Growth of Juvenile Coho Salmon (Oncorhynchus ...

Oncorhynchus kisutch Years of Differing Co

Can. J. Fish. Aquat. Sci. Downloaded from www.nrcresearchpress.com by Renmin University of China on 06/06/13 For personal use only.

I. P.

Fisher and W. G. Pearcy

Oregon State University, College of Oceanography, Corvallis, 0 8 9733 7, USA

Fisher, ). P., and W. G. Pearcy. 1988. Growth of juvenile coho salmon (Oncorhynchus kisutch) off Oregon and Washington, USA, in years sf differing coastal upwelling. Can. ). Fish. Aquat. Sci. 45: 1036-1 044. Estimated growth rates, condition, and stomach fullness of juvenile coho saimon (Bwco~hynehuskisutch) caught in the ocean in early summer, when mortality was most variable, were as high in 1983 and li 984, years of very low survival and low early upwelling, as in 1981, 1982, and 1985, years of higher survival and higher early upwelling. Chronic food shortage leading to starvation, poor condition, or slow growth apparently was not the cause of the increased mortality of juvenile coho salmon in 1983 and 1984. Survival of juvenile coho salmon was positively correlated with purse seine catches of fish in lune and with early summer upwelling, 1981-85. Hence, year-class success probably was determined early in the summer, soon after most juvenile coho salmon entered the ocean. Spacing of the first five ocean circuli, which was positively correlated with growth rate, was not significantly different for fish caught early in the summer and those caught late in the summer, suggesting that growth rate selective mortality in the ocean was not strong. The increase in morta%ityin 1983 and 1984 may have been caused by increased predation on juvenile coho salmon due to decreased numbers of alternative prey for predators. bes taux estimatifs de croissance, le facteur de condition et le contenu stomacal de jeunes saumons cohos (Oncorhynchus kisuech) captures dans ['ocean au d6but de l'ete, alors que la mortalit6 etait trPs variable, ont et4 aussi klevks en 1983 et 1984, annees caracteris6es par un taux de survie trgs bas et une faible rernontee hstive d'eau froide, qu'en 1981, 1982 et 1985, annkes oh la survie etait plus elevee et la remontee hstive plus forte. La penurie chronique de nourriture menant A la privation, A une mauvaise condition ou a une croissance lente n'a apparemment pas 6t6 la cause de la mortalit6 accrue des jeunes saurnons coho en 1983 et 1984. De 1981 1985, il y a eu une csrr6latiesn positive entre la survie des jeunes saumons cohos et les prises de poisson 3 la senne coulissante en juin et la rernontee d'eaur au debut de IP&t6.Ainsi, le succes des classes annuelies a probablement 6te determink au debut de l'etk, peu apres que la plurpart des jeunes saumons cohos eurent gagne I'ocean. L'espacement des cinq premiers anneaux de croissance en mer, montrant une correlation positive avec le taux de croissance, n'etait pas different de f a ~ o nsignificative pour les pissons captures au debut de I'4t6 et pour ceux captures tard dans la saison, donnant A genser que la mortalit4 selective dans I'ocean liee au taux de croissance n16taitpas forte. L'accroissement de la mortalit6 en 1983 et 1984 psurrait avoir 6te cause par la predation accrue sur de jeunes saumons cohos en raissn d'un rnoins grand nombre d'autres espPces de proie dispsnibles aux pr6dateurs. Received March 9, 1987 Accepted February 10, 8 988 (J91 71)

T

otal numbers sf coho salmon (Oncsrhynchus kis ch) srnolts released from hatcheries from southern Washington through California (the Oregon Production Index (6PH) area) increased from less than 10 million in the early 1960's to more than 66 million by the early 1980's (Nickelson and Lichatowich 1984). Abundance of adult fish increased concugfentHywith increased smolt abundance the previous year until the period 1970-76, when production of adults began to fluetuate independently of the number sf smolts released. From 1977 to 1985, production o f adults declined despite increased releases of hatchery-reared smolts (Nickelson and Lichatowich 1984; Pacific Fisheries Management Council 1986, their table III- 1).

