Winter distribution and movements of northern ... - Wiley Online Library

FISHERIES OCEANOGRAPHY

Fish. Oceanogr. 4:2, 128-146, 1995

Winter distribution and movements of northern Atlantic cod ( Gadus morhua) along the Newfoundland-Labrador continental shelf edge derived from observations on commercial trawlers

J. S. WROBLEWSKI,' D. W. KULKA,' S. NARAYANAN,' A. M. O A K E , ~ A. G. COLLIER,' AND B. D. MCGRATH' 'Fisherit., Oceunopuphy Group, Ocean Sciences Centre, Memorid Unicersity of Newfiiundland, S t john's. Netefoutidhnii, Canada ,41B 3x7 'Science Branch, Department of Fisheries and Oceans, Northu:est Atlantic Fisheries Centre, PO Box 5667, St john's, Neufmmdhnd, C'mudu A 1 C 5x1 'Fisheris, Resource Decelopment Ltd, 202 1 Brunswick Street, Sum 3 i 7. Halifax, Nova Scotia, C a d B3K t Y 5

ABSTRACT Catch-effort d x a recorded by observers aboard commercinl trdwlers hshing the Newfoundland-Labrador continental shelf during the winters of 1980-1991 were used t o define the distribution and rnmeinents of presp~wningand spanning Atlantic cod, C d u ~ rnorhw. Although cod were widely distributed over the outer continental bhelf in the months of January to April, commercial concentrations were consistently located in three 'ireas along the shelf edge. These three areas, ~ o n ~ t i t u t i nonl) g 16% of the 190000 km2 fishing ground tor northern cod, were centred north-east of Belle Isle Bank, north-east of Funk Island Bank, and between Funk Island Bank and the northern Grand Bank Tow positions with catch rates 2 5000 kg h - ' were plotted on a weekly basis to define movements of cod schools during the winters of 1986-1988. I t was assumed that Lhange in the collective position of trawlers taking large quantities of cod reflected shoal movements. Analyses indicate that cod moved southeast along the shelf edge in January and February of each year, crossing trom NAFO Division 2J into 3K Each March, cod in Division 3K apparently moved northwest dgninst prevailing ocean currents into the Hamilton Bank region of 21. These movements do not appear to be a direct response to ocean temperature or pack ice Received h r publication 29 December 1994 Accepted for publication 17 February

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conditions, although cod moving along the shelf break were generally in bottom waters of 2 4 ° C . W e suggest these patterns reflect the migratory behaviour of prespawning and spawning cod.

Key words: northern cod, winter distribution, spawning migration, Newfoundland Fisheries Observer Program

INTRODUCTION Knowledge of the distribution and migratory behaviour of an exploited stock, particularly during its reproductive phase, is required for proper management of the resource. Northern cod, i.e. Atlantic cod, Gudus morhua, in North Atlantic Fisheries Organization (NAFO) Divisions 21, 3K and 3L off Labrador and eastern Newfoundland (Fig. l ) , was the most important marine resource of Atlantic Canada, until the fishery collapsed in 1992 (Lear and Parsons, 1993). However, there has been no long-term study of the distribution and migration of northern cod during its winter (January to April) prespawning-early spawning period. After feeding along the coast during the summer, most fish migrate offshore to overwinter and spawn in the spring (Lear and Green, 1984). Tagging studies determined that most cod migrate to offshore bank areas (Templeman, 1979; Lear, 1984), while some fish overwinter in the major bays (Wroblewski et al., 1994, 1995). T h e postspawning migration of northern cod to the coasts of Labrador and Newfoundland is well documented (Templeman and Fleming, 1963; Templeman, 1966; Rose, 1993). Relatively little information exists on the distribution and movements of northern cod during the winter period offshore (Rose et al., 1994). Research surveys, the primary source of information on groundfish distribution off Labrador and Newfoundland (Baird et al., 1992; Bishop et al., 1993), take place primarily during the late fall (November-December) when fish may still be migrating across the shelf (Lilly, 1994). The commercial fishery for northern cod has pro@ 1995 Blackwell Science Ltd.

Winter migration of northern Atlantic cod

129

Figure 1. Newfoundland-Labrador continental shelf bathymetry (depths in m), major banks and NAFO Divisions. The diamond marks the location of the 1986 current meter mooring, and the star shows the position of the 1987 current meter mooring of Wright et al. (1988). Boxes 1, 2 and 3 refer to the same areas delineated in Fig. 2.

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LONGITUDE vided detailed scientific data since 1980, when observers were first deployed on offshore trawlers (Kulka and Firth, 1987). We have used geo-referenced, catch data recorded by observers aboard trawlers fishing the Newfoundland-Labrador continental shelf during the winter months of January to April, 1980-1991 to define the distribution of prespawning and early spawning cod. March-May was identified as the peak spawning period for Hamilton, Belle Isle, and Funk Island Bank cod by Myers et al. (1993). January-April corresponds to the height of the fishery, with maximum data collected. To determine movements of cod, trawler fishing positions were examined on a weekly basis during the winters of 1986-1988. Virtual population analysis indicated that a severe decline in the stock biomass of the northern cod began in 1989 (Baird et d., 1992; Bishop et al., 1993). Data sufficient for detailed analyses exist @ 1995 Blackwell Science Ltd., Fish. Oceanogr., 4, 128-146.

for the 3 years prior to this decline in stock size. Temporal changes in the collective position of trawlers taking large quantities of cod are assumed to reflect movement of cod schools. In deriving cod migration patterns, we considered the influence of fishing regulations, harvesting strategies and pack ice conditions on trawler movements. Having found evidence of schools of cod moving along the shelf break bathymetry, we then considered whether these fish might be responding to bottom currents, temperature gradients or the ice edge.

