with te m p e ra tu re ; the low er m e an stom ach co n te n t weights o f cod o ff the n o rth e a ste rn U n ite d States are th o u g h t to result from higher am bien t w ater te m p eratu res (IC E S , 1992). T h e fraction o f fish in th e diet of G eo rges B an k /G u lf o f M aine cod is also generally lower than o bserv ed in o th e r N o rth A tlantic cod stocks (IC E S , 1992). M ore investigations on the effects o f prey size and prey type (as well as te m p e ra tu re ) on digestion rates are considered to be essential in resolving th ese a p p are n t parad ox es, as well as to obtain m ore accurate estim ates o f consu m p tio n rates by cod and o th e r species (U rsin et al., 1985). A m e asure o f the relative im p ortance o f cod as a piscivore on G eo rges B an k is provided within the fram e w o rk o f energy budget calculations for G eo rges B ank. Fish consu m ption by cod rep resen ted only 3% o f the
I C E S m a r . Sei. S y m p ., 198 (1994)
total estim ated prod uctio n o f fish (including pre-recruit stages) and just 4% of the pre-recru it fish p roduction c o m p o n en t for the period s 1964—1966 and 1973-1975 (Sissenwine et a l. , 1984). By co m parison, the co n su m p tion o f fish by silver h ak e re p resen ted 55% and 67% of the total estim ated fish p ro du ctio n and 69% and 93% of the pre-recru it fish pro du ction . W hile th e re is con sider able uncertain ty ab o u t the accuracy o f th e absolute values of th e energy b u d get calculations, they provide a reasonable basis for concluding th a t cod is n o t the controlling fish p re d a to r in the G eorges B an k and G ulf o f M aine regions. G iven tha t the biom ass o f b o th cod stocks has recently sharply declined while o th e r piscivor ous species (notably spiny dogfish) have m arkedly increased (U S D e p a rtm e n t of C o m m e rce, 1992), it is believed th a t cod plays only a m inor role in regulating the current production and composition of the finfish bio mass in the G eo rges B a n k /G u lf o f M aine ecosystems.
Environmental factors affecting cod Seasonal an d in teran n u a l changes in th e enviro nm ental conditions o n G eo rges B a n k and in th e G ulf o f M aine exert influences on the cod stocks - in term s o f bo th their distribution an d a b u nd ance. E n v iro n m en tal factors affecting th e ab un dan ce of cod m ost likely occur at the early life history stages. W hile a n u m b e r o f physical and biological p rop erties a n d processes can potentially influ ence grow th and survival o f cod larvae an d juveniles, different p ro p ertie s o r physical processes m ay be im p o rtan t during each of th e different life stages. M any hypotheses have b een pro p o sed linking env iro nm en tal conditions and cod recru itm ent success, but so far few relationships have b een established based on actual ob servations and no ne have yet led to an in-depth u n d erstand in g o f the actual mechanism s involved. T e m p e ra tu re T he environ m ental influence m ost easy to d ocu m e n t is the effect o f te m p e ra tu re changes on stock distributions. D uring N E F S C spring trawl surveys, w hen b o tto m w ater te m p e ra tu re s a re 4-6°C , adult cod are widely dispersed across G eo rges B a n k (Fig. 3). In au tu m n , w hen b o tto m w a te r te m p e ra tu re s are above 10°C, the distribution o f cod shifts with th e highest concentratio ns o f adults found in the cooler, d e e p e r w aters off the n o rth e rn side of the B an k and in the n o rth e rn p a rt o f the G re a t South C h an n el (Fig. 3). A similar seasonal p a t te rn is also evident for juvenile fish (W igley and Serchuk, 1992). In tera n n u al changes in te m p e ra tu re also affect cod distribution p attern s. M o un tain an d M urawski (1992) co m p ared the average b o tto m w ater te m p e ra tu re on G eo rges B an k during spring N E F S C
I C E S m a r . Sei. S y m p ., 198 ( 1994)
G eorges B a n k a nd G u lf o f M aine A tlantic co d stocks
surveys with the average te m p e ra tu re at w hich cod were caught in the surveys. T h e tw o te m p e ra tu re series were related (p < 0.05) with a regression slope of 0.58, such th a t for each degree increase in G eo rges B ank t e m p e ra ture th e average catch te m p e ra tu re increased by 0.58°C. T hese findings suggest that cod c om p ensated for about 40% o f the in terann ual changes in te m p e ra tu re by changing th eir spatial distribution. H o w ever, this in ter annual te m p e ra tu re com p ensation could not simply be explained by m ovem ents o f fish betw een d epths (e.g., shallow to deep ) o r across latitudes (e.g ., n o rth -so u th shifts). O ptim um spaw ning and hatching te m p e ratu res for cod are betw een 5 and 7°C, which typically occur on G eo rges Bank in March and A pril (H ey erdah l and L ivingstone, 1982). H o w ev er, w ater te m p e ra tu re s and p eak spawning vary from y ear to yea r (Sm ith, 1985). In warm to m o d e rate w inters, peak spaw ning can occur