R e p le 9 mars 1987 Accept4 le 10 fkvrier I 988

Several hypotheses have been proposed to explain the low survival rates of coho salmon after the mid- 1970's. One possible explanation is that the poor ocean conditions in the mid1970's resulted in low survival rates o f juvenile coho. As evidence for this hypothesis, several investigators have shown a positive relationship between coastal upwelling during the spring and summer that juvenile coho enter the ocean and resulting adult abundance the following year (Gunsolus 1978; Scamecchia 1981; Clark md MeCarl 1983; Nickelson 3983; Nickelson and Licahtswich 1984). Nickelson (1986) found that survival sf hatchery fish in yeas sf strong upwelling was about twice that in yews s f weak upwelling and that survival s f both hatchery and wild coho was inversely related to sea-surface Can.J. Fish. Agrpat' Sci., VoE. 45, 688


temperatures during years sf strong upwelling. Coastal upwelling from the mid-1970's to 1985, a period of low adult abunh c e , was generally less than from the mid-1960's to the mid1970's, when adult production was high (Nickelson 1986). Another hypothesis is that survival is density dependent; i.e. low survival rates occur when large numbers of smolts are released (McGie 1981; McCarl and Wettig 1983). However, McGie (1984) and Peteman and Woutledge (1983) found a nonlinear relationship between number of smolts and resulting number of adults only in low upwelling yem, which suggests that my density-dependent mortality may be associated with the ocean environment. Nickelson (1986) reported linear relationships between smolts released and adult production the following year for separate analyses of wild, public hatchery, and private hatchery fish. He concluded that ocean conditions had greater effect on coho survival than density-dependent factors. Because abundance of aduIt coho salmon is correlated with strength sf upwelling that occurs a year earlier and because the number of precocious males (jacks) returning to hatcheries after r in the mean is generally an excellent predictor of the production of adult coho (Gunsolus 1978; Pacific Fisheries Management Council 1986, their fig. 111-11, ocean conditions during the fist s u m e r in the ocean appear to significantly affect survival of juvenile coho salmon. Low primary and secondary productivity in the ocean, leading to less food for juvenile fish, is a logical mechanism for the higher mortality of juvenile coho salmon during years of low upwelling. This reduced productivity in years sf low upwelling would be expected to result in slower growth rates, lower fish condition, and increased vulnerability to predation, starvation, or disease (Ricker and Foerster 1948; Barker 1971; Scmecchia 1981). If this mechanism is important, the larger, faster growing fish should survive at higher rates than smaller, slower growing fish. To evaluate the linkage between ocean productivity and growth of coho salmon during ther first summer in the ocean, we compare growth rates, condition, and stomach fullness of marked and unmarked juvenile coho caught in the ocean in y e m of low survival and low upwelling (1983 and 1984) with those in y e m of higher survival and higher upwelling (1981, 1982, and 1985). We also examine the evidence of size-selective and growth rate selective mortality in the ocean during these years.

Materials and Methods Collections We collected juvenile coho salmon in purse seine sets in the wean off Oregon and Washington in May, June, July, and August 1981, May, June, and September 1982 and 1983, June, July, and September 1984, and from late May though June 1985. Sets were made in the upper 2&30 m a b u t every 9 h along east-west trmsects from about the 37-m depth contour (27 m in 1985) out to about 40 to 50 h offshore where juvenile salmonids were r a e or absent. The transects were generally 37 h (20 nautical miles) apart between the mid-Oregon and northern Washington coasts (southern Washington coast in 1981). See Pearcy (1984), Pearcy et al. (1985), or Pearcy and Fisher (1988) for a detailed description of the sampling. A large part of our study area was within the OPI area. Ocean Conditions md Survival We used the mean monthly coastal upwelling indices at 45"N, 125"W (Mason and B&un 1986) as measures of upwelIing Can. Js Fish. Aqbcut. Sci., Vo'ol. 45, 1988