PHYSICAL OCEANOGRAPHIC ENVIRONMENT The continental shelf off Labrador and north-eastern Newfoundland consists of a series of offshore banks

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punctuated by deep, cross-shelf trenches. The banks are separated from the coast by a channel (marginal trough) which spans the length of the shelf in an intermittent fashion. At the shelf edge, the water depth increases rapidly to over 1000 m, resulting in a sharp shelf break (Fig. 1). The oceanographic climate of the NewfoundlandLabrador shelf is strongly influenced by the Labrador Current, which transports low-salinity Arctic waters, ice and icebergs to lower latitudes. This current has two branches. The main current occurs along the shelf break, and flows more or less parallel to the local bathymetry as a strong western boundary current. A weaker branch carrying approximately 15% of the total transport is located in the marginal trough, and communicates with the offshore branch through the deep saddles that separate the banks (Lazier and Wright,

1993). Coincident with the offshore branch is a strong front between the cold, low-salinity waters of the shelf and the warmer, more saline waters of the Labrador Sea. O n the offshore banks, the front is located more or less at the shelf edge. However, the subsurface intrusions of the Labrador Sea water through the saddles result in a permanent layer of warm (> OOC) water at the bottom, even though in the upper layers, the subzero-degree water extends to the shelf edge (Narayanan et al., 1991). The oceanographic conditions along the shelf edge, near the fishing locations in the winter, have significant variability induced by shifts in the position of the Labrador Current, and by disturbances (shelf waves and eddies) generated upstream propagating to lower latitudes. These oscillations are primarily storm-induced, and consequently have a time scale of a few days to a week (Narayanan et al., 1991). An environmental factor that affects fishing activity is the pack ice. During the winter months, most of the northern cod habitat is covered by ice formed at higher latitudes and transported to the region by the Labrador Current. The ice season begins in late autumn, and by January most of the Labrador Shelf is covered in ice. With the onset of spring (usually April), the ice edge begins to retreat. By July the shelf areas off eastern NewfOundland and southern Labrador are mostly free of ice (Narayanan et ul., 1994).

NEWFOUNDLAND FISHERIES OBSERVER PROGRAM The Newfoundland Fisheries Observer Program was implemented for surveillance, and for the collection of biological data and catch-effort statistics (Kulka and

Table 1. Number of Newfoundland Fisheries Observer Program records in January to April, 1986 to 1988.

Year

January

February

March

April

1986 1987 1988

487 2459 2968

493 3280 2739

1123 3161 2268

458 2322 2989

Firth, 1987). Since 1980, observers stationed on a proportion of the offshore trawler fleets of all countries have recorded information on net tow positions, hours towed and amount of each fish species caught. For the fishery directed toward northern cod inside Canada’s 200 mile economic zone in NAFO Divisions 2J, 3K and 3L, the level of fishing activity observed (i. e. the proportion of the total sets fished by the fleets in a year for which there is a scientific record) was about 20% from 1980 to 1986. Beginning in 1987, coverage was expanded to 100% of the offshore 2J3K3L cod fishery. Table 1 shows the number of observed tows in which cod were caught during January-April in the years 1986 to 1988. There was little or no fishing activity directed at northem cod during June-October. The greatest fishing effort by trawlers (and maximal observer data collected) occurred during January to April when the cod were concentrated near the shelf edge.

METHODS Winter distribution of northern cod Observer data from the foreign and domestic northern cod fishery, and from fisheries directed toward other groundfish species, were used to examine the distribution of cod inside Canada’s 200 mile economic zone. Using this additional information from cod by-catches provided for a more widely sampled area. The underlying assumption in using these data for a winter distributional study of cod is that commercial fishing activity encompassed the main area over which the stock was spread at this time of year. A comparison of fall research survey data with observer data from the years 1981 to 1991 by Rose et al. (1994), suggests this assumption is valid, except for the winter of 1985 when there was little fishing activity on the northern Labrador shelf. Plots of cod distribution in winter were constructed for each of the years 1980 to 1991, using the geographical information system SPANS (Intera TydacTM) to convert the geo-referenced data (catch per hour from

01995 Blackwell Science Ltd., Fish. Oceanogr., 4,128-146.

Winter mgmtiun of northern Atlantic cod

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Figure 2. Areas of commercial concentrations of northern cod derived from Newfoundland Fisheries Observer Program data collected during the winter fishery for the years 1980-1991. Commercial concentrations were indicated where catch rates exceeded 1500 kg h-I. Regions where these concentrations reoccurred for several years are shaded darker (see key).

each fishing set) into a description of where cod were most abundant. These annual winter distribution maps were then examined to determine where commercially significant densities of cod occurred consistently. A summary chart (Fig. 2) was constructed showing areas where commercial concentrations persisted for 1-3, 46, 7-9, and 10-12 years of the study period. Monthly estimates of cod biomass (tonnes) were then calculated (Kulka, 1990) for each of these regions (Fig. 3).

