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Changes in the abundance and distribution of walleye pollock (Theragra chalcogramma) in the western Gulf of Alaska (1961–1984)
Fisheries Research, 5 (1987) 185-197 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
185
Changes in the Abundance and Distribution of Walleye Pollock ( T h e r a g r a chalcogramma) in the Western Gulf of Alaska ( 1 9 6 1 - 1 9 8 4 ) MILES S. ALTON, MARTIN O. NELSON and BERNARD A. MEGREY
National Oceanic and Atmospheric Administration, Northwest and Alaska Fisheries Center, National Marine Fisheries Service, Resource Assessment and Conservation Engineering Division 7600 Sand Point Way NE, Seattle, WA 98115 (U.S.A.)
ABSTRACT
Alton, M.S., Nelson, M.O. and Megrey, B.A., 1987. Changes in the abundance and distribution of walleye pollock (Theragra chalcogramma) in the western Gulf of Alaska (1961-1984). Fish. Res., 5: 185-197. Changes in the abundance and recruitment of western Gulf of Alaska walleye pollock ( Theragra chalcogramma) during 1961-1984 were examined. Bottom trawl survey CPUE estimates increased between the early 1960s and early 1970s. Independent biomass estimates from acoustic surveys and catch-at-age analyses of foreign and joint venture fisheries indicate that another increase in abundance occurred during 1978-1981, but that stock size has declined since 1981 or 1982. Biomass was estimated at 950 000 metric tons (t) in 1973-1974, 2.3-3.8 million t in 1981 and 1.8 million t in 1984. The fluctuations in stock size reflect substantial changes in year-class strength. These include the occurrence of strong year classes in 1967, 1970, 1972 and 1975-1979. Weak year classes apparently occurred in 1980-1982. Based on spawner-recruit estimates since 1976, pollock recruitment may be strongly density dependent. It is believed that there has been a shift, since 1977, in the abundance of pollock from the Kodiak-Chirikof area, westward towards the Shumagin Islands.
INTRODUCTION
T h e objective of this p a p e r is to review t h e c h a n g e s t h a t have o c c u r r e d in t h e a b u n d a n c e a n d d i s t r i b u t i o n o f w e s t e r n G u l f o f A l a s k a walleye pollock ( Theragra c h a l c o g r a m m a ) since 1961. T h i s species has a c o n t i n u o u s g e o g r a p h ical d i s t r i b u t i o n t h a t e x t e n d s f r o m off California, n o r t h w a r d to A l a s k a a n d w e s t w a r d to w a t e r s o f f t h e U.S.S.R., J a p a n a n d K o r e a ( B a k k a l a et al., 1986). S t o c k b o u n d a r i e s over t h e r a n g e have f r e q u e n t l y b e e n b a s e d o n statistical r e p o r t i n g areas r a t h e r t h a n o n p o p u l a t i o n d i s t i n c t i o n s . W e have defined t h e ' w e s t e r n G u l f o f A l a s k a ' as t h e region b e t w e e n t h e Fox I s l a n d s a n d t h e K e n a i
0165-7836/87/$03.50
© 1987 Elsevier Science Publishers B.V.
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Fig. 1. WesternGulfof AlaskashowingINPFC statistical areas and geographicallocations. Peninsula (Fig. 1 ) and have assumed that pollock within this region can be treated as a single stock. The information presented in this paper is based on reports by Hughes and Hirschhorn (1979), Alton and Deriso (1983), Nelson and Nunnallee (1985) and Megrey (1986) and on further examination of fisheries and research vessel survey data on the western Gulf of Alaska pollock resource. CATCH STATISTICS Prior to 1982, Gulf of Alaska pollock was primarily harvested by fisheries from Japan, the U.S.S.R. and other foreign nations. The most complete time series of reported pollock catches is from the Japanese fisheries, which began in the Gulf of Alaska in 1963. As a member of the International North Pacific Fisheries Commission (INPFC), Japan reported annual catches of its fisheries in a prescribed manner - - by species, month, gear type and vessel tonnage size for statistical blocks of 1 ° longitude × 1/2 ° latitude. When the Magnuson Fisheries Conservation and Management Act was passed in 1977, all foreign nations were required to report their catches by INPFC standards. Before 1977, the U.S.S.R. and other nations, excluding Japan, had reported their catches in the Gulf of Alaska in an inconsistent manner. In fact, there are no adequate estimates of the U.S.S.R. pollock catch for the years 1964-1971 when U.S.S.R. trawl fisheries were very active in the Gulf of Alaska.
187
In addition to the foreign nations' annual reports, the U.S. Observer Program, which began in 1978, provides estimates of the annual foreign catches of pollock and other groundfish by broad INPFC areas (Fig. 1 ). These estimates are determined using procedures described by Wall et al. (1981). Pollock catches by U.S. vessels were of little consequence until the 1980s, when joint ventures 1 began to intensify in the Gulf of Alaska. Estimates of the pollock catch in such fisheries come from the U.S. Foreign Fisheries Observer Program. LENGTH AND AGE COMPOSITION OF CATCHES
Length and age data come from U.S. observer sampling of foreign and joint venture catches and are available for the years 1976-1984. Length measurements are taken by the observer, when pollock is the target species, from a random sample of ~ 150 fish per day. However, only one age sample is obtained per observer cruise (which usually lasts 1-2 months). This sample consists of otoliths removed from a maximum of five fish per sex per 1-cm length interval. The length and age sampling procedures are described in detail by Nelson et al. (1981b). Following procedures described by Allen {1966), an age-length key assembled from the length-stratified age sample is used to convert length frequency samples to age compositions. Otoliths are read by the ageing unit of the Northwest and Alaska Fisheries Center using methods described by LaLanne (1975). Estimates of the catch in numbers at age were obtained in a step-wise manner, beginning with weighting the age composition of samples from a specific nation, vessel class, sub-area and time period by the catch and then summing as required to obtain the annual catch at age by the nation and sub-area (Alton and Deriso, 1983). RESEARCH SURVEYS
In the early 1960s, the International Pacific Halibut Commission (IPHC) conducted bottom trawl surveys throughout the Gulf of Alaska to obtain estimates of the abundance of halibut relative to other bottomfish in order to estimate the impact of a bottom trawl fishery on the halibut stock ( International Pacific Halibut Commission, 1964). U.S. commercial trawlers of 200-300 horsepower were chartered for the survey and all used a commercial type trawl (400-mesh eastern). Trawl stations were pre-determined and systematically arranged for depth and area coverage. The duration of trawling at each station was 1 h. Only data from the 1961 surveys are used in this report because the 1Ajoint venture is an arrangement between U.S. and foreign interests, in which U.S. vessels catch and sell fish to foreign processing vessels operating in the fishery conservation zone (FCZ).