as early as D e cem b er, b u t during unusually cold w inters the spaw ning peak may be delayed until the end o f A pril (Sm ith et al., 1981). A lth ou gh te m p e ra tu re clearly affects the timing o f peak spaw ning, th e re is so far no indication th a t overall egg prod uction is affected by te m p era tu re. D u rin g the p eriod 1978-1987, B errien and Sibunka (1993) re p o rte d a do w nw ard tren d in cod egg ab u nd ance o ver the entire n o rth east shelf region (M id dle A tlantic to the G u lf o f M aine). T e m p e ra tu re data (H olzw arth and M ou ntain , 1992) indicate that the second h alf of this decade was ab o u t 1°C w arm er th a n the first half; w ater te m p era tu res in the 1983-1987 p erio d were com parable to the w arm mid-1970s, p a r ticularly on G eorges B ank (Fig. 7). H ow ever, the d e cline in cod egg prod uctio n is m ost likely due to the sharp decline in spawning-stock biom ass th a t occurred b e tw een 1980 and 1986 (Fig. 11). In several stocks o f A tlantic cod, higher w ater te m p eratu res have b e en associated with higher grow th rates (Taylor, 1957). In the au tu m n (during the la tte r p art of the growing season for cod o ff the U S coast), bottom te m p eratu re s on G eorg es B ank (H o lzw arth and M o u n tain, 1992) are w arm er (8.6-13.4°C) tha n in the G u lf of M aine (5.8-9.2°C). T he accelerated grow th an d m a tu ratio n rates o f G eo rges B an k cod co m pared to G u lf of M aine cod may thus be partly due to higher average te m p era tu res on G eo rges Bank. G eorg es Bank also app ears to have higher fish production than the G u lf of M aine; this production is rep resen te d largely by juvenile fish which w ould be potential prey for cod (C oh en and G rosslein, 1987). H ow ev er, as n o te d earlier, n o co rre lation exists betw een m a tu ratio n rates and b o ttom w ater te m p e ratu re s. T he significant correlations found in both stocks betw een m a tu ratio n a nd abu nd ance ( O ’Brien, 1990) suggest that the acceleration of m atu ratio n is a com pensatory response associated with reduced stock size levels in recent years.
103
Based on ichthyoplankton d ata collected during the 1977-1984 M A R M A P surveys, M orse (1989) rep o rted that b oth larval grow th of cod and le n gth-d epen den t larval m ortality were positively co rrelated with w ater tem p e ratu res in the G eo rges B a n k -G u lf o f M aine re gion, and po stulated th a t th e increased m etabolic requirem en ts o f pred ato rs w ere primarily responsible for the higher larval m ortality. Larval m etabolic re q u ire m ents also increase with te m p eratu re. B uckley et al. (1992) rec ord ed larval cod m ortalities ranging from 2 to 10% p e r day in p red ator-free tanks at a constant te m p eratu re of 7°C, and at prey densities o f 10-500 plankters/1 which prov id ed norm al larval cod grow th rates o f 8 -10 % p e r day. T hu s, even u n d er near-optim al conditions, som e cod larvae will not survive. Since both larval m etabolic requ irem en ts and larval grow th (at food densities above a critical level) increase w ith te m p e ra tu re, te m p e ra tu re m ust also play a role - in dep end ently of p red atio n - in regulating larval m ortality . Studies of the co pep od p opulations sam pled with 0.333 m m mesh during the 1977-1984 M A R M A P su r veys show ed: (1) no correlation betw een w a te r column te m p e ra tu re and total co pep od ab un dan ce on G eorges B ank and in the w estern G u lf o f M aine; (2) n o c o rre lation betw een te m p e ra tu re and C. finm archicus on G eorges B an k; a nd (3) a negative co rrelation b etw een te m p e ra tu re and C. finm archicus in th e G u lf o f M aine (M eise-M unns et al., 1990). T hese results seemingly indicate th a t during spring, food o f th e larger cod larvae is less plentiful (or, at least, no different) at hig he r w ater te m p era tu res. It is recognized th a t m ost of the diet of sm aller cod larvae com prises sm aller-bodied c op epo d species n o t fully sam pled by th e 0.333 m m mesh. H o w ever, no evidence exists to suggest that foo d supplies at higher w ater te m p eratu re s are insufficient to sustain higher grow th rates. A s such, part of the increase in m ortality with te m p e ra tu re is likely attribu tab le to in trinsic te m p e ra tu re effects. Koslow et al. (1987) considered the influence o f a n u m b e r o f climatic variables o n recruitm en t in various cod stocks in th e N orth w est A tlantic, and fo und that both local and large-scale winds w ere significant co rre lates for a n u m b e r o f stocks b u t sea-surface te m p e ra tures generally w ere not. A lth o u g h the G eorg es B an k cod stock was not included in this study, subsequent analysis o f G eorges B ank recru itm en t (Serchuk et al., 1993) and spring and fall te m p e ra tu re anom aly estim ates (H olzw arth an d M o un tain, 1992) revealed no co rre lation betw een recru itm ent and th e local interan nu al te m p e ra tu re patterns.