intensity and potential ocean productivity that might affect survival, growth, and condition of juvenile coho salmon. An index of survivd of juvenile coho salmon in each year was derived by dividing the number of jacks returning to public hatcheries in the OPI area in the fall (Pacific Fisheries Management Council 1987, their table V-5) by the totd number of smolts released from public hatcheries earlier in the spring (T. Lichatowich, Oregon Dep. Fish Wildl., Portland, OR, pers. co jack index was used as a measure of juvenile survival rather thm one based on adult catches and escapement because of the exceptionally high mortality expressed by OBI coho salmon during their final year in the ocean during the 1983 El Nifio (Pearcy et al. 1985; Johnson 1984; Pearcy and Schoener 1987; Pacific Fisheries Management Council 1986). Growth sf Marked Fish To estimate growth rates of marked fish, we subtracted mean fork Iength (FL) of the mark group at time of hatchery release from FL at capture in the ocean and divided by days between release and capture*Certain mark groups of coho released in the Columbia river were sampled during their downstream migration by the National Marine Fisheries Service at river kilometre 75 (Dawley et al. 1985a). Because these data on Iengths were collected closer to time of ocean entrance, they were used instead sf the lengths at time of hatchery release to estimate growth rates for fish from these groups that were collected later in the ocean. We also estimated weight-specific growth rates of marked coho from the slope of the linear regression of In (weight at capture/mean weight at release) on days between release from the hatchery and capture in the ocean. Growth of Unmarked Fish Two age groups sf juvenile coho were found in the ocean off Oregon and Washington during our sampling: yealing fish that had spent one winter in fresh water before migrating to the ocean and subyearling fish that entered the ocean about 6-9 mo after hatching. Most wild and public hatchery coho smoIts were yearling fish, while many coho smolts released from private hatcheries on the Oregon Coast were subyealing fish. The majority of yearling fish entered the ocean over a relatively short time period between March and June. A very large fraction entered from the Columbia River between mid-April and Iate June with a peak in late May (Dawley et al. 1985b, their tabIes 4-61. However, large numbers of subyearling fish entered the ocean over a more extended period from May through September. These two age groups were distinguished by the spacing of the first 21 scale ridges (circuIi). The subyearling smolts, because they experienced accelerated growth in the hatchery, had more widely spaced circuli and thus a greater radius to the 2 1st ckculus. Scales were removed from a consistent area above the laterd line behind the dorsal fin (Clutter and Whitesel 1956) from up to 25 fish per purse seine set. Acetate impressions of the scales were made and magnified 88 times for measurement. Because yearIing fish entered the ocean over a fairly short time period, mostly before our sampling in the summer, we were able to make estimates of mean growth rate of these fish from change in mean length over time. For fish classified as yearlings, we estimated mean growth rates ktween cruises by change in mean length divided by mean days between cruises. Mean spacing of scale circuli formed during ocean residence was also used as a measure of relative p w t h rates of unmarked yearling fish caught in the mean. We had found that sede eir-

TABLEI . Regressions of FL (m)ow scale radius (mm at 88 x for yearling juvenile coho salmon caught in the ocean.


Year

P%

r2

A.

CatchBSeO in June Vs. Jack S u r v i v a l I n d e x

Regression equation

culus spacing was positively correlated (n = 83, r = 0.8 1 , p < 0.01) with p w t h rate for a group of individually marked juvenile coho smolts held for about 60 d in saltwater tanks (unpublished data). We made the assumption that spacing of circuli formed in the wean was also positively correlated with growth rate in the ocean. Based on a comparison of scales from fish collected during downstream migration or at hatcheries and fmm fish collected in the ocem, we concluded that ocean entrance occurred at about the same time as the abrupt transition on the scale from nmowly or moderately spaced circuli to widely spaced circuli. We measured the radius to the first, fifth, and last circulus formed in the ocean and calculated mean spacing of the first five and d l ocean ckculi for yearling fish caught in each cmise.

2.8

S u m sf Mean M o n t h l y U p w e l l i n g Indices Sep l. Aug.

Sep t. July

June

Condition Fish were frozen at sea individually in marked plastic bags to prevent dehydration md later weighed in the laboratory. Weight was compared with measured at sea to determine the weight-length relationships in each yea. Stomach Fulness We removed stomachs from about 10 fish from each purse seine set and weighed the contents to the nearest 6.1 g. Mean weight of stomach contents as a percentage of total body weight was then determined for each cmise in order to make between. sets were made mainly during daylight year c o m p ~ s o n sShce hours on all cruises of each year, diel fluctuations in feeding s stomach fullness. should not bias between-year c o m p ~ s o n of Growth of Sacks Growth rates of marked precocious mdes (jacks) returning ta the Anadromsus, Inc. facility at Coos Bay, Oregon, between 1988 and 1985 were estimated from the differences between mean lengths at release and return. We also compared mean lengths of jacks returning to Fa91 Creek and Siletz hatcheries on the Oregon coast between 1978 and 1985 with survival m d upwelling in each year. Selective Mortality If large ocem entrants survived at a higher rate than smaller wean entrants, then there should be a shift to a larger mean length at ocean entrmce with time in our samples. We backcalculated FL at time sf ocean entrance for yearling coho salmon caught in the ocean from the distance along the scale radius 20' ventrad to the long axis of the scale from the focus to the ocean entrance mark. For each yearling fish, distance to the ocean entrance mark was converted to FL using regressions derived from yearling coho caught in the scem in the same year (Table 1). Mean E s at the time of mean entrance for fish