188
coverage by season and area that year was comparable to that of surveys conducted by the National Marine Fisheries Service ( NMFS ) during 1973-1975. The 1973-1975 NMFS surveys were confined to the western Gulf of Alaska and were aimed at assessing the abundance of pollock and other bottomfish (Hughes and Hirschhorn, 1979). These surveys were conducted using the NOAA research vessel "John N. Cobb", a vessel similar in size and horsepower to the trawlers used for the 1961 IPHC surveys. Trawl stations were selected randomly within broad bottom-depth strata ( ~ 100-m intervals from 50 to 450 m). The same type of trawl used during the earlier IPHC surveys was employed. However, trawling duration was 1/2 h rather than 1 h as in the IPHC surveys. Acoustic and midwater trawl surveys ( hereafter referred to as acoustic surveys) of pollock in the western Gulf of Alaska began in 1980, following recognition of the unusually large aggregation of spawning pollock in Shelikof Strait (Nelson et al., 1981a). These surveys, which are designed to estimate the biomass and age composition of spawning pollock in the Strait area, continued in 1981, 1983 and 1984. In 1983 and 1984, acoustic surveys were also conducted outside Shelikof Strait in an attempt to locate other pollock spawning areas in the western Gulf. The design of the surveys and the methods of collecting and analyzing the acoustic and biological data are described by Nelson et al. (1981a), Nunnallee et al. (1982) and Nelson and Nunnallee (1985). All echo-integrator data on the Shelikof pollock have been scaled to estimates of density, using an average target strength of - 3 1 . 3 dB kg-1. This estimate is based on Bering Sea pollock target strength measurements reported by Traynor and Williamson (1983). Biomass estimates were obtained by multiplying average density pe~ unit surface area by the area surveyed. Age-specific biomass and population estimates were calculated using length frequency data, survey-specific length-weight relationships and age-length keys. STOCK DELINEATION The actual relationships among Gulf of Alaska pollock stocks and between Gulf and Bering Sea stocks are not clear. Biochemical genetic studies by Grant and Utter (1980) revealed some distinction between pollock of the Bering Sea and those sampled off the Kenai Peninsula and in the eastern Gulf of Alaska. One sample of pollock from the Shelik of Strait area showed more affinity with Bering Sea pollock than with fish from off the Kenai Peninsula and the eastern Gulf of Alaska. These results suggest an east vs. west separation of pollock in the vicinity of Kodiak Island. This is in agreement with Hughes and Hirschhorn (1979), who also found evidence of such a separation based on regional differences in recruitment and mean size at age for two year classes. However, we have treated the pollock of the western Gulf of Alaska as one stock because of evidence from acoustic surveys (Nelson and Nunnallee, 1985) and ichth-
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Fig. 2. Geographical distribution ( percent of total removals) of pollock catch by the foreign nations trawl fisheries in the western Gulf of Alaska, 1964-1983. A dash refers to < 1% of total removals.
yoplankton surveys (Dunn et al., 1984) that most pollock of the region spawn in one locality, Shelikof Strait. RESULTS
1961-I976 Between 1962 and 1971, foreign trawlers in the Gulf of Alaska targeted mainly on rockfish, particularly Pacific ocean perch (Sebastes alutus) ; pollock were a minor part of the total catch until 1972. Rockfish apparently were an important component by weight in the bottomfish community during the 1960s, as indicated by the results of bottom trawl surveys (Alverson, 1968) and by their importance in the foreign fisheries ( Chikuni, 1970 ). Since the distributions of adult Pacific ocean perch and pollock overlap, we assume that much of the pollock taken by the fisheries in the 1960s was by-catch. The average annual pollock catch by Japan during the peak years of the rockfish fisheries was ~ 5000 t. Japanese fishing was concentrated in the Chirikof-Kodiak region and south of the Fox Islands, where pollock by-catch was highest ( Fig. 2 ). The size of the pollock by-catch in the intense U.S.S.R. rockfish fishery prior to 1971 is unknown. The 1972 catch of pollock by foreign fisheries was almost four times larger than the 1971 catch (Table I). Whether this was because of a declining rockfish resource or due to an increase in pollock abundance is not known. The annual pollock catch continued to increase through the 1970s.