A d v e c tio n T he characteristic circulation p attern on G eo rges Bank transpo rts gadid ichthyoplankton from spaw ning areas
104
F. M . Serchuk et al.
on the n o rth e astern p a rt o f th e B ank to th e southw est. T h e ra te o f w ater m o v em ent is such th a t, by the time w aterm asses reach th e G re a t South C h ann el, many larvae have m oved shoalw ards and are thus retain ed on the B ank. V ariability in the circulation p a tte rn , ho w ever, can result in a g reate r o r lesser advective losses of larvae from the B an k. P olacheck et al. (1992) d o cu m e n ted an unusual w est w ard drift of ha d d o ck larvae from G eo rges B ank into the M iddle A tlantic region in th e spring o f 1987. T he larvae a p p e a re d to have survived th rou gh th e su m m er and au tu m n becau se o f unusually cool b o tto m w ater te m p e ratu res o n the o u te r half of the shelf. G eo rges B an k cod larvae w ere n o t similarly advected into the m id-A tlantic a re a in 1987, b u t this may occur in som e years. In general, how ever, advective losses o f larvae w ould be less significant to re cru itm en t success for cod th a n h ad do ck, becau se cod has a m o re p rotracted spaw ning season. G ulf S tream w arm -co re eddies n e a r th e so u th ern edge o f G eorg es B an k m ay play an im p o rta n t role in some years in b o th en training larvae and in moving w ater on and off the shelf (L ough, 1982; Joyce and W iebe, 1983). T h e so u th east flank o f G eo rges B a n k is particularly vulne rable to advective losses during period s o f strong no rtherly w indstress (W alford, 1938; C o h en et al., 1986). M yers an d D rin k w a te r (1989) investigated the relationship b etw een w arm -core ring frequency an d recru itm en t in a n u m b e r o f N orthw est A tla ntic fish stocks. A lth o u g h th e re was a tenden cy for decreased re cru itm en t with increased ring freq uen cy, no relatio n ship w as found b etw e en w arm -core rings and cod re cruitm ent on G eo rges B ank. Difficulties in identifying the influence o f e n v iron m ental processes on larval survival w ere illustrated by C o h e n et al. (1986). C om parisons w ere m ade of the d evelo pm ent o f the 1981, 1982, and 1983 cod and h a d dock y ear classes o n G eo rg e s B a n k in relation to various physical processes which could affect larval survival. In 1982, after w hat a p p e a re d to be n orm al spaw ning, low larval abu n d a n c e suggested unusually high egg and lar val mortality. This m ortality was m o st likely du e to: (1) a large sto rm e v ent in early A p ril, which satellite im agery indicated d rov e a large am o u n t o f w ater off th e so u th eastern flank o f the B ank; (2) a G ulf Stream ring which e n train ed a large volum e o f w ater from the sou th eastern p a rt o f the B a n k during M arch and A pril; o r (3) a co m bination o f these events.
Stratification T h e d ev elo p m en t o f w ater colum n stratification during spring o n th e so u th ern flank o f G eorg es B an k leads to increased p h y to p lan k to n an d zo o p lan k to n in the vicinity o f th e pycnocline. This concentratin g m echanism for the
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larval fo od organism s is believed to be im p o rtan t for the grow th and survival o f gadid larvae. B uckley a n d L ough (1987) investigated the effect o f w ater colum n stratifi cation on G eorg es B an k on the short-term grow th and condition of h addock and cod larvae, and found that h add ock larvae from a site w here th e w ater was well m ixed w ere in p o o r condition, while larvae from a nearby site w here th e w ater was stratified w ere in good condition. C o d larval condition did n o t show as strong a relationship with stratification as did hadd ock . C od collected at the sam e sites as h ad do ck w ere in b e tte r condition and w ere faster growing. C o d larvae a p p e a r to be b e tte r ad a p te d to w in te r conditions - w h en prey densities are generally low er. H a d d o c k larvae require higher prey densities tha n cod an d seem m o re ad ap ted to spring conditions w hen prey are co n cen trated by stratifi cation (L ough, 1984). T h e findings suggest th a t in teran nual changes in stratification conditions on G eo rges B an k could have a m a rk ed influence on th e survival of h ad do ck larvae and th e reb y affect recru itm en t, but p robably a lesser effect o n cod.