M~Y MUE -Apr

1981

1982

1983

I984

1985

FIG. 1. (A) Mean catch per set in June of yearling juvenile coho salmon vs. jack survival index (see hfakrids and Methods); (B) sum of mean monthly upwelling indices at 45% latitude from March though September.

caught in each cmise were compared to see if there was a shift to a larger value with time. We also back-calculated length at ocean entrmce for marked fish sampled during downstream migration in the Columbia River and later caught in the ocean. We compared back-calculakd length at mean entrmce for each fish with mean length for the s m e mark group when sampled during downstream migration in the Columbia River (Dawley et al. 1985a). Consistently larger back-calculated lengths at ocean entrance than m a n lengths during downstream migation would indicate that size-selective mortality or growth had occuued between sampling in the river m d ocem entrance. If fish that grew rapidly after entering the mean survived at a higher rate than those that grew more slowly, then the proportion of the faster growing fish should increase with time. Since scde circulus spacing appears to be positively correlated with growth rate, there should also be a shift to more widely spaced eirculi with t h e , We compared the mean spacing of the f i t five scde circuli laid dawn in the ocem in different cruises to see if spacing increased with time.

Results Ocean Conditions and Juvenile Survival The suwivd of juvenile coho between 1981 and 1985 was highly variable. The index of survival sf juvenile coho based on jacks returning to hatcheries 4 5 mo after release v i e d Can. J. Fish. Aquat. Sci., Vob. 45, 1988

TABLE2. Upwelling index, survival index, and growth rates of marked juvenile coho caught in the ocean (20 d or more after release or after river sampling) in the pekods May-June md July-Sepkmkr. Higher survival years

Lower survival years 1983


Upwelling index (cumulative) Mach-June Muarch-September

1984

1982

1985

-

Survival index

1.14

Growth rates (mm-d- I ) May-June catch July-September catch

n

12 5

R

0.91 SD

1.42 0.50 1.2264.65

n. 6 12

R

Ealy Summer Growth Rates Growth rates of juvenile coho salmon early in ocean life appeared to be as high in the low survival yeas ($983a d 1984) as in higher survival years (1982 and 19851, based on estimates of growth of marked fish a d unmarked yearling fish and from spacing of scale circuli. The mean growth rate of marked fish caught during the early s u m e r (May-June) of 1983, a year of 45?1988

2.03 n

SD

0.68 0.27 1.280.42

threefold between 1981 and 1985 and was highest in 1982 and 1985 and lowest in 1983 and 1984 (Fig. 1A). Mean catch per set of yealing juvenile coho in June along eight transects between 46"40% and 44'20'N varied more than fivefold between 1981 and 1985 and followed the same trends as the jack survival index (Fig. 1A). Mortality of juvenile coho was apparently variable very early in the s u m e r , before June, within 1 mo of the peak outmigration of coho smolts from the Columbia River (Bawley et al. 1985b), the major source of smolts in this area. Therefore, ocean conditions prior to or during June appear to have a great effect on juvenile survival. Juvenile survival and abundance in June was csmelated with coastal upwelling during these years. Cumulative monthly upwelling indices at 45"N, 125"W (Mason and Bakun 1986) were low during early summer (March-June) in 1983 and 1984 and were much higher in 1982 m d 1985 (Fig. 1B). Although low in the early summer, upwelling in 1984 intensified later in the s u m e r . Upwelling during the entire spring and summer (Mach-September) was lowest in 1983 and highest in 1982. The jack survival index was more strongly correlated with cumulative upwelling between Mach and June (aa = 5, r = 0.86, 0.05 < p < 0.10) than with the index between July and September (n = 5, r = -0.O1,p > 0.50) (Fig. 1A and 1B). A similar correlation was found for the survival of hatchery coho salmon in the years 1970-85. Survival from smolt ts d u l t for public hatchery fish was more strongly correlated with the cumulative coastal upwelling index between March m d June than between July and September of the smolt outmigration y e x ( n = 1 5 , r = 0 . 5 9 , p < 0 . 8 2 5 v s a pz = 1 5 , r = 0.43, 0.1 < p < 0.2, respectively; s w i v d data from 8. Kaiser, Oregon b p . Fish Wildl., Newport, OR 97365; data were excluded for the adult production year 1983 when adult mortality was unusually high). Thus, ocean conditions early in r of most years seem to be an important factor determining abundance of adults the following year.