190 TABLE I Catch (1000 t) of pollock in the Gulf of Alaska, by fishery category, 1971-1984a Year
1971 1972 1973 1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984
Fishery category Foreign
Joint venture
Domestic
Total
9.5 34.1 36.8 61.9 59.5 86.5 117.8 96.3 103.8 113.0 130.3 92.6 81.4 99.3
0 0 0 0 0 0 0 0 0.6 1.1 16.8 73.9 134.1 207.1
~ 9 ? ~ ~ ~ 0.2 1.0 2.0 0.9 0.6 2.2 0.1 0.3
9.5 34.1 36.8 61.9 59.5 86.5 118.0 97.3 106.4 115.0 147.7 168.7 215.6 306.7
~Sources: Foreign and joint venture catches: Berger et al., 1986. Domestic catches 1978-1980: Rigby, 1984. Domestic catches 1981-1984: Pacific Fishery Information Network (PacFIN), Pacific Marine Fisheries Commission, 305 State Office Building, 1400 SW Fifth Avenue, Portland, OR 97201, U.S.A. N M F S surveys of 1973-1975 revealed t h a t pollock was a d o m i n a n t species in the b o t t o m f i s h c o m m u n i t y of t h e w e s t e r n G u l f of Alaska a n d t h a t its a b u n dance was significantly higher t h a n indicated by I P H C surveys c o n d u c t e d 12-14 years earlier ( R o n h o l t et al., 1978). As i n d i c a t e d in T a b l e II, t h e 1973-1975 c a t c h per s t a n d a r d tow was 4 to 24 t i m e s g r e a t e r t h a n t h a t of t h e earlier I P H C surveys in 1961, even w i t h o u t c o r r e c t i n g for t h e d i f f e r e n c e in tow d u r a t i o n b e t w e e n t h e two s u r v e y periods. T h e 1973-1975 surveys p r o v i d e d a crude first e s t i m a t e of the exploitable biomass (age 3 a n d older) of pollock in t h e w e s t e r n G u l f of Alaska. A range was e s t i m a t e d b y a s s u m i n g t h a t t h e c a t c h a b i l i t y coefficient of t h e s u r v e y trawl was 1.0 for a m i n i m u m value o f b i o m a s s ( 0.95 million t ) a n d 0.5 for a m a x i m u m value (1.90 million t ) ( A l t o n a n d Deriso, 1983). T h e 1973-1975 surveys also i n d i c a t e d t h a t the 1967, 1970 a n d 1972 y e a r classes were relatively m o r e a b u n d a n t t h a n the 1968, 1969 a n d 1971 year classes ( H u g h e s a n d H i r s c h h o r n , 1979). 1977-1984 T h e e s t i m a t e s of t h e exploitable biomass d e r i v e d f r o m t h e 1973-1975 s u r v e y results p r o v i d e d a basis for d e t e r m i n i n g a c o r r e s p o n d i n g range of equilibrium yield of 152 000-305 000 t ( A l t o n a n d Deriso, 1983). T h e a n n u a l allowable
191 TABLE II Comparison of pollock catch rates between 1961 International Pacific Halibut Commission and 1973-1975 NMFS research trawl surveys. Catch rates are only from those stations that were located at bottom depths between 100 and 300 m Region
Kodiak (151°-154 ° W. long)
Chirikof (154°-159 ° W. long)
Shumagin (159°-167 ° W. long)
Period
Number of stations
Catch per standard haul (kg) a
F (df}
July-Sept. 1961 July-Sep. 1973
16 21
123 494
4.7* (1,35}
June-July 1961 June-July 1975
46 27
37 417
4.3* (1,71)
July-Aug. 1961 July-Aug. 1974
44 36
19 454
10.7" {1,78)
aFor the 1961 surveys a standard tow was 1.0 h in duration and for surveys in 1973-1975 the duration was 0.5 h. *Significant at the 95% level.
catch for 1977 was conservatively set at the lower value. In that year, the catch rose sharply to 118 000 t, as foreign trawlers intensified their efforts on pollock ( Table I ). In 1981, the pollock catch approached the allowable catch of 152 000 t. Although there were relatively few years of catch-at-age data (1976-1981), Alton and Deriso (1983) performed a catch-at-age analysis using a modified version of the method described by Doubleday (1976). The model provided a good fit between observed and predicted annual catch at age and indicated that exploitable biomass and surplus production had increased sharply between 1978 and 1979 and continued to increase in 1980 and 1981. Average annual exploitable biomass for the period 1976-1981 was estimated at 1.040 million t. When 1982 data became available and included in the catch-at-age analysis, poor correspondence between observed and predicted catch at age resulted. This was attributed to changes that had occurred in age-specific selectivity during 1982. Changes in selectivity were also found by Megrey (1986) when 1983 data were included in a later analysis. Megrey (1986) applied three different catch-at-age models to 1976-1983 catch-at-age data. Results were similar for all models and showed the same sharp 1979-1981 rise in pollock abundance found by Alton and Deriso (1983). However, abundance declined in 1983. This decline is also reflected in independent biomass estimates taken from acoustic surveys of pre-spawning and spawning fish in Shelikof Strait in
192 TABLE III Changes in biomass (million t, age 3 and older) of western Gulf of Alaskapollockas estimated by (1) catch-at-age analysis using two values of natural mortality (M) (Megrey, 1986) and (ii) acoustic trawl surveys (Nelson and Nunnallee, 1986) Year
1976 1977 1978 1979 1980 1981 1982 1983 1984
Estimates from catch-at-age analysis with 95% confidenceinterval M = 0.32
M = 0.40
0.74 _ 0.07 0.64 ± 0.07 0.96 ± 0.14 1.65~+0.30 2.08 ± 0.39 2.46 ± 0.50 2.63 ± 0.54 2.23 ± 0.49
0.95 ± 0.09 0.80 ___0.09 1.12 ± 0.17 1.85 ± 0.35 2.17 ± 0.43 2.29 ± 0.48 2.23 ± 0.47 1.71 _ 0.39
Acoustic trawl survey estimate with 95% confidence interval
3.77_+0.91 No survey 2.43 ± 0.74 1.84 ± 0.63
1983 and 1984 (Table III). The decline did not appear to result from a change in distribution, because surveys outside the strait during the same period in 1983 and 1984 did not detect any substantial concentrations of pollock. The decline of pollock is attributed, in part, to the fact t h a t the 1980 year class was below average in abundance. As 4-year-olds, this year class was poorly represented in samples from both the commercial catch and the acoustic survey in 1984 (Alton, 1985; Nelson and Nunnallee, 1985). This was the first time t h a t 4-year-old fish were a relatively minor component in the Shelikofpollock fisheries ( Fig. 3 ). In addition, findings from the 1985 acoustic survey ( Nelson and Nunnallee, 1986), indicated t h a t the 1981 and 1982 year classes were poorly represented in the population. The results of catch-at-age analysis (Megrey, 1986) and acoustic survey estimates (Nelson and Nunnallee, 1985) show t h a t pollock biomass had increased substantially, starting in 1979, peaked in 1981 or 1982 and is currently declining (Table III). The increase in abundance is attributed to a succession of five a b u n d a n t year classes (1975-1979) t h a t followed the weak year classes of 1973 and 1974. The population estimates of age 3 fish for these exceptional year classes range from 1.2 to 2.1 billion, 4-10 times the estimated number of 3-year-olds from the 1973 and 1974 year classes (Megrey, 1986). The 1980 year class is considered comparable in size to the weak 1973 and 1974 year classes. In 1984, the pollock biomass in the western Gulf of Alaska was comprised of mainly fish of the 1978 and 1979 year classes. The rise in pollock abundance since 1978 has coincided with changes in the geographical distribution of the foreign pollock catch. Before 1977, catches were primarily from the Fox Island and Chirikof-Kodiak regions. During
193 30F
1980
I
10
APRIL MAY
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30