T u rb ulent mixing T u rb u len t mixing g e n e rated by strong tidal curren ts and surface windstress on G eorg es B ank may affect the en c o u n ter rate b etw een larvae and prey organism s (R othschild and O sb o rn , 1988). Studies from o th e r regions have indicated a relationship b etw een w atercolum n tu rbu len ce and larval feeding (Sundby and Foss u m , 1990). A ltho ug h in form ation is n o t yet available on w ater turbu len ce a nd the feeding o f cod larvae in the G eorg es B a n k -G u lf o f M aine region, M eise-M unns et al. (1990) fo und a significant positive correlation b e tw een total co p ep o d ab un dan ce and w indstress during F e b ru a ry -A p ril on bo th G eorges B ank and in the w est ern G ulf of M aine. W indstress could conceivably affect larval grow th and m ortality via effects on p rey density and te m p e ra tu re , b u t specific linkages have not yet b een established.
Fish food production A ltho ug h m a tu ra tio n rates o f b o th G e o rg es B an k and G ulf o f M aine cod have b een influenced by stock size, the faster grow th and m a tu ra tio n rates of G eo rges B an k cod relative to G ulf of M aine cod may be due to e n v iron m ental differences. A s n oted previously, w ater te m p e ra tu re s an d ap p a re n t fish prey p ro du ctio n on G eorges B ank are higher than in th e G u lf o f M aine. H o w ever, secondary p roduction (zo o plan kton and m a crob entho s) a p p ears to be a b o u t the sam e in both areas (C o h en an d G rosslein, 1987) and th e p ro p o rtio n of fish in th e diet o f G u lf o f M aine cod is as high - o r higher - tha n for G eorg es B ank cod (L angton an d B o w m an ,
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Georges B a n k a nd G u lf o f M aine A tlantic cod stocks
1980; B ow m an and M ichaels, 1984). O nce again the pieces of the puzzle do n o t fit neatly into place. A n o th e r p roblem atic aspect is th a t som atic and gona dal d ev elop m ent a p p ear to be de-coupled in G eorges B ank cod but not for cod in th e G u lf o f M aine. M ean lengths-at-age for G eorges B ank cod do not show any tren d over tim e, despite declines in age/size at m a turity, w hereas in the G u lf o f M aine th e re is evidence for increased grow th o f cod along with faster m aturation ( O ’B rien, 1990). If G eo rges B an k cod w ere already at o r n e a r their physiological limit for grow th, it is plausible tha t any excess energy might be diverted tow ards gonadal grow th, resulting in acceleration of m atu ratio n. A l though changes in m a tu ratio n are usually associated with changes in grow th, this is n o t always the case (R off, 1982). If the G ulf of M aine is a less productive environ m en t for cod (relative to G eorg es B ank ), then an in crease in available prey might enable the stock to shunt excess energy into b o th som atic and gonadal grow th, th e reb y accelerating b o th grow th and m a tu ratio n rates. H o w ever, th e re are no d ata th a t indicate th a t secondary pro du ctio n in the G u lf o f M aine has increased in recent years.
Predator/prey biomass changes Large-scale fluctuations in ab un dan ce o f m ackerel, h er ring, and sand lance have occurred in the G eorges B a n k -G u lf o f M aine region during the past several decades (S h erm an, 1986). T hese species are n o t only prey for cod but also significant p red ato rs o f cod eggs and larvae (G rosslein et al.. 1980; M ichaels, 1991). T he relative occurrence o f m ackerel/herring/sand lance in the diet o f cod (Fig. 17) broadly reflects changes in the ab un dan ce levels o f these prey. Em pirical evidence exists that m ackerel and herring exert pred ato ry control on sand lance abu n d an c e (Fogarty et al., 1991). G iven these interactions, the p re d a to r-p re y linkages am ong just these fo u r species may be extraordinarily com plex. Presently, th e re are insufficient da ta to adequately quantify p red atio n m ortality o n cod eggs, larvae, or juveniles. T o obtain quantitatively reliable m easu res of critical rates, intensive studies o f the first year of life including a life table along with co ncu rrent studies of physical/biological processes - are urgently need ed.