01995 Blackwell Science Ltd., Fish. Oceanogr., 4, 128-146.

In 1988observers recorded information on the sexual maturity of cod caught. Fifty fish were taken from a tow and an average maturity was assigned to the set. The fish were assigned to one of four categories: fish that were immature or spent (category 1); had enlarging gonads (category 2); had fully enlarged gonads, i.e. females with opaque eggs and males with solid white testes (category 3); or were spawning, i.e. females with clear eggs and males with running milt (category 4). This

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Figure 3. Monthl, trends in ~ o dhiomnss index (tonnes) uithin the wharea5 of the fi\hing ground shown in Fig 2, averag!edti\er 1980 to 1991 North, middle and \outli refer to the \tlhdrt'a\ delineated a5 hoxe\ 1, 2 and 3 h i u n in Fig

2

Jan

cl,iwttc,rtion I \ n simplihed version ot the maturit) st,igc\ ~ k h n e dhi Templeinan et '11 (1978). Obser\?ers t~\,iiiiincJ265 bets norrh,e,ist of Belle Isle Bank, 218 sets norrh c*ct\t (it Funk I\l,ind Rank and 220 sets between h i i k i i l m d R m k mil the northern Grmd Rank

U - ' I ~ L ~mlpfcI~J71 'T patterns of northern

cod

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twtder mowrwnts

While on the fishing grounds, trawler captains attempt to t ~ ) I concenrrations h o f cod to maximize their catch rates. Trawler fleets can maintain position over a concentration of cod for setveral weeks. Vessels of one fishing company often replace each uther as they fill their holds w i t h fish. Therefore, movements of schools ot cod m;ty be reflected in the change c i the collective positions of trltwlers making high catches. The pohiticrns of trawlers catching northern cod during the nioiiths of January t(: April, 1986 to 1988 ,ainineLi.Each month contained several hundred co thousmJx of trawler positions (Tahle 1 ) . A n example ot 7hciu.n i n Fig. 4. Some areas of the shelf were iiircn\ii,cly t i s h d hut surrounding areas were sampled >I> M C l l .

TO rc'sol\'c inuvements of schools of cctd, we have plottcJ rhc positions of trawlers making large catches (T;hlc 2 ) . T o n s Lvith catch rates 2 5 t o m e s h - ' were plotrcd hy ~veckand boundaries were around the clwxw p.)ints. It &as assumed that thest clusters iiiclicatcii that ;I school of cod was present. This assumption is >upported by the much lower catch rates of tows m;& in surroiinding areas. Clusters that remained colic.;ive over successive weeks were assumed to show rhe mcnwnrnt at schools of cod. C I ~ c t t ~ i o ~ ~ L~., mtli l ~ ) tions h i c nrur fishing positions Oce,iri iiirrent observations (Wright et ctl. 1988) are tor the winter months of 1986 and 1987 frrmi c~vLiilahlc

Feb

Mar

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current meters placed near Hamilton Bank (Fig 1). These observations were used to assess the influence of near-bottom currents on the apparent movements of cod schools During the period December 1986 to March 1988, the National Sea Products Ltd trawlers Cube North, Cope Brier, Cape Fox and C u p e l o h were equipped with expendable bathythermograph (XBT) systems which enabled surface-to-bottom temperature profiles to be taken while fishing O n occasion, scouting trips were made tc) locate schools that were not being fished. Temperature profile9 were taken along the track of these scouting expeditions ab well. T h e XBT coverage was most extensive for late January and early February in 1987 and 1988.

Pack ice contlitions near fishing positions Winter pack ice charts prepared by Ice Forecasting Central-Ottawa were examined in relation to the Observer Program data to determine whether trawler fishing positions (and the apparent movements of cod) were influenced by the presence of pack ice. The position of the ice edge at the beginning of each week was plotted on a chart along with all the observer data for that week, It was noted where heavy pack ice concentration might have prevented tishing.

RESULTS

Winter distribution Figure 2 shows where cod were found by trawlers in commercially significant concentrations (indicated where catch rates exceeded 1 . 5 tonnes hK') during January to April, 1980-1991. This 12 year compcisite rek-ealscod distributed over most of the outer shelf from 48"N to 55"N latitude. T h e fishing grounds covered ahout 19L7 000km' of the continental shelf. @ 1 W Aleckwell Science Ltd., Fish. Oceunogr., 4, 128-146.

133


Figure 4. Plot of Newfoundland Fisheries Observer Program data for March 1987, showing all 3161 positions of otter trawl tows that caught northern cod during that month. Different symbols are used for each week of the month: x, week 1; X, week 2; o, week 3; 0, week 4.

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Table 2. Number of tow positions with catch rates 2 5000 kg h-'. In parentheses is the number of positions after grouping tows within a radius of 0.1" latitude and longitude. Week

1

3

4

6) 5) 6) 5)

28 (10) 44 ( 8 ) 80 (15) O ( 0)

103 (17) 32 (10) 44 (12) 13 ( 6)

129 (21) 435 (12) 295 (15) 277 (14)

148 (20) 235 (21) 151 (22) 202 ( 1 1)

344 (23) 209 (22) 117 (16) 100 (15)

445 (18) 124 (17) 219 (20) 190 (19)

107 (13) 413 (18) 158 (22) 321 (29)

488 (31) 433 (30) 179 (28) 204 (20)

446 (34) 318 (29) 107 (20) 164 (11)

582 (29) 346 (23) 305 (37) 140 (18)

Year

Month

1986

January February March April

7 ( 3) 78 (13) 11 ( 4) 28 ( 7)

1987


1988


Cod tended to aggregate in three areas along the shelf edge which constituted only 16% of the total fishing ground (boxes I , 2 and 3 in Fig. 2). The first area was defined by lat. 53"28'N, long. 52'34'W (north-west

01995 Blackwell Science Ltd., Fish. Ocennogr., 4,128-146.

2 12( 36( 19 ( 16(

corner) and lat. 52'16'N, long. 51'19'W (south-east corner). A second area was defined by lat. 51"59'N, long. 51"lO'W and lat. 50"35'N, long. 50"15'W. The third area was defined by lat. 50"1@'N, long. 5@"3@'W

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J. S. Wrubkwski rt al.