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~
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1981
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10
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Fig. 3. Age composition of pollock in joint venture catches from the Shelikof Strait region, 1980-1984. 1977-1980, catches south and east of Shumagin Islands increased and by 1981-1983, ~ 75% of the total foreign pollock catch came from there (Fig. 2). Preliminary fisheries information for 1984 suggests that the Chirikof-Kodiak grounds are resuming their importance in the foreign pollock fisheries. Most of the foreign pollock catch is taken in the late summer and autumn. Coincident with this geographical shift of the foreign pollock harvest, has been the development of joint venture fisheries targetting mainly on pollock. Such fisheries have been centered in Shelikof Strait on pre-spawning and spawning pollock during January-April. In 1983 and 1984, the joint venture fisheries catch exceeded that of the foreign fisheries (Table I). DISCUSSION It is apparent that since 1961, five important changes have occurred with respect to the western Gulf of Alaska pollock resource. These are: (i) the substantial increase in stock size between the early 1960s and 1973-1975, which resulted largely from the success of the 1967, 1970 and 1972 year classes, (ii) the succession of five abundant year classes (1975-1979) that caused an exceptional increase in pollock abundance during 1978-1982, i.e., to a level about four times that of the mid-1970s. (iii) the abundance decline observed since
194 1981-1982, (iv) a regional shift in pollock catches from the Chirikof-Kodiak region to the Shumagin area that began in 1977, and (v) the development of a fishery on spawning fish in Shelikof Strait. The lack of complete and comparable time-series data on the abundance of pollock and other species and on the physical and biological environment, make it difficult to examine the extent to which different biotic and abiotic factors have contributed to the major changes in pollock abundance that have been observed since the early 1960s. However, two processes, species replacement (Cushing, 1980; Daan, 1980) and density-dependent control of recruitment, may have been important. The evidence for replacement is circumstantial and relates to the decline of Pacific ocean perch. The pronounced increase in the size of the pollock population that was detected by the 1973-1975 surveys occurred during and after a period in which exploitation caused a major reduction in Gulf of Alaska stocks of Pacific ocean perch. In the early 1960s, before there were intensive fisheries for Pacific ocean perch, the adult biomass of this species was estimated at 1.1 million t (Kimura et al., 1984). Then, during a relatively short period (1963-1969), its biomass was reduced by ~ 70% ( Ito, 1982 ). Pollock biomass, in turn, increased to a 1973-1975 level comparable to that of the original biomass of Pacific ocean perch. Pacific ocean perch, as well as some other species of rockfish, are potential competitors of pollock because they and pollock feed on similar prey (euphausiids and other nekton) and overlap in distribution. If, however, replacement involving competition was operating alone, the second increase in pollock biomass to > 2 million t in the late 1970s and early 1980s appears excessive. If replacement did not occur between the 1960s and 1970s, then the abundance of Pacific ocean perch and that of pollock fluctuated independently of one another. The fisheries may have begun in the Gulf of Alaska during a period of high abundance of Pacific ocean perch. Because it is such a long-lived species with low fecundity, one would expect the Pacific ocean perch population to be composed of many sub-dominant age groups. However, Westrheim (1970) found from trawl surveys in the early 1960s, that three age groups comprised >~69% of the population. He raises the possibility that ocean perch abundance in the early 1960s was at an unusually high level because of the recruitment of a small number of abundant cohorts over a relatively brief period ( S.J. Westrheim, personal communication, 1986). Pollock is currently in a period of decline because of the emergence of a series of poor year classes (1980, 1981, 1982 and possibly, 1983). Since these weak year classes were spawned when pollock were at high levels of abundance, density-dependent control of recruitment may be operating on this stock. This would agree with Megrey (1986), who suggests that Western Gulf of Alaska pollock may have a Ricker type spawner-recruit relationship. The recent emergence of a strong year class (1984) when population was low in abundance
195 further suggests a Ricker type relationship. This year class was found in exceptional numbers as 1-year-old fish in Shelikof Strait during an acoustic survey in 1985 ( Nelson and Nunnallee, 1986). We can only speculate as to the mechanisms involved in density-dependent recruitment. For eastern Bering Sea pollock, cannibalism of young fish has been proposed as a factor controlling recruitment (Laevastu and Larkins, 1981, p. 106; Francis and Bailey, 1983), but limited studies of the diet of Gulf of Alaska pollock (Simenstad, 1979; Blackburn et al., 1983; Clausen, 1983 ) show no evidence of this phenomenon. The increase since 1977 in the pollock catch by the foreign trawl fisheries in the Shumagin area and the corresponding decrease in catch in the previously more important Chirikof-Kodiak region can be intepreted as a regional shift in availability. There is no evidence that this regional change in catch was caused by quotas or other regulations. In recent years (1981-1983), ~ 75% of the foreign trawl catch of pollock came from the Shumagin area and 37% from only two statistical blocks in that area (Fig. 2). This localization of large removals indicates that pollock abundance was centered in this region in those years when pollock biomass was high. These removals took place principally during July-November, a period of intense feeding, indicating that pollock availability was probably related to the abundance and behavior of prey. This is further supported by the fact that the catch of other species (Atka mackerel, Pleurogrammus monopterygius and Pacific ocean perch) having similar prey preferences (euphausiids and other nektonic organisms) was also disproportionately higher in the Shumagin area than in other regions. Furthermore, the Japanese setline fishery for Pacific cod ( Gadus macrocephalus) has been centered in the Shumagin area since 1977. Cod feed on both benthic and nektonic organisms, including pollock (Jewett, 1978). The concentration offish removals from the Shumagin area suggests that this area contained a large food base for fish populations. ACKNOWLEDGMENTS The authors thank George Hirschhorn and other members of the Ageing Unit of the Northwest and Alaska Fisheries Center in Seattle for providing timely information on the ages of pollock. They also thank Russell Nelson, Jr. and his colleagues in the U.S. Foreign Fisheries Observer Program in Seattle for their advice and information on the foreign and joint venture fisheries of the Gulf of Alaska.