Future research A lth ou gh m any attem pts have b e en m ade to correlate en vironm ental variables with variations in stock a b u n dance o f cod, nearly all o f these lack long-term predic tive po w er and usually b re ak dow n w hen re-exam ined with new d ata (M yers et al., 1993a). Large-scale synch rony in recruitm en t p attern s o f cod stocks from
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L ab ra d o r to N o va Scotia w ere re p o rte d by Koslow (1984) and linked to large-scale m eteorological pressure fields a nd offshore winds (Koslow et al., 1987). T h o m p son and Page (1989), using m ore ap p ro p riate analytical m e th od s, also detec ted a synchrony in recru itm e n t p a t terns for these sam e cod stocks but show ed th a t the co rrelation with large-scale w e ath er p attern s was not significant. M o re recently M yers et al. (1993a) rep o rted a strong relationship b etw een salinity an d recru itm ent for cod stocks in the N ew fo un dlan d region, b u t the causal oceanographic an d food chain m echanism s link ing these variables w ere not identified. In a n o th e r study o f n o rth ern cod stocks (G ra n d B an k o f N ew foundland, L a b ra d o r, an d W est and E ast G re e n la n d Shelves), Myers et al. (1993b) exam ined cod spaw ning tim es in relation to seasonal cycles o f p la n kton ab u n d an ce , ocean te m p e ra tu re , w ater colum n stability, and o ceanic tran sp ort; no consistent relationships b etw een these features and cod spawning times were d etected , and only a very general m atch existed b etw een tim ing of larval prod uction and planktonic food. C o h en et al. (1991) u p d a te d and re-exam ined the time series o f rec ru itm en t estim ates for cod (a n d hadd ock ) stocks th ro u g h o u t th e entire N orthw est A tlan tic (includ ing the G ulf o f M aine and G eo rges B a n k stocks) and found th a t these stocks fell into tw o groups: a n o rth e rn g roup in the L ab ra d o r/N ew fou nd land region, a nd a sou thern group extending from the G u lf o f St L aw rence to G eorg es B ank. R e cru itm e n t synchrony was stron ger w ithin the groups than b etw een th e m , suggesting th a t local-scale physical factors exert m ore control o v er re cruitm ent th an large-scale climatic effects. A tte m p ts to link recru itm ent to physical en v iro n m en tal factors on G eorg es B ank have so far been unsuccess ful (Fogarty et al., 1987). Since cod recruitm en t appears to b e a co m p o u n d function o f largely density-inde p en d en t factors op erating in the egg and larval stages and density-d epen den t factors op eratin g during the juvenile period , correlation analysis alone will certainly be in a d eq u a te to sort out th e controlling m echanism s. A chieving predictive capability will req u ire a b e tte r quantitative u nd erstand ing o f th e dynam ics o f key p ro cesses controlling grow th and m ortality th ro u g h o u t th e first y ear o f life. T hese processes are th e focus o f two new research initiatives on G eorg es B a n k being con ducted u n d e r the auspices of: (1) the US G L O B E C P rogram ; and (2) the N O A A C oastal O cean P rogram . T he goal o f the US G L O B E C N W A tlantic/G eorges B ank Study is to predict the potential effects o f climate change on G eorg es B ank gadid stocks (U S G L O B E C , 1992). T h e P ro gram will involve intensive investigations o f those processes believed to be m ost susceptible to alteration by climate change and which are likely to be im p o rtan t to the p op ulatio n dynam ics o f gadid larvae and the ir prim ary prey organism s, C alanus a nd Pseudo-
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calanus. T h re e physical processes will be studied in detail: (1) stratification of the w ater colum n; (2) re te n tion and loss o f w ater from the B an k; and (3) frontal exchange processes. C o n cu rre n t biological studies will be co n d ucted on feeding, grow th (an d physiological co ndition), ab u n d an ce , distribution, and m ortality of cod and h addock eggs/larvae (as well as principal zo o p la n kton species). Identification of m ajor in ve rteb rate p red ato rs and estim ation of p red atio n rates o n pelagic stages o f 0-group gadids will also be attem p ted . Studies o f fish p re d a tio n on larval and juvenile gadids will be co nd ucted u n d e r the N O A A C oastal O cean P ro gram (C O P ) research initiative on pred ato r/p rey interactions in th e G eo rg es B a n k ecosystem . T h e chief goal o f th e C O P G eorg es B an k study is to u n d erstan d the interactive effects o f exp loitation and interspecific interactions on the fish co m m unity dynam ics o f G eorg es B ank. This p ro g ram involves: (1) retrospective studies o f changes in fish biom ass, species com position, and p re d a to r-p r e y interactions; (2) p rocess-oriented studies in th e field and la b o rato ry o n p red atio n on 0-group stages, digestion rates, etc.; an d (3) multispecies m o d e l ling to test and ev aluate h yp otheses a b o ut controlling m echanism s, and to exam ine lo ng-term effects of differ en t fishery m a n a g e m e n t strategies on the productivity of the G eorg es B a n k ecosystem .