Figure 5 . (a) Clusters of tows catching 2 5000 kg h- of cod from the foiith week of January through the first week of March, 1986 The value of n IS the number of tows in the cluster Note the progressive southeast movement of the clusters, presumably following concentrations of cod (b) A progressive vector diagram of near-bottom (985 m ) currents measured east of Hamilton Bank (location marked by the diamond in Fig 1 and Fig. 5a) during the period 22 Janudrt to 3 I March 1986 Data of Wright etul (1988)

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01995 Blackwell Science Ltd., Fish. Oceunop., 4, 128-146.

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Figure 6. (a) Clusters of tows with catch rates 25000 kg h-' moving south-east along the shelf edge during January and the first week of February 1987. (b) Clusters moving north-west from the fourth week of February through the fourth week of March 1987. (c) Clusters moving south-east from the fourth week of March through the fourth week of April 1987.

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Figure 6. Continued

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and lat. 48"46'N, long. 49'56'W. All three aggregation areas are located near the offshore termini of cross-shelf channels that separate the major banks (Fig. 1). Biological data collected by observers during the winter of 1988 indicate that most adult cod near the shelf edge were at o r approaching spawning condition. Narrative trip summaries often reported that eggs and milt were discharged by netted cod on trawlers' afterdecks. The proportion of spent cod in the samples examined by observers increased with time between January and April. The presence of female cod in spawning condition occurred in a few sets in January and February, but the majority of sets with spawningcod were observed in March and April, north of lat. 48"N along the shelf break. Most fish in the northern area (box 1 in Fig. 2) were spent after the first week of April. These data confirm that commercial trawlers were fishing prespawning and spawning cod. T h e biomass of cod at each of the three aggregation areas decreased between January and April (Fig. 3 ) , suggesting a departure of spawning or postspawning cod. Winter migration patterns Position\ of tows with catch rates 2 5 tonnes h - ' plotted each week indicate alongshelf movement of cod

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during the winter months of 1986-1988 (Figs 5, 6, 9). Shoals of cod apparently moved unidirectionally for several weeks, stopped, then continued or reversed direction, resulting in a displacement of several degrees of latitude along the shelf edge bathymetry. Migration patterns in 1986 With only 20% observer coverage, the data from 1986 are not as extensive as for the subsequent 2 years when 100%of the offshore fleets carried observers (Table 1). Nevertheless, our analysis indicates a south-easterly movement of cod along the shelf edge during January and February of 1986 (Fig. 5a). Trawlers fishing south of Hamilton Bank at 53'50'N, 52'50'W moved (presumably with the cod) south-east during the first 3 weeks of January. These trawlers joined another group successfully fishing at 52"12'N, 51'10'Wduringthe IastweekofJanuary. This assemblage of trawlers then continued south-east along the 400-600 m isobaths during the 4 weeks of February and the first week of March (Fig. 5a). During the last 2 weeks of March trawlers moved back north-west, fishing along the shelf edge. T h e direction of near-bottom currents measured along the 1000 m isobath off Hamilton Bank (Fig. 1) was primarily south-east during the period of January to April, 1986 (Fig. 5b). Assuming the current meter

01995 Blackwell Science Ltd., Fish. Oceanogr., 4,128-146.

Winter migration ofnorthern Atlantic cod

Figure 7. A progressive vector diagram of near-bottom (200 m) currents measured on Hamilton Bank (location of star in Fig. 1 and Fig. 6) during the period (a) 1 January to 14 February, (b) 2 2 February to 31 March, and ( c )22 March to 30 April, 1987. These periods correspond to the weeks shown in Fig. 6a, 6b and 6c, respectively. Data of Wright et al. (1988).

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01995 Blackwell Science Ltd., Fish. Oceanogr., 4, 12&146.

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Figure 7. Continued

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measurements represent the characteristics of the bottom currents in the vicinity of the cod schools, one may conclude that during their south-east movement in January and February, the cod were swimming with the current. However, on the journey north-west during the last 2 weeks of March, the cod were apparently swimming against the current. Because the dominant variability in the currents and temperature at the shelf edge occurs at time scales of about a week, while cod movements varied on a longer time scale of about a month, the general migration pattern of the cod in winter may be unrelated to the shorter-time-scale variability in oceanographic conditions. During January 1986, fishing positions along the shelf edge were seaward of the pack ice boundary, as defined in the ice forecasts. Then, from the first week in February to the first week of April, fishing took place within the pack ice. National Sea Products Ltd cap, tains' logs indicate that ice conditions did not significantly restrict fishing activity.