Can. J. Fish. &wt. Sci., Val.

1981

f

2.78 SD

15 0.91 0.62 3% 1 . 3 3 0 . 4 6

n

34 21

f

3.19 SD

0.90 0.68 1.700.39

pa

75 -

R

SD

0.88 0.47 -

-

low early upwelling and low survival, was greater than in all other years (8-test, p < 0.05, Table 2). In the other poor survival yew of 1984, mean growth rate was low, but not significantly lower (gs > 8.051, than in the better survival yeas 1981, 1982, and 1985. Similxily, growth rate of unmxked yewling fish between May and June estimated from change in mean length was higher in 1983 (0.73 m - d - 9 than in 1982 (0.36 m m d - I). Moreover, mean length in June I983 (182 mm) was higher than in the June of any other year (Table 3). Mean spacing of all ocean circuli of juvenile coho caught in May 1983, when survival was very low, was greater (t-test, p < 8.05) than for cshs caught in May 1981, 1982 and 1985, yeas of much better survival (Table 4). For fish caught in June, mean spacing of the first five ocean circuli, representing about the first 2835 d sf growth in the ocean (9. P. Fisher, unpubl. data), was very similar in all yeas (t-test, p > 0.5) except in 1984 when the sample size was very low. Mean spacing of all ocean circuli for fish caught in June was also similar in all yeas, except in the y e a of highest juvenile survival (1985) when it was lower (gs < 0.05) than in 1981 or 1982. Thus, evidence is lacking for slower growth of juvenile coho in low survival years during the ealy summer period when survival of fish is highly variable.

L~~~Summer Growth Rates In contrast with the trends in early s u m e r , mean growth rate of tagged coho caught late in the summer (July-September) was significantly greater (t-test, p < 0.05) in 1982, the year of highest Mach-September upwelling m d moderate juvenile survival, than in other yeas (Table 2). The mean growth rate in 1982 (1.70 mm=d-I) was 39, 33, and 28% higher than in 1983, 1984, and $981, respectively. Mean growth rate of marked fish caught in late summer sf 1983 was not significantly different (t-test, p < 0.05) than growth rates of those caught in late summer sf 1981 or 1984, despite much higher MachSeptember upwelling in these later two yeas. Growth rates estimated from change in mean length of yearling coho between June a d August or September were also higher in the good survival y e x 1982 (1.76 m m d - I) than in the yeas of low juvenile coho survival 1983 and 1984 (1.37 and 1.33 mm=d-B, respectively, Table 3). Similarily, mean spacing sf all ocean c k u l i for fish caught in August or September of 1982 m d 1981 was grater than in 1983 and 1984, yeas ,of low survival (ttest, p < 0.05, Table 4). 1839

TABLE3. Mem P;E (rnm), capture day (May 15

=

day 01, and estimated growth rate between cruises for unm&ed juvenile coho classified as

yearlings.

I% Year

Cruise

pa

3

Estimated growth rates ( m v d - I ) Mean day of capture

SD

May-June

June-July

July-Aug. or Sept.

June-Aug. or Sepa.


Higher survival years

1981

Magi June July Aug .

1982

May June Sepb.

Lower survival years

1983

May dune Sept.

1984

dune July Sept .

TABLE4. Mean spacing of the first five ocean circuli and d l ocean circuli for marked and unmarked fish classified as yea1ings.