REFERENCES Allen, K.R., 1966. Determination of age distribution from age-length distributions, IBM 7090, 7094, FORTRAN IV. Trans. Am. Fish. Soc.,95: 230-231.
196 Alton, M., 1985. Walleye pollock fisheries of the Gulf of Alaska, 1983-1984. In: R.L. Major (Editor), Condition of Groundfish Resources of the Gulf of Alaska Region as Assessed in 1984. U.S. Dep. Commer., NOAA Tech. Memo. NMFS F/NWC-80, pp. 1-10. Alton, M. and Deriso, R., 1983. Pollock. In: D. Ito and J. Balsiger (Editors), Condition of Groundfish Resources of the Gulf of Alaska in 1982. U.S. Dep. Commer., NOAA Tech. Memo. NMFS F/NWC-52, pp. 1-62. Alverson, D.L., 1968. Fisheries resources in the northeastern Pacific Ocean. In: The Future of the Fishing Industry of the United States. Univ. Wash. Publ. Fish. New Ser., 4, pp. 86-101. Bakkala, R., Maeda, T. and McFarlane, G., 1986. Distribution and stock structure of pollock ( Theragra chalcogramma) in the North Pacific Ocean. Int. North Pac. Fish. Comm. Bull., 45: 3-20. Berger, J.D., Smoker, J.E. and King, K.A., 1986. Foreign and joint venture catches and allocations in the Pacific Northwest and Alaska fishing area under the Magnuson Fishery Management and Conservation Act, 1977-1984. U.S. Dep. Commer., NOAA Tech. Memo. NMFS F/NWC99, 53 pp. Blackburn, J.E., Anderson, K., Hamilton, C.I. and Starr, S.J., 1983. Pelagic and demersal fish assessment in the lower Cook Inlet estuary system . In: Environmental Assessment of the Alaskan Continental Shelf, Final Reports, Biological Studies 17: 107-450. U.S. Dep. Commer., Natl. Oceanic Atmos. Admin., Natl. Ocean Serv., Off. Oceanography Mar. Sci., Juneau, Alaska. Chikuni, S., 1970. Data on the Pacific ocean perch in the northeast Pacific. Development and history of the Japanese trawl fishery through 1969. Fish. Agency Japan, Shimizu, INPFC doc. 1354 (unpublished,), 34 pp. Clausen, D.M., 1983. Food of walleye pollock, Theragra chalcograrama, in an embayment of southeast Alaska. U.S. Natl. Mar. Fish Serv. Fish. Bull., 81: 637-642. Cushing, D.H., 1980. The decline of the herring stocks and the gadoid outburst. J. Cons. Int. Explor. Mer, 39: 70-81. Daan, N., 1980, A revieW of replacement of depleted stocks by other species and the mechanisms underlying such replacement. Rapp. P.-V. R~un. Cons. Int. Explor. Met, 177:405-421. Doubleday; W.G., 1976. A least squares approach to analyzing catch at age data. Int. Comm. Northwest Atl. Fish. Res. Bull., 12: 69-81. Dunn, J.R., Kendall, A.W., Jr. and Bates, R.D., 1984. Distribution and abundance patterns of eggs and larvae of walleye pollock ( Theragra chalcogramma) in the western Gulf of Alaska. NWAFC Processed Rep. 84-10. Northwest and Alaska Fish. Cent., Natl. Mar. Fish. Serv., NOAA, Seattle, Wash. (unpublished), 66 pp. Francis, R.C. and Bailey, K.M., 1983. Factors affecting recruitment of selected gadoids in the northeast Pacific and east Bering Sea. In: W.S. Wooster (Editor), From Year to Year. Univ. Wash., Seattle, Coll. Ocean Fish. Sci., Sea Grant Program, Sea Grant Publ. WSG-WO 83-3, pp. 35-60. Grant, W.S. and Utter, F.M., 1980. Biochemical genetic variation in walleye pollock, Theragra chalcogramraa: population structure in the southeastern Bering Sea and the Gulf of Alaska. Can. J. Fish. Aquat. Sci., 37: 1093-1100. Hughes, S.E. and Hirschhorn, G., 1979. Biology of walleye pollock, Theragra chalcogramma, in the western Gulf of Alaska. U.S. Natl. Mar. Fish. Serv., Fish. Bull., 77: 263-274. International Pacific Halibut Commission, 1964. Catch records of a trawl survey conducted by the International Pacific Halibut Commission between Unimak Pass and Cape Spencer, Alaska, from May 1961 to April 1965. Rep. Int. Pac. Halibut Comm., 36:524 pp. Ito, D.H., 1982. A cohort analysis of Pacific ocean perch stocks from the Gulf of Alaska and Bering Sea regions. NWAFC Processed Rep. 82-15.Northwest and Alaska Fish. Cent., Natl Mar. Fish. Serv., NOAA, Seattle, Wash. (unpublished), 157 pp. Jewett, S.C., 1978. Summer food of the Pacific cod, Gadus macrocephalus, near Kodiak Island, Alaska. Fish. Bull., 76: 700-706.