Migration patterns in 1987 Concentrations of cod were present along the southern slope of Hamilton Bank and the northern part of Funk Island Bank in early January 1987. The cod near Hamilton Bank appeared to move south-east along the bathymetry during January, just as in 1986 (compare Figs 5a and 6a). During the second week of February the concentration of cod apparently split into two schools, one continuing to move south-east, the other proceeding back north-west. In the first week of March, the school moving north-west was fished at about 53"N, 52"W (Fig. 6b). During the second and third week of March, fishing occurred along the eastern slope of Hamilton Bank and in Hawke Saddle. In the last week of March, fishing was concentrated along the eastern slopes of Hamilton Bank from 54"N to 55"N. Thereafter, the main concentration of cod began to move south-east (Fig. 6c), and continued to do so until late April, when the school apparently halted at about 51"30'N,

50"30'W.

01995 Blackwell Science Ltd., Fish. Oceanogr.. 4, 128-146.


139

Figure 8. Bottom temperatures from bathythermographs taken by National Sea Products Ltd trawlers during (a) the fourth week of January 1987, and (b) the first week of February 1987. Also shown is the location of all tows (0,January week 4;x, February week 1) with catch rates 25000 kg h-'. Note that bottom temperatures along the shelf edge are similar within and north of the school of cod located at 51"30'N, 50"30'W. The bold dashed line marks the furthest offshore position of the pack ice edge at the beginning of the week.

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Near-bottom currents measured along the 200 m isobath on the eastern side of Hamilton Bank (Fig. 1) during the period January to April 1987 are shown in Fig. 7. These data lend additional support to the suggestion from the 1986 analysis that the alongshelf movements of cod during the winter months may be independent of the currents. Just as in 1986, the current amplitude fluctuated at a period of about a week, but for the most part the direction was to the south-east. The temperature data recorded by the same current meter indicated an average temperature of about 2.5"C during January to April, although it fluctuated between 0.2"C and 3.5"C on a time scale of about a week. This measurement site (Fig. 1) was in shallower water than the cod schools. Cod near the shelf edge would likely experience currents more unidirectional than what is shown in Fig. 7. Bathythermographs deployed by National Sea Products Ltd trawlers during scouting trips showed bottom temperatures ranged from 2°C to 4°C along the bathymetry from 49"N (Funk Island Bank) to 55"N (Hamilton Bank). This range of bottom temperatures is typical for the shelf edge because of the cross-shelf movement of the slope water. There were n o major differences in the temperature along the 400-500 m bathymetric contours from one month to the next. Furthermore, the offshore edge of the pack ice did not extend into these fishing areas (Fig. 8a,b), so the exploration for commercial quantities of cod was not restricted by pack ice. Thus the alongshelf movement of cod schools during the winter does not seem to be driven by environmental fluctuations.

Mipution patterns in 1988 In early January 1988, a concentration ofcod centred just east of Hamilton Bank apparently moved south-east along the shelf edge (Fig. 9a). By the first week in February, pursuing trawlers had moved south to 51"30'N, SO"30'W. A second concentration o f cod located between Belle Isle Bank and northern Funk Island Bank displayed a similar southeast movement in late January (not illustrated in Fig. 93). This school of cod, centred near 51"30'N, 50"50'W in the first week of January, moved south-east along the 350-500 m contours, reaching 49"30'N, 50'30'W during early February. T h e school from Hamilton Bank appeared to blend with this school during the third week of February. XBT data taken during early February along the 400500 m isobaths between Hamilton Bank and Funk Island Bank showed bottom temperatures ranging from 2.4"C to 4.0"C (Fig. 10a). Scouting trips northward to Hamilton Bank by National Sea Products Ltd trawlers in late February measured similar bottom temperatures,

but n o commercial concentrations of cod were discovered (Fig. lob). In early March a north-westward movement began (Fig. 9b). A large assemblage of successful fishing positions moved steadily north-west along the shelf edge, starting from 52"N in the first week of March and reaching 55"N by the end of March. This group then changed directions during early April and moved southeast, back toward Belle Isle Bank and into Hawke Saddle (Fig. 9c).

DISCUSSION T h e winter distribution of cod derived from 12 years of Observer Program data shows concentrations along the shelf edge between Hamilton Bank, Belle Isle Bank, Funk Island Bank and the northern Grand Bank. Interestingly, the three locations where the fish tended to consistently aggregate are the same areas where cod likely began their inshore migrations (Fig. l a in Rose, 1993). Cod from the southernmost area may have moved along the bottom > 300 m toward Cape Bonavista (Fig. 1). The other two aggregation areas lie directly to the east of troughs in bottom topography which lead toward the coast, i.e. Hawke Saddle and the depression between Belle Isle Bank and Funk Island Bank. In 1988 observers found adult fish in these winter concentrations to be in prespawning and spawning condition, the latter particularly in March and early April. Contrary to previous views (Serebryakov, 1968; Fitzpatrick and Miller, 1979; Templeman, 1981) Hutchings et al. (1993) concluded that cod off Newfoundland and Labrador spawn primarily on the continental shelf. Their analysis of research trawl data showed spawning only o n the eastern slope of Hamilton Bank and a small section of the northern Grand Bank and Flemish Cap. However, their conclusion was based on 79 research trawls conducted along the shelf edge north of 50"N latitude between 1942 and 1992. Hundreds of observer records from the winter of 1988 indicate that north of 48"N latitude, cod were spawning along the shelf edge. The movements of cod inferred from Observer Program data are consistent with indications from tagging data (Templeman, 1979; Lear, 1984) that northern cod migrate during the winter months between the bank areas. Our analysis indicates that cod moved south-east along the shelf edge during the month of January in each of the years 1986-1988 (compare Figs 5a, 6a and 9a). Patterns of movement during January to April in both 1987 and 1988 are remarkably similar (compare Figs 6 and 9). In March of both years there appeared to be a

01995 Blackwell Science Ltd., Ftsh. Ocrunogr., 4,128-146.


Figure 9. (a) Clusters of tows catching cod at rates 25000 kg h-' moving southeast along the shelf edge from 8 January to 7 February 1988. (b) Clusters moving northwest from 22 February to 31 March 1988. ( c ) Clusters moving south-east from 22 March to 30 April 1988. Note the similarities to Fig. 6 in these apparent cod migration patterns.