References Anthony, V. C. 1990. The New England groundfish fishery after 10 years under the Magnuson Fishery Conservation and Management Act. N. Am. J. Fish. M anage., 10: 175-184. Auditore, P. J., Bolz, G. R., and Lough, R. G. 1988. Juvenile haddock, M elanogrammus aeglefinus, and Atlantic cod, Gadus m orhua, stomach contents and morphometric data, from four recruitment surveys (1984-1986) in the Georges Bank-Nantucket Shoals area. Northeast Fisheries Center, W oods Fiole Lab. Ref. 88-05. 105 pp. Auditore, P. J., Lough, R. G ., and Broughton, E. A. 1994. A review of the comparative development of Atlantic cod and haddock based on an illustrated series of larvae and juveniles from Georges Bank. N A FO Sei. Coun. Stud., 20: 7-18. Azarovitz, T. R. 1981. A brief historical review of the Woods Hole Laboratory trawl survey time series. In Bottom Trawl Surveys, pp. 62^67. Ed. by W. G. Doubleday and D. Rivard. Can. Spec. Publ. Fish, aquat. Sei., 58. 273 pp. Bailey, K. M., and Houde, E. D. 1989. Predation on eggs and larvae of marine fishes and the recruitment problem. Adv. Mar. Biol., 25: 1-83. Beacham, T. D. 1982. Some aspects of growth, Canadian exploitation, and stock identification of Atlantic cod (Gadus morhua) on the Scotian Shelf and Georges Bank in the northwest Atlantic Ocean. Can. Tech. Rep. Fish, aquat. Sei., 1069. 43 pp. Beacham, T. D. 1983. Variability in median size and age at sexual maturity of Atlantic cod, Gadus morhua, on the Scotian Shelf in the Northwest Atlantic Ocean. Fish. Bull., 81:303-321. Berrien, P., and Sibunka, J. 1993. Distribution patterns of fish eggs in the northeast continental shelf ecosystem. N O A A Tech. Memo. NMFS-F/NEC (in press).
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Bolz, G. R., and Lough, R. G. 1988. Growth through the first six months of Atlantic cod Gadus morhua and haddock M elanogrammus aeglefinus based on daily growth incre ments. Fish. Bull., 86: 223-235. Bowen. D. (E d.). 1987. A review of stock structure in the Gulf of Maine area: a workshop report. CA FSA C Res. Doc., 87/ 21. 51 pp. Bowman, R., and Michaels, W. 1984. Food of seventeen species of Northwest Atlantic Fish. US Dept. Commerce. N O A A Tech. Memo. NMFS-F/NEC-28. 183 pp. Buckley, L. J., and Lough, R. G. 1987. Recent growth, biochemical composition, and prey field of larval haddock (Melanogrammus aeglefinus) and Atlantic cod (Gadus m or hua) on Georges Bank. Can. J. Fish, aquat. Sei., 44: 14—25. Buckley, L. J., Smigielski, A. S., H alavik,T . A ., Burns, B. R., and Laurence, G. C. 1992. Growth and survival of the larvae of three tem perate marine fish species at discrete prey densities. II. Cod (Gadus morhua), winter flounder (Pseudopleuronectes americanus), and silver hake (Merluccius bilinearis). In Physiology and biochemistry of fish larval development, pp. 183-195. Ed. by B. T. W althers and H. J. Fyhn. University of Bergen. Norway. Butman, B., Loder, J. W. and Beardsley, R. C. 1987. The seasonal mean circulation: observation and theory. In Georges Bank, pp. 125-138. Ed. by R. H. Backus. MIT Press, Cambridge, MA. 593 pp. CAFSAC (Canadian Atlantic Fisheries Scientific Advisory Committee). 1989. Advice on the m anagement of groundfish stocks in 1990. CA FSA C Adv. Doc., 89/12. 82 pp. Chapman, D. C., and Beardsley, R. C. 1989. On the origin of shelf water in the Middle Atlantic Bight. J. phys. Oceanogr., 19: 384-391. Cohen, E. B., and Grosslein, M. D. 1981. Food consumption in five species of fish on Georges Bank. ICES CM 1981/G: 68. Cohen, E. B., and Grosslein, M. D. 1987. Production on Georges Bank compared with other shelf ecosystems. In Georges Bank, pp. 383-391. Ed. by R. H. Backus. MIT Press, Cambridge, MA. 593 pp. Cohen, E. B., Mountain, D. G ., and Lough. R. G. 1986. Possible factors responsible for the variable recruitment of the 1981, 1982, and 1983 year-classes of haddock (M elano grammus aeglefinus L.) on Georges Bank. N A FO SCR Doc., 86/110. 27 pp. Cohen, E. B.. Mountain. D. G ., and O'Boyle, R. 1991. Localscale vs. large-scale factors affecting recruitment. Can. J. Fish, aquat. Sei., 48: 1003-1006. Cohen, E. B., Mountain, D. G ., and Smith, W. G. 1984. Comparison of cod and haddock spawning in 1982 and 1983 on Georges Bank. N A FO SCR Doc., 84/VI/75. Colton, J. B., Smith, W. G ., Kendall, A. W .. Berrien, P. L., and Fahay, M. P. 1979. Principal spawning areas and times of marine fishes, Cape Sable to Cape Hatteras. Fish. Bull., 76: 911-915. Daan, N. 1973. A quantitative analysis of the food intake of North Sea cod, Gadus morhua. Neth. J. Sea Res., 6: 479517. Durbin, E. G ., Durbin, A. G ., Langton. R. W ., and Bowman, R. E. 1983. Stomach contents of silver hake, Merluccius bilinearis and Atlantic cod, Gadus m orhua, and estimation of their daily rations. Fish. Bull., 81: 437—454. Earll, R. E. 1880. A report on the history and present condition of the shore cod-fisheries of Cape Ann, Mass., together with notes on the natural history and artificial propagation of the species. Report of the US Fish Commission 1878, vol. VI: 685-740. Edwards, R. L., and Bowman. R. E. 1979. Food consumed by continental shelf fishes. In Predator-prey systems in fish communities and their role in fisheries management, pp.