197 Kimura, D.K., Balsiger, J.W. and Ito, D.H., 1984. Generalized Stock Reduction Analysis. Can. J. Fish. Aquat. Sci., 41: 1325-1333. Laevastu, T. and Larkins, H.A., 1981. Marine Fisheries Ecosystem. Fishery News Book Ltd., Farnham, Surrey, England, 162 pp. La•anne, J.J., 1975. The validity and consistency of age determinations from otoliths of Pacific pollock (Theragra chalcogramma). Int. North Pac. Fish. Comm. Annu. Rep., 1977: 99-107. Megrey, B., 1986. Application of three Catch-at-Age Models to stocks of walleye pollock ( Ther agra chalcogramma) in the Western Gulf of Alaska. In: M. Alton {Compiler), Proceedings of a Workshop on Comparative Biology, Assessment, and Management of Gadoids from the North Pacific and Atlantic Oceans. U.S. Natl. Mar. Fish. Serv., Northwest and Alaska Fish. Cent., Seattle, Wash. (unpublished), 29 pp. Nelson, M.O. and Nunallee, E.P., 1985. Acoustic-midwater trawl surveys of spawning walleye pollock in the Shelikof Strait region, 1980-1981 and 1983-84. In: R.L. Major (Editor), Condition of Groundfish Resources of the Gulf of Alaska Region as Assessed in 1984. U.S. Dep. Commer., NOAA Tech. Memo. NMFS F/NWC-80, pp. 179-206. Nelson, M.O. and Nunnallee, E.P., 1986. Results of acoustic-midwater trawl surveys for walleye pollock in Shelikof Strait, 1985. In: R.L. Major (Editor), Condition of Groundfish Resources of the Gulf of Alaska Region as Assessed in 1985. U.S. Dep. Commer., NOAA Tech. Memo., NMFS, in press. Nelson, M.O., Williamson, N.J. and Nunnallee, E.P., 1981a. A preliminary report on 1980 and 1981 acoustic-trawl surveys of spawning walleye pollock (Theragra chalcogramma) in the Shelikof Strait-Chirikof Island region of the Gulf of Alaska. Northwest and Alaska Fish. Cent., Natl. Mar. Fish. Serv., NOAA, Seattle, Wash. (unpublished), 33 pp. Nelson, R., Jr., French, R. and Wall, J., 1981b. Sampling by U.S. observers on foreign fishing vessels in the eastern Bering Sea and Aleutian Island region, 1977-1978. U.S. Natl. Mar. Fish. Serv. Mar. Fish. Rev., 43: 1-19. Nunnallee, E.P., Williamson, N.J. and Nelson, M.O., 1982. Acoustic-trawl surveys of spawning walleye pollock (Theragra chalcogramma) in the Shelikof Strait-Chirikof region of the Gulf of Alaska in 1980 and 1981. Northwest and Alaska Fish. Cent., Natl. Mar. Fish. Serv., NOAA, 2725 Montlake Blvd. E., Seattle, Wash. (unpublished), 31 pp. Rigby, P.W., 1984. Alaskan domestic groundfish fishery for the years 1970 through 1980 with a review of two historic fisheries - - Pacific cod (Gadus macrocephalus) and Sablefish (Anoplopoma fimbria). Alaska Dep. Fish Game, Tech. Data Rep. 108, 446 pp. Ronholt, L.L., Shippen, H.H. and Brown, E.S., 1978. Demersal fish and shellfish resources of the Gulf of Alaska from Cape Spencer to Unimak Pass, 1948-1976. (A Historical Review). Processed Rep., Northwest and Alaska Fish. Cent., Natl. Mar. Fish. Serv., NOAA, Seattle, Wash. {unpubl.), 4 Vols., 955 pp. Simenstad, C.A., 1979. ADF & G-OCS fish food habits analysis. In: Environmental Assessment of the Alaskan Continental Shelf, Annual Reports of Principal Investigators 4: 411-446. U.S. Dep. Commer., Natl. Oceanic Atmos. Admin., Environ. Res. Lab., Boulder, Colo. Traynor, J.J. and Williamson, N.J., 1983. Target strength measurements of walleye pollock (Theragra chalcogramma) and a simulation study of the dual beam method. In: O. Nakken and S.C. Venema (Editors), Symposium on Fisheries Acoustic, 21-24 June 1982, at Bergen, Norway. Food Agric. Org. U.N., Rome, FAO Fish Rep. 300, pp. 112-124. Wall, J., French, R. and Nelson, R., Jr., 1981. Foreign Fisheries in the Gulf of Alaska, 1977-78. U.S. Natl. Mar. Fish. Serv. Mar. Fish. Rev., 45: 20-35. Westrheim, S.J., 1970. Survey of rockfishes, especially Pacific ocean perch, in the Northeast Pacific Ocean, 1963-66. J. Fish Res. Board Can., 25: 1781-1809.
185
Changes in the Abundance and Distribution of Walleye Pollock ( T h e r a g r a chalcogramma) in the Western Gulf of Alaska ( 1 9 6 1 - 1 9 8 4 ) MILES S. ALTON, MARTIN O. NELSON and BERNARD A. MEGREY
National Oceanic and Atmospheric Administration, Northwest and Alaska Fisheries Center, National Marine Fisheries Service, Resource Assessment and Conservation Engineering Division 7600 Sand Point Way NE, Seattle, WA 98115 (U.S.A.)
ABSTRACT
Alton, M.S., Nelson, M.O. and Megrey, B.A., 1987. Changes in the abundance and distribution of walleye pollock (Theragra chalcogramma) in the western Gulf of Alaska (1961-1984). Fish. Res., 5: 185-197. Changes in the abundance and recruitment of western Gulf of Alaska walleye pollock ( Theragra chalcogramma) during 1961-1984 were examined. Bottom trawl survey CPUE estimates increased between the early 1960s and early 1970s. Independent biomass estimates from acoustic surveys and catch-at-age analyses of foreign and joint venture fisheries indicate that another increase in abundance occurred during 1978-1981, but that stock size has declined since 1981 or 1982. Biomass was estimated at 950 000 metric tons (t) in 1973-1974, 2.3-3.8 million t in 1981 and 1.8 million t in 1984. The fluctuations in stock size reflect substantial changes in year-class strength. These include the occurrence of strong year classes in 1967, 1970, 1972 and 1975-1979. Weak year classes apparently occurred in 1980-1982. Based on spawner-recruit estimates since 1976, pollock recruitment may be strongly density dependent. It is believed that there has been a shift, since 1977, in the abundance of pollock from the Kodiak-Chirikof area, westward towards the Shumagin Islands.