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migration to Hamilton Bank, followed in April by a return south-cast to the point of departure for the inshore migration. These movements along the shelf edge were likely associated with prespawning-spawning behaviour. A near-zero gut fullness index suggests that cod were not actively feeding during the winter months (Lilly et al., 1984; Lilly 1987). The factors controlling the timing and direction of movements are not obvious. Examination of ice charts prepared by Ice Forecasting Central-Ottawa indicate that the direction of apparent migration (south-east or north-west) O t cod was unrelated to the movement of the pack ice. The cod seemed to follow the bottom topography rather than the pack ice edge above them. Figures 8 and 10 show the trawlers (and presumably cod schools) were consistently located along the continental shelf break whether or not pack ice was present. When the extent of pack ice was greatest during the winter months of 19861988, trawlers were often located within the pack ice pursuing schools of cod. National Sea Products captains' logs show that pack ice seldom prevented the trawlers from reaching the schools of cod present along the shelf edge. Other studies had suggested that an association be-

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tween northern cod and the pack ice might exist. Crawford and Jorgenson (1993) found that the distribution of arctic cod, Boreogadus saida, was influenced by the amount and location of ice cover in Resolute Bay, NWT. Templeman and May (1965) reported large catches of northern cod, Gadus morhua, on the southeastern slope of Hamilton Bank near the ice edge. Frechet (1990) found that large catches of Gadus morhua occur in the marginal ice zone in the Gulf of St Lawrence, but the depth (180-275 m) and bottom temperature (3.9-5.3"C) also had to be suitable for the presence of cod. We have found a similar result, i.e. concentrations of northern cod can occur where pack ice is present, but other factors (depth, temperature and spawning migration behaviour) may better explain the presence of large numbers of fish. The XBT data taken by National Sea Products Ltd trawlers show that bottom temperatures ranged between 2°C and 4°C along the shelf edge (Figs 8 and 10). Variations are due to intrusions of warm slope water onto the shelf (Narayanan et al., 1991). Temperatures over the length of the shelf edge appeared suitable for the presence of cod. The ground speeds of the cod schools apparent in our


Winter migration of northan Atlantic cod

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Figure 10. Bottom temperatures from bathythermographs taken by National Sea Products Ltd trawlers during (a) the first week of February 1988, and (b) the fourth week of February 1988. Also tows (x, week 1; 0, week 4) with catch rates 25000 kg

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analysis are plausible. Adult northern cod can maintain a swimming speed of approximately one body length S - ' at temperatures near 1°C (He, 1991). With no substantial component due to movement of the water, a 50 cm cod could theoretically move 40 km day-'. In our analysis, schools of cod appeared to migrate unidirectionally for several weeks. Maximum distances travelled were about two degrees of latitude in one month, or about 7 km day-'. The 1986 and 1987 current meter data from Hamilton Bank (Figs 5b and 7) suggest that the fish moved at times with the bottom currents, and at other times against the currents. Typical bottom current speeds are 5-15 cm s - ' , or 4-13 km day-'. In January and February of both 1986 and 1987, cod appeared to move south-east along the shelf break in the same direction as the alongshelf current. However, in March 1987 the cod swam north-west against the south-easterly-flowing currents for several weeks during their migration to Hamilton Bank (compare Figs 6b and 7b). Even swimming against a 15 cm s - ' Labrador Current, the cod could have travelled several times faster than their apparent ground speed of 5 km day- I . The weekly patterns of movement by trawlers (and presumably by cod) along the shelf edge were not created by company-directed harvesting strategies. The Canada Department of Fisheries and Oceans instituted a regulation in 1987 that annual offshore trawler harvests be divided equally among NAFO Divisions 2J, 3K and 3L. There was no such requirement in 1986. For the years 1987 and 1988, offshore trawler quotas for northern cod were not filled in Divisions 2J or 3K until the fall of the year. Thus the continuity in movement of the trawlers from week to week in a north-westerly or southeasterly direction along the shelf break was likely a consequence of trawler captains' efforts to fish moving schools of northern cod. Rose (1993) pointed out that the clues that cod use in their migrations are unknown. In our analysis, the direction of wintertime movements (south-east or north-west along the shelf edge) could not be directly related to bottom temperature gradients, currents or pack ice conditions. However, the cues for migration during early spring may be linked to the reproductive cycle. Movements apparent in our study fit the classic pattern of fish migrating to a spring spawning area (Harden Jones, 1968). During the prespawning months of January and February, schools of northern cod appeared to meander along the shelf-edge bathymetry, moving in the same direction as the mean Labrador Current (south-east). Then during March in a northwestward movement toward Hamilton Bank, northem cod may have performed a contranatant migration to

their spawning area. Spawning and postspawning fish would have been carried back south-east along the shelf edge with the mean current in a denatant migration during April, finally arriving at the point of departure for the inshore migration to summer feeding grounds. Similar contranatant migrations have been reported for Arcto-Norwegian cod which move from Bear Island to the Lofoten Islands spawning area (Trout, 1957; Nakken, 1994), and for West Greenland cod which migrate to spawning grounds south-west of Iceland (Harden Jones, 1968; Schopka, 1994).