Month of capture in the ocean May Pg

2

June SD

PZ

.f

August or September SD

1

f2

SD

First five ocean circesli

Higher survival yeas 1981 1982 1985 Lower suwiva1 years 1983 1984 All ocean circuli

Higher survival years 1981 1982 1985 Lower survival yeas 1983 1984

Growth in Weight Instantaneous growth rates in weight of make8 juvenile coho salmon in 1983 and 1984 were 2.3 and 2.2% of body weight per day, respectively, rates as high as in 1982, 1983, and 1985 (Table 5). Fish caught both in early and late summer ape included in these regressions. 1W

Growth Rate and Size of Jacks Jacks returning in the fall (September-November) to Coos Bay (Anadrornous, Inc.) and the Fall Creek and Siletz hatcheries were smallest in 1983, the year of lowest March-September upwelling, 1978-85 (Table 6). Mean growth rates of jacks Can. J. Fish. Aquat. Scd.. Vol. 4.5, 6988

TABLE5. Growth in weight of ocean-caught marked juvenile coho, where w l = mean weight at release, w2 = weight of fish when collected in the ocem, and d = days between release md recovery.


1982 (dl mark groups)

difference was not significant @I > 8.85); however, there was a significant difference between June 1983and 1985 @ < 8.85). Food was apparently as available in 1983 as in the higher survival years of 1982 and 1985. In all years and months, stomach content weight as a percentage of body weight was higher than that reported by Hedey (1980) for coho in the Strait of Georgia caught between July and September. Size-Selective Mortdity

1982 (Columbia R. only)

reaming to Coos Bay were also lower in 1983 than in d l other yeas. Although survival, as measured by the percent of the smolts released returning as jacks, was very low in both 1983 and 1984 at the Fall Creek md Siletz hatcheries, mean length at both hatcheries in 1984 was as great or greater than in any other y e a (Table 6). Perhaps the large size of jacks in 1984, relative to 1983, was the result of the much higher upwelling that occurred late in the summer sf 1984. The Mach-September cumulative upwelling index was 150 in 1984 versus only 40 in 1983 (Table 6). Interestingly, unlike for jacks, growth rates for marked and unmarked juvenile coho in 1983 and 1984 were very similar (Tables 2 and 3). Perhaps the large size of jacks in 1984 was due to a late summer growth spurt, after the end of our ocean sampling in September for juvenile fish. Condition and Stomach Fullness Fish collected in the poor survival years (1983 and 1984) in 1983 and 1984 showed no signs of stmation. Condition was as high as in the better juvenile survival yeas 1981, 1982, and I985 (Table 7). Additionally, weight of food as a percentage sf body weight was significantly higher (t-test, p < 0.85) in May 1983 than in May 1985 and in September 1983 than in September 1982 or 1984 (Table 8). Mthsugh mean stomach content weight was lower in June 1983 thm in June 1982, the

(a

Back-cdculated lengths at wean en higher in June than in May 1983 a d June, or July 1981 (t-tests, p < 0.01, Table 9). These differences could be a result of size-selective mortality: the larger fish entering the ocean surviving at a higher rate causing a shift ength at ocean entrance among fish caught er. However, the back-calculated length at mean entrance for fish caught in September 1983 was the same as in May, suggesting that the increase in mean length seen in June may have been an artifact. There was no trend for significant (p < 0.05) increases with time in back-calculated lengths at we& entrance in 1982, 1984, or 1985. Hence, consistent differences indicating size-selective mortality were lacking. Average back-calculated length at ocean entrance of marked Columbia River coho in 1982 and 1983 was slightly greater thm mean lengths sf these same tag groups when sampled during downstream migration at river kilometre 75 (1982: n = 22, 2 = 5.8 m, SB = 5.9; 1983: 8a = 8 , Z = 7.1 m).These differences in size could have resulted from sizeselective mortality, but they could dso have resulted from estuarine growth or a bias in back-calculation of length at time of ocean entrance. Over 98% of coho sampled in the Columbia River had some food in their stomachs and the stomachs averaged half full (Dawley et d. 1986), so it is likely that growth occurred during downstream migration. Average downstream migration rates ranged &om 6 to 30 kmed - "Dawley et al. 1985a), so about 2-1 H d of growth could occur between river kilometre 75 and the ocean. There is little evidence that fish that grew fastest immediately after ocean entrance had higher survival. If this was the case, then mean circulus spacing of the first few ocean circuli should be greater for fish caught late in the summer than for fish caught early in the summer. However, among fish caught in late summer (August or September), spacing of the first five ocean circuli, representing the first 2G-35 d of wean growth, was not significantly different than mean spacing of all ocem circuli

TABLE6. Growth rates of jacks returning to Coos Bay and mean lengths of marked jacks from releases in Mach and percent of the release returning a jacks to two coastal Oregon hatcheries in years of varying strength of upwelling. Coos Bay (Anadromous, Inc.)