INTRODUCTION
T h e objective of this p a p e r is to review t h e c h a n g e s t h a t have o c c u r r e d in t h e a b u n d a n c e a n d d i s t r i b u t i o n o f w e s t e r n G u l f o f A l a s k a walleye pollock ( Theragra c h a l c o g r a m m a ) since 1961. T h i s species has a c o n t i n u o u s g e o g r a p h ical d i s t r i b u t i o n t h a t e x t e n d s f r o m off California, n o r t h w a r d to A l a s k a a n d w e s t w a r d to w a t e r s o f f t h e U.S.S.R., J a p a n a n d K o r e a ( B a k k a l a et al., 1986). S t o c k b o u n d a r i e s over t h e r a n g e have f r e q u e n t l y b e e n b a s e d o n statistical r e p o r t i n g areas r a t h e r t h a n o n p o p u l a t i o n d i s t i n c t i o n s . W e have defined t h e ' w e s t e r n G u l f o f A l a s k a ' as t h e region b e t w e e n t h e Fox I s l a n d s a n d t h e K e n a i
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Fig. 1. WesternGulfof AlaskashowingINPFC statistical areas and geographicallocations. Peninsula (Fig. 1 ) and have assumed that pollock within this region can be treated as a single stock. The information presented in this paper is based on reports by Hughes and Hirschhorn (1979), Alton and Deriso (1983), Nelson and Nunnallee (1985) and Megrey (1986) and on further examination of fisheries and research vessel survey data on the western Gulf of Alaska pollock resource. CATCH STATISTICS Prior to 1982, Gulf of Alaska pollock was primarily harvested by fisheries from Japan, the U.S.S.R. and other foreign nations. The most complete time series of reported pollock catches is from the Japanese fisheries, which began in the Gulf of Alaska in 1963. As a member of the International North Pacific Fisheries Commission (INPFC), Japan reported annual catches of its fisheries in a prescribed manner - - by species, month, gear type and vessel tonnage size for statistical blocks of 1 ° longitude × 1/2 ° latitude. When the Magnuson Fisheries Conservation and Management Act was passed in 1977, all foreign nations were required to report their catches by INPFC standards. Before 1977, the U.S.S.R. and other nations, excluding Japan, had reported their catches in the Gulf of Alaska in an inconsistent manner. In fact, there are no adequate estimates of the U.S.S.R. pollock catch for the years 1964-1971 when U.S.S.R. trawl fisheries were very active in the Gulf of Alaska.
187
In addition to the foreign nations' annual reports, the U.S. Observer Program, which began in 1978, provides estimates of the annual foreign catches of pollock and other groundfish by broad INPFC areas (Fig. 1 ). These estimates are determined using procedures described by Wall et al. (1981). Pollock catches by U.S. vessels were of little consequence until the 1980s, when joint ventures 1 began to intensify in the Gulf of Alaska. Estimates of the pollock catch in such fisheries come from the U.S. Foreign Fisheries Observer Program. LENGTH AND AGE COMPOSITION OF CATCHES
Length and age data come from U.S. observer sampling of foreign and joint venture catches and are available for the years 1976-1984. Length measurements are taken by the observer, when pollock is the target species, from a random sample of ~ 150 fish per day. However, only one age sample is obtained per observer cruise (which usually lasts 1-2 months). This sample consists of otoliths removed from a maximum of five fish per sex per 1-cm length interval. The length and age sampling procedures are described in detail by Nelson et al. (1981b). Following procedures described by Allen {1966), an age-length key assembled from the length-stratified age sample is used to convert length frequency samples to age compositions. Otoliths are read by the ageing unit of the Northwest and Alaska Fisheries Center using methods described by LaLanne (1975). Estimates of the catch in numbers at age were obtained in a step-wise manner, beginning with weighting the age composition of samples from a specific nation, vessel class, sub-area and time period by the catch and then summing as required to obtain the annual catch at age by the nation and sub-area (Alton and Deriso, 1983). RESEARCH SURVEYS
In the early 1960s, the International Pacific Halibut Commission (IPHC) conducted bottom trawl surveys throughout the Gulf of Alaska to obtain estimates of the abundance of halibut relative to other bottomfish in order to estimate the impact of a bottom trawl fishery on the halibut stock ( International Pacific Halibut Commission, 1964). U.S. commercial trawlers of 200-300 horsepower were chartered for the survey and all used a commercial type trawl (400-mesh eastern). Trawl stations were pre-determined and systematically arranged for depth and area coverage. The duration of trawling at each station was 1 h. Only data from the 1961 surveys are used in this report because the 1Ajoint venture is an arrangement between U.S. and foreign interests, in which U.S. vessels catch and sell fish to foreign processing vessels operating in the fishery conservation zone (FCZ).