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387^106. Ed. by H. Clepper. Sport Fishing Institute, Wash ington. DC. Fogarty, M. J., Cohen, E. B., Michaels, W. L., and Morse, W. W. 1991. Predation and the regulation of sand lance popu lations: an exploratory analysis. ICES mar. Sei. Symp., 193: 120-124. Fogarty, M. J., Sissenwine, M. P., and Grosslein, M. D. 1987. Fish population dynamics. In Georges Bank, pp. 494-507. Ed. by R. H. Backus. MIT Press, Cambridge, MA. 593 pp. Garrison, L. E ., and McMaster, R. L. 1966. Sediments and geomorphology of the continental shelf off southern New England. Mar. Geol., 14: 273-289. Grosslein. M. D., Langton, R. W., and Sissenwine. M. P. 1980. Recent fluctuations in pelagic fish stocks in the Northwest Atlantic, Georges Bank region, in relation to species inter actions. Rapp. P.-v. Réun. Cons. Int. Explor. Mer, 177: 374405. Halliday, R. G ., and Pinhorn, A. T. 1990. The delimitation of fishing areas in the Northwest Atlantic. J. Northw. Atl. Fish. Sei., 10: 1-51. Heyerdahl, E. G ., and Livingstone, Jr., R. 1982. Atlantic cod, Gadus morhua. In Fish distribution, pp. 70-72. Ed. by M. D. Grosslein and T. R. Azarovitz. M ESA New York Bight Atlas Monograph 15, New York Sea Grant Institute, Albany, New York. 182 pp. Holzwarth, T., and Mountain, D. 1992. Surface and bottom temperature distributions from the Northeast Fisheries C enter spring and fall trawl survey program, 1963-1987, with addendum for 1988-1990. Northeast Fisheries Science Center Ref. Doc. 90-03. 62 pp. Hopkins, T. S., and Garfield III, N. 1981. Physical origins of George Bank water. J. mar. Res., 39: 465-500. H oude. E. D. 1987. Fish early life dynamics and recruitment variability. Am. Fish. Soc. Symp., 2: 17-29. H oude, E. D. 1989. Subtleties and episodes in the early life of fishes. J. Fish Biol., 35: 29-38. Hunt. J. J. 1988. Status of the Atlantic cod stock on Georges Bank, N A FO Division 5Z and Subarea 6, in 1987. CAFSAC Res. Doc., 88/73. 50 pp. Hunt, J. J. 1989. Status of the Atlantic cod stock on Georges Bank in unit areas 5Zj and 5Zm, 1978-88. CAFSAC Res. Doc., 89/47. 26 pp. Hunt, J. J. 1990. Status of the Atlantic cod stock on Georges Bank in unit areas 5Zj and 5Zm, 1978-89. CA FSA C Res. Doc., 90/80. 37 pp. Hunt, J. J., and Buzeta, M.-l. 1992. Status of the Atlantic cod stock on Georges Bank in unit areas 5Zj and 5Zm, 1978-91. C AFSAC Res. Doc., 92/48. 23 pp. Hunter, J. R. 1981. Feeding ecology and predation of marine fish larvae. In Marine fish larvae, pp. 33-77. Ed. by R. Lasker. University of Washington Press, Seattle, WA. 131 pp. ICES. 1992. Report of the Multispecies Assessment Working Group. ICES CM 1992/Assess: 16. ICES. 1993a. Report of the North-Western Working Group, May 1993. ICES CM 1993/Assess: 18. ICES. 1993b. Report of the Working group on the Assessment of Demersal Stocks in the North Sea and Skagerrak, Octobcr 1992, ICES CM 1993/Assess: 5. ICNAF. 1973. Standing Committee on Research and Statistics. Appendix I - Report of Assessments Subcommittee. ICNA F Redbook 1973, Part I, 133 pp. ICNAF. 1976. Standing Committee on Research and Statistics. Part C. R eport of Standing Committee on Research and Statistics (STACRES). ICNAF Redbook 1976, 219 pp. Ingham, M. C., Armstrong, R. S., Chamberlin, J. L., Cook, S. K.. Mountain, D. G ., Schlitz, R. J., Thomas, J. P., Bisagni, J. J., Paul, J. F., and Walsh, C. W. 1982. Summary of the physical oceanographic processes and features pertinent to