Yea

Mar.-Sept . upwelling index

B;E

Can. J. Fish. Aquat. Sci., Vo'si. 45, 1988

Mean FL (m)

Mean growth rate (mm.d-l)

Fall Creek

n

Mean FL (mm)

Siletz 96 returning as jacks

% n

Mean returning FL (m) as jacks

1041

TABLE7. Fork length vs. weight relationships for ocean caught juvenile coho salmon. K = condition factor = wt (g)

X

FL (warn)-%

1H05.


Predicted weight m d K for t h e e fork lengths Yea

n

B2

5981 1982 1983 1984 1985

287 17 1 146 505 783

0.98 0.99 0.98 0.99 0.97

Regression equation wt (g) wt (g) wt (g) wt (g) wt (g)

155 rn

= 1-44 x = 2.61 x = 3-02 x

10-6FL (n~n)"~" 38.9 FL ( n ~ n ) ~ . ~ 40.0 ~ FL 4.3 = 2.81 x FL (m)i3.27 4.0 = 1.21 X FL (m)13.42 38.0

1 1.07 1.08 1.07 1.02

230 rn 147.5 146.2 145.6 145.0 146.5

1.21 1.20 1.20 1.19 1.20

300 m 365.2 349.5 345.7 345.3

1.35 1.29 1.28 1.28

TABLE8. Weight of stomach contents as a percentage of body weight for juvenile coho sdmsn caught in the ocem in different months a d years.

Maya June July August September

1982

1983

1984

136 2.18 1.21 296 2.60 1.79

79 2.21 1.20 144 2.33 1.63

138 1.75 1.49

102 2.42 1.87

156 2.01 1.63 100 1.21 1.34 145

I985

136 1.50 1.50 228 2.72 1.67

1.63 1.49

aAll sets close to the mouth of the Columbia River in May 1985. TABLE9. Back-calculated FL (mm)at time of ocean entrance of yearling cshs (marked and unmarked combined) caught in the ocem in different months. Data ovencored are significantly different (&test, p < 0.05). June

(mostly five or fewer circuli) for fish caught in June (Table 4). Thus, my growth rate or size-selective rnortdity probably occurs prior to June, very soon after ocean entrance.

Discussion The similar intermnual variations in catch per set in June m d jack survival index suggest that survival of juvenile coho stdmon in the OPI area was most variable during a short period during or right after entry into the ocean. During this early su period, stomach fullness, condition, and estimated growth rates of juvenile coho were as high in the poor survival yeas of 1983 and 1984 as in the better survival yeus of 1981, 1982, and 1985. This suggests &at the unusually high rnortdity of Quvenile coho in these y e m was not a direct result of chronic food shortage in the ocean leading to poor condition or starvation. Low growth rates do not appear to be associated with low survival rates. Although growth rates were lower in late summer of the poor survival years (1983 and 1984) than in the better survival years (198 1 and 1982), growth rates in all years (198 184) of surviving juvenile coho were quite high, exceeding those found by Healey (1980) for juvenile coho from the Strait of Georgia and Mathews and Buckley (1976) for coho from h g e t Sound and the Strait of Georgia. Additionally, growth of jacks in the y e a of low survival 1984 was greater thm in many other years of better survival.

July

August

September

Other workers have found that wean growth rates and survival rates or abundance we not positively comelated. Bottom (1985) found no correlation between total growth during the first summer and winter in the ocean m d survival of coho returning to a coastal Oregon lake system in 13 different years. In fact, he found that spacing of thk first six ocean circuli and distance to the first ocean annulus on scales were above average in 1983, a year of very low survival. Mathews and Ishida (S .B . Mathews, Fisheries Research Institute, University of Washington, Seattle, WA, pers. c o r n . ) found that growth rates of poorly surviving coho released early in the summer from the Columbia River and Coos Bay were comparable with those for fish that survived at a high rate released later in the summer. Several authors (Peteman 1984; Rogers 1980; Mathews 1980) found negative correlations between marine growth and abundmee of salmonids, suggesting that growth was density dependent. Bottom (1985) suggested that the better than average growth of coho salmon smolts he found in the very poor survivd y e a 1983 may have been due to the large-scale mortality that ooccumed that yea, reducing competition for food. McGie (1984) found that size of adult coho salmon in the OPI area was negatively comelated with density in the years 195776, but eontray to expectations if growth were density dependent, both average size m d abundance of adult coho salmon were very low in 1983. McGie argued that the very low average size &E.