188
coverage by season and area that year was comparable to that of surveys conducted by the National Marine Fisheries Service ( NMFS ) during 1973-1975. The 1973-1975 NMFS surveys were confined to the western Gulf of Alaska and were aimed at assessing the abundance of pollock and other bottomfish (Hughes and Hirschhorn, 1979). These surveys were conducted using the NOAA research vessel "John N. Cobb", a vessel similar in size and horsepower to the trawlers used for the 1961 IPHC surveys. Trawl stations were selected randomly within broad bottom-depth strata ( ~ 100-m intervals from 50 to 450 m). The same type of trawl used during the earlier IPHC surveys was employed. However, trawling duration was 1/2 h rather than 1 h as in the IPHC surveys. Acoustic and midwater trawl surveys ( hereafter referred to as acoustic surveys) of pollock in the western Gulf of Alaska began in 1980, following recognition of the unusually large aggregation of spawning pollock in Shelikof Strait (Nelson et al., 1981a). These surveys, which are designed to estimate the biomass and age composition of spawning pollock in the Strait area, continued in 1981, 1983 and 1984. In 1983 and 1984, acoustic surveys were also conducted outside Shelikof Strait in an attempt to locate other pollock spawning areas in the western Gulf. The design of the surveys and the methods of collecting and analyzing the acoustic and biological data are described by Nelson et al. (1981a), Nunnallee et al. (1982) and Nelson and Nunnallee (1985). All echo-integrator data on the Shelikof pollock have been scaled to estimates of density, using an average target strength of - 3 1 . 3 dB kg-1. This estimate is based on Bering Sea pollock target strength measurements reported by Traynor and Williamson (1983). Biomass estimates were obtained by multiplying average density pe~ unit surface area by the area surveyed. Age-specific biomass and population estimates were calculated using length frequency data, survey-specific length-weight relationships and age-length keys. STOCK DELINEATION The actual relationships among Gulf of Alaska pollock stocks and between Gulf and Bering Sea stocks are not clear. Biochemical genetic studies by Grant and Utter (1980) revealed some distinction between pollock of the Bering Sea and those sampled off the Kenai Peninsula and in the eastern Gulf of Alaska. One sample of pollock from the Shelik of Strait area showed more affinity with Bering Sea pollock than with fish from off the Kenai Peninsula and the eastern Gulf of Alaska. These results suggest an east vs. west separation of pollock in the vicinity of Kodiak Island. This is in agreement with Hughes and Hirschhorn (1979), who also found evidence of such a separation based on regional differences in recruitment and mean size at age for two year classes. However, we have treated the pollock of the western Gulf of Alaska as one stock because of evidence from acoustic surveys (Nelson and Nunnallee, 1985) and ichth-
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yoplankton surveys (Dunn et al., 1984) that most pollock of the region spawn in one locality, Shelikof Strait. RESULTS
1961-I976 Between 1962 and 1971, foreign trawlers in the Gulf of Alaska targeted mainly on rockfish, particularly Pacific ocean perch (Sebastes alutus) ; pollock were a minor part of the total catch until 1972. Rockfish apparently were an important component by weight in the bottomfish community during the 1960s, as indicated by the results of bottom trawl surveys (Alverson, 1968) and by their importance in the foreign fisheries ( Chikuni, 1970 ). Since the distributions of adult Pacific ocean perch and pollock overlap, we assume that much of the pollock taken by the fisheries in the 1960s was by-catch. The average annual pollock catch by Japan during the peak years of the rockfish fisheries was ~ 5000 t. Japanese fishing was concentrated in the Chirikof-Kodiak region and south of the Fox Islands, where pollock by-catch was highest ( Fig. 2 ). The size of the pollock by-catch in the intense U.S.S.R. rockfish fishery prior to 1971 is unknown. The 1972 catch of pollock by foreign fisheries was almost four times larger than the 1971 catch (Table I). Whether this was because of a declining rockfish resource or due to an increase in pollock abundance is not known. The annual pollock catch continued to increase through the 1970s.
190 TABLE I Catch (1000 t) of pollock in the Gulf of Alaska, by fishery category, 1971-1984a Year
1971 1972 1973 1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984
Fishery category Foreign
Joint venture
Domestic
Total
9.5 34.1 36.8 61.9 59.5 86.5 117.8 96.3 103.8 113.0 130.3 92.6 81.4 99.3
0 0 0 0 0 0 0 0 0.6 1.1 16.8 73.9 134.1 207.1
~ 9 ? ~ ~ ~ 0.2 1.0 2.0 0.9 0.6 2.2 0.1 0.3
9.5 34.1 36.8 61.9 59.5 86.5 118.0 97.3 106.4 115.0 147.7 168.7 215.6 306.7
~Sources: Foreign and joint venture catches: Berger et al., 1986. Domestic catches 1978-1980: Rigby, 1984. Domestic catches 1981-1984: Pacific Fishery Information Network (PacFIN), Pacific Marine Fisheries Commission, 305 State Office Building, 1400 SW Fifth Avenue, Portland, OR 97201, U.S.A. N M F S surveys of 1973-1975 revealed t h a t pollock was a d o m i n a n t species in the b o t t o m f i s h c o m m u n i t y of t h e w e s t e r n G u l f of Alaska a n d t h a t its a b u n dance was significantly higher t h a n indicated by I P H C surveys c o n d u c t e d 12-14 years earlier ( R o n h o l t et al., 1978). As i n d i c a t e d in T a b l e II, t h e 1973-1975 c a t c h per s t a n d a r d tow was 4 to 24 t i m e s g r e a t e r t h a n t h a t of t h e earlier I P H C surveys in 1961, even w i t h o u t c o r r e c t i n g for t h e d i f f e r e n c e in tow d u r a t i o n b e t w e e n t h e two s u r v e y periods. T h e 1973-1975 surveys p r o v i d e d a crude first e s t i m a t e of the exploitable biomass (age 3 a n d older) of pollock in t h e w e s t e r n G u l f of Alaska. A range was e s t i m a t e d b y a s s u m i n g t h a t t h e c a t c h a b i l i t y coefficient of t h e s u r v e y trawl was 1.0 for a m i n i m u m value o f b i o m a s s ( 0.95 million t ) a n d 0.5 for a m a x i m u m value (1.90 million t ) ( A l t o n a n d Deriso, 1983). T h e 1973-1975 surveys also i n d i c a t e d t h a t the 1967, 1970 a n d 1972 y e a r classes were relatively m o r e a b u n d a n t t h a n the 1968, 1969 a n d 1971 year classes ( H u g h e s a n d H i r s c h h o r n , 1979). 1977-1984 T h e e s t i m a t e s of t h e exploitable biomass d e r i v e d f r o m t h e 1973-1975 s u r v e y results p r o v i d e d a basis for d e t e r m i n i n g a c o r r e s p o n d i n g range of equilibrium yield of 152 000-305 000 t ( A l t o n a n d Deriso, 1983). T h e a n n u a l allowable
191 TABLE II Comparison of pollock catch rates between 1961 International Pacific Halibut Commission