CONCLUSIONS Data collected by observers aboard commercial trawlers have provided detailed information on the distribution and migration patterns of northern cod in winter. A composite of data from the winters of 1980-1991 shows northern cod were concentrated in three areas along the offshore edge of the Newfoundland-Labrador continental shelf. Rose (1993) predicted three inshore migration routes where shoreward-leading depressions in bottom topography hold water of 2-3°C. The offshore termini of these three migration 'highways' correspond with the three locations where our analysis shows cod aggregate in winter. Our analysis of trawler positions during JanuaryApril 19861988 indicates that schools of cod moved along the shelf edge, at times crossing NAFO Division boundaries. Migrating cod apparently maintained direction for several weeks, paused and then continued or reversed direction. Sexual maturity data for 1988 indicate that fish spawned during a contranatant migration toward Hamilton Bank in March, and during a denatant migration along the shelf edge in April. The fish then apparently dispersed in a postspawning, feeding migration to the Newfoundland and Labrador coasts. The patterns of movement derived for 19861988 are very similar, suggesting that these cod migrations occurred annually. Recently, there has been a change in the migration patterns described here. The extensive movements of schools along the shelf edge apparent for the years 1986-1988 have diminished (Kulka et al., in press). This change is likely related to the severe decline in the abundance of northern cod which began in 1989, and the southward shift in cod distribution over the Newfoundland-Labrador shelf (de Young and Rose, 1993; Rose et al., 1994; Taggart et al., 1994).

ACKNOWLEDGEMENTS Funding for this research has been provided by the Natural Sciences and Engineering Research Council of @ 1995 Blackwell Science Ltd., Fish. Oceanogr., 4, 128-146.


Canada (NSERC), National Sea Products Ltd and Fishery Products International Ltd under NSERC's Research Partnerships Program, and by the Atlantic Fisheries Adjustment Program's (AFAP) Northern Cod Science Program. We acknowledge the contribution of the scientific observers who collected data aboard commercial trawlers, often under adverse sea conditions. W. Bailey and A. Simms provided advice on data analysis. F. Perry and D. Pitcher assisted in computing graphics. We thank C. Bishop, J. Green, G. Rose and two anonymous reviewers for their comments on the manuscript.

REFERENCES Baird, J.W., Bishop, C.A., Brodie, W.B., and Murphy, E.F. (1992) An assessment of the cod stock in NAFO divisions 2]+3KL. NAFO SRCR Doc. 92/18. Bishop, C. A., Murphy, E.F., Davis, M.B., Baird, J. W., and Rose, G.A. (1993) An assessment of the cod stock in NAFO Divisions 2J+3KL. NAFO SRCR Doc. 93/86. Crawford, R.E., and Jorgenson, J.K. (1993) Schooling behaviour of arctic cod, Boreogadus saida, in relation to drifting pack ice. Enw. Biol. Fishes 36:345-357. Fitzpatrick, C., and Miller, R. (1979) Review of spawning time and locations for some commercial finfish on the Newfoundland and Labrador Coasts. Fish. Mar. Sew. tech. Rep. No. 905. 14 PP. Frechet, A. (1990) Catchability variations of cod in the marginal ice zone. Can. 1. Fish. aquat. Sci. 47:167&1683. Harden Jones, F.R. (1968) Fish Migration. London: Edward Arnold, 325 pp. He, P. (1991) Swimming endurance of the Atlantic cod, Gadus morhua L., at low temperatures. Fish. Res. 12:65-73. Hutchings, J.A., Myers, R.A., and Lilly, G.R. (1993) Geographic variation in the spawning of Atlantic cod, Gadus morhua, in the northwest Atlantic. Can. J. Fish. aquat. Sci. 50:2457-2467. Kulka, D. W. (1990) Areal patterns in the 2K3KL offshore witch fishery, 1979-89. CAFSAC Res. Doc. 90182. Kulka, D.W., and Firth, J.R. (1987) Observer program training manual - Newfoundland region. Can. tech. Rept. Fish. aquut. Sci. No. 1355 (Revised). 197 pp. Kulka, D.W., Wroblewski, J.S., and Narayanan, S. (1995) Recent changes in the winter distribution and movements of northern Atlantic cod (Gadusmurhua Linnaeus, 1758) on the Newfoundland-Labrador shelf. ICES J. mar. Sci. (in press). Lazier, J.R.N., and Wright, D.G. (1993) Annual velocity variations in the Labrador Current. J. phys. Oceanogr. 23:659678. Lear, W.H. (1984) Discrimination of the stock complex of Atlantic cod (Gadus morhuu) off southern Labrador and eastern Newfoundland, as inferred from tagging studies. J. Northw. Atl. Fish. Sci. 5 : 143-159. Lear, W.H., andGreen, J.M. (1984) Migrationofthe"northern" Atlantic ccd and the mechanisms involved. In: Mechanisms of Migration in Fishes. J.D. McCleave, G.P. Arnold, J.J. Dodson and W.H. Neil1 (eds). New York: Plenum, pp. 309-315. Lear, W.H., and Parsons, L.S. (1993) History and management of the fishery for northern cod in NAFO Divisions 21, 3K and