J. Fish. A ~ U Q PSci.. . VoI. 45. 1988


of adult coho salmon in 1983 was caused by the large numbers of juvenile fish surviving the previous fall (the 1982 jack survival index was high) competing for a very limited food s in the El Nifio y e a 1983. The relatively high growth r found for juvenile coho salmon caught in 1983 and 1984 indicate that food supply in these poor survival years was adequate ort good growth for reduced nu reduced primary and secondary everd authors indicated that size wcur for chum (0. keta), pink (0. g s r b u ~ h a )and , sockeye n (0. nerka) (Parker 1971; Hedey 1982; West and Larkin . These authors indicated that selective mortality was most e for a certain size range of fish or during a specific life history period. The size of fish for which they found size-selective mortality was much smaller than the size of a typical coho smolt. However, Mathews and Ishida (S .B. Mathews, pers. comrn.) also found that mean back-calculated lengths at release of surviving adult coho salmon released from Coos Bay were 5-10 rnm greater than mean lengths at release, suggesting that mortality may have been greater for smdler fish. We found evidence for size-selective mortality of coho salmon in the wean between May md late summer only in I98 1, suggesting that if size-selective mortality occurs, it probably most often during May, the period when mortality is usule . that the decreased survival of juvenile coho in 1983 and 1984 was not directly the ~ s u l of t decreased food condition, or slow growth. supply leading to stmatis Our data indicate that the v mortality determining yeaclass success occurs very during the time that coho smolts are entering the ocean. Growth rates, condition, m d stomach fullness of juvenile coho caught in this early summer riod were similar in all yeas. However, the lower primary d secondary productivity indirectly may have caused higher mortality in 1983 and I984 through increased predation on juvenile coho salmon as they entered the ocean due to decreased abundances of alternative prey for daters. Shifts in ocean currents in yeas of low upwelling also may have changed the distribution and increased the vulnerability of m o l t s to predators.

We thank L. Bst

mkis, and two monyrng8ass

improving the manuscript. , R. Gowan (Anadromous,

B o m o ~ B. , k. 1985. Research and development of Oregon's c o a t d salmon stocks: Job 1, Study 2: coho salmon model. Oreg. Bep. Fish Wildl. Annu. Rog. Rep., Portland, OR. 11 p. CLARK, I., AND B. M c C ~ L 1983. . An investigation of the relationship between Oregon coho salmon (Oncordf_vnchuskis~tch)hatchery releases and d u l t production utilizing law of the minimum regression. Can. J. Fish. Aquat. Sci. 40: 5 16-523. GLWTTEJP, W. I., AND L. E. WHITESEL.1956. Collection and interpretation of sockeye salmon scales. Int. Pac. Salmon Fish. C o r n . Bull. 9: 1-159. BAWLEY,E. M., R. B.L E W E R W ~T. D ,H. BLBKIM, C. W. SIMS,J. T. Dmem, R. A. K r w , A. B. RANKIS,G . E. MONAN,AND F. J. ~SSLANDER. 1986.

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P~WCY W,. G . , AND A. S C H O E 1987. N ~ . Changes in the marine biota coincident with the 1982-1983 El Nifics in the northeastern subarctic Pacific Ocean. J. Geophysical Res. 92: 14417-14428. PETERMAN,R. M. 1984. Interaction mong sockeye salmon in the Gulf of Alaska, p. 187-199. In W. G. Pearcy [ed.] The influence of ocean conditions on the production of sdmsraids in the North Pacific: a workshop. Oregon State Univ.. Sea Grant Coll. h o g . OWU-W-83-0Q1,Corvallis, OR. PETERMAN,R. M . , AND R. D.ROUTLEDGE. 1983. Experimental management of Oregon coho salmon (Oncorhynchsts kisubch): designing for yield of infomation. Can. J. Fish. Aquat. Sci. 40: 1212-1223.

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