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pollution distribution in the coastal and offshore waters of the northeastern United States, Virginia to Maine. N O A A Tech. Memo. NMFS-F/NEC-17. 166 pp. Jensen. A. C. 1972. The cod. Thomas Y. Crowell Co., New York. 182 pp. Joyce, T ., and Wiebe, P. 1983. Warm-core rings of the Gulf Stream. Oceanus, 26: 34-44. Kane, J. 1984. The feeding habits of co-occurring cod and haddock larvae from Georges Bank. Mar. Ecol. Prog. Ser., 16: 9-20. Koslow, J. A. 1984. Recruitment patterns in northwest Atlantic fish stocks. Can. J. Fish, aquat. Sei., 41: 1722-1729. Koslow, J. A ., Thompson, K. R., and Silvert, W. 1987. Recruitment to Northwest Atlantic cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) stocks: influence of stock size and climate. Can. J. Fish, aquat. Sei., 44: 26-39. Langton, R. W ., and Bowman, R. E. 1980. Food of fifteen northwest Atlantic gadiform fishes. US Dept. Commerce, N O A A Tech. Rep. NMFS-SSRF-740. 23 pp. Laurence, G. C., and Rogers, C. A. 1976. Effects of tem pera ture and salinity on comparative embryo development and mortality of Atlantic cod (Gadus morhua L.) and haddock (M elanogrammus aeglefinus (L.)). J. Cons. int. Explor. Mer, 36: 220-228. Laurence, G. C., Smigielski, A. S., H alavik.T . A ., and Burns, B. R. 1981. Implications of direct competition between larval cod (Gadus morhua L.) and haddock (M elanogrammus aeg lefinus) in laboratory growth and survival studies at different food densities. Rapp. P.-v. Réun. Cons. int. Explor. Mer, 178: 304-311. Livingstone, R ., Jr., a n d D e ry , L. 1976. A n observation on the age and length at maturity of cod in the Georges Bank and Browns Bank stocks. ICNAF Res. Doc., 76/VI/42. 2 pp. Loder, J. W. 1980. Topographic rectification of tidal currents on the sides of Georges Bank. J. phys. Oceanogr., 10: 13991416. Lough, R. G. 1982. Observations on the impingement of warm core eddy 81-C on Georges Bank. E OS 63: 59. Lough, R. G. 1984. Larval fish trophodynamic studies on Georges Bank: sampling strategy and initial results. In The propagation of cod Gadus morhua L., pp. 395-434. Ed. b y E . Dahl, D. S. Danielssen, E. Moksness, and P. Solemdal. Flødevigen rapportser., 1, 1984. Lough, R. G ., and Bolz, G. R. 1989. The movement of cod and haddock larvae onto the shoals of Georges Bank. J. Fish Biol., 35: 71-79. Lough, R. G ., and Potter, D. C. 1993. Vertical distribution patterns and diel migrations of larval and juvenile haddock, M elanogrammus aeglefinus, and Atlantic cod, Gadus m or hua, on Georges Bank. Fish. Bull., 91: 281-303. Lough, R. G ., Valentine, P. C., Potter, D. C., Auditore, P. J., Bolz, G. R., Neilson, J. D., and Perry, R. I. 1989. Ecology and distribution of juvenile cod and haddock in relation to sediment type and bottom currents on eastern Georges Bank. Mar. Ecol. Prog. Ser., 56: 1-12. Mayo, R. K., Fogarty, M. J., and Serchuk, F. M. 1992. Aggregate fish biomass and yield on Georges Bank, 1960-87. J. Northw. Atl. Fish. Sei., 14:-59-78. Mayo, R. K., O ’Brien, L., and Serchuk, F. M. 1993. Assess ment of the Gulf of Maine cod stock for 1992. Northeast Fisheries Science Center Ref. Doc. 93-04. 54 pp. Meise-Munns, C., Green, J.. Ingham, M., and Mountain, D. 1990. Interannual variability in the copepod populations of Georges Bank and the western Gulf of Maine. Mar. Ecol. Prog. Ser., 65: 225-232. Michaels, W. L. 1991. The impact of mackerel predation on the survival of pelagic age-zero sandlance, cod, and haddock on Georges Bank during spring of 1986. Master of Science
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