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3L. In: Perspectives on Canadian Marine Fisheries Management. L.S. Parsons and W.H. Lear (eds). Can. BuU. Fish. aquut. Sci. No. 226, pp. 55-89. Lilly, G.R. (1987) Interactions between Atlantic cod (Gadus morhua) and capelin (Mallotus willows) off Labrador and eastern Newfoundland: a review. Can. tech. Rep. Fish. aquut. Sci. No. 1567. 37 pp. Lilly, G.R. (1994) Predation by Atlantic cod on capelin on the southern Labrador and Northeast Newfoundland shelves during a period of changing spatial distributions. ICES mar. Sci. Symp. 198:600411. Lilly, G.R., Almeida, M.A., and Lear, W.H. (1984) Food of Atlantic cod (Gadw morhua) from southern Labrador and eastern Newfoundland (Div. 2J, 3K, and 3L) in winter. NAFO SCR Doc. 84/88. Myers, R.A., Mertz, G., and Bishop, C.A. (1993) Codspawning in relation to physical and biological cycles of the northern North-west Atlantic. Fish. Oceanogr. 2:154-165. Nakken, 0. (1994) Causes of trends and fluctuations in the Arcto-Norwegian cod stock. ICES mar. Sci. Symp. 198212228. Narayanan, S., Carscadden, J., Dempson, J.B., O'Connell, M.F., Prinsenberg, S., Reddin, D.G., and Shackell, N. (1994) Marine climate off Newfoundland and its influence on salmon (Salmo safur) and capelin (Mallotus villosus). Can. J. Fish. aquat. Sci. (in press). Narayanan, S., Colbourne, E.B., and Fitzpatrick, C. (1991) Frontal oscillations on the northeast Newfoundland shelf. Atmos. -Ocean 29:547-562. Rose, G.A. (1993) Cod spawning on a migration highway in the north-west Atlantic. Nature 366:458461. Rose, G.A., Atkinson, B.A., Baird, J., Bishop, C.A., and Kulka, D.W. (1994) Changes in distribution of Atlantic cod and thermal variations in Newfoundland waters, 198G1992. ICES mar. Sci. Symp. 198542-552. Schopka, S. (1994) Fluctuations in the cod stock off Iceland during the twentieth century in relation to changes in the fisheries and environment. ICES mar. Sci. Symp. 198:175193. Serebryakov, V.P. (1968) Cod reproduction in the northwest Atlantic. Fish. Res. Bd Can. Transl. Ser. No. 1133. Taggart, C.T., Anderson, J., Bishop, C., Colbourne, E., Hutchings, ]., Lilly, G., Morgan, ]., Murphy, E., Myers, R., Rose, G., and Shelton, P. (1994) Overview of cod stocks, biology, and environment in the Northwest Atlantic region of Newfoundland, with emphasis on northern cod. ICES mar. Sci. Symp. 198:14G157. Templeman, W. ( 1966) Marine resourcesof Newfoundland. Fish. Res. Bd Can. Bull. No. 154. 170 pp. Templeman, W. (1979) Migration and intermingling ofstocks of Atlantic cod, Gadw mmhua, of the Newfoundland and adjacent areas from tagging in 1962-66. ICNAF Res. Bull. No. 14. 50 PP. Templeman, W. (1981) Vertebral numbers in Atlantic cod, Gadus morhuu, of the Newfoundland and adjacent areas, 1947-1971, and their use for delineating cod stocks. J . Northw. Atl. Fish. Sci. 2:21-45. Templeman, W., and Fleming, A.M. (1963) Longlining experiments for cod off the east coast of Newfoundland and southern Labrador, 1950-1955. Bull. Fish. Res. Bd Can. No. 141. 65 pp. Templeman, W., andMay, A.W. (1965) Researchvesselcatches of cod in the Hamilton Inlet Bank area in relation to depth and temperature. ICNAF spec. Publ. No. 6:149-165.

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Templeman, W., Hodder, V.M., and Wells, R. (1978) Sexual maturity and spawning in haddock, Melanogrammu aegkfinus, of the southern Grand Bank. ICNAF Res. BuU. 13:53-65. Trout, G.C. (1957) The Bear Island cod: migrations and movements. Fishery Invest., Lond., Ser. 2, 2 1: 5 1 pp. Wright, D.G., Lazier, J.R.N., and Armstrong, W. (1988) Moored current and pressure data from the Labrador/ Newfoundland shelf, June 1985-July 1987. Can. Data Rep Hydrogr. and Ocean SCI.No. 62. 258 pp. Wrohlewski, J.S., Bailey, W.L., and Howse, K. A. ( 1994)Obser-

vations of adult northern cod (Gadusmurhua) overwintering in nearshore waters of Trinity Bay, Newfoundland. Can. J. Fish. q u a t . Sci. 51:142-150. Wroblewski, J.S., Goddard, S.V., Smedbol, R.K., and Bailey, W.L. (1995) Movements of Atlantic cod (Gadusmurhua) within the spring thermocline in Trinity Bay, Newfoundland. .I.mar. Biol. Ass. U.K. 75:265-284. deYoung, B. and Rose, C.A . (1993) O n recruitment and distribution of Atlantic cod (Gadus morhua) off Newfoundland. Can. 1. Fish. q u a t . Sci. 932729-2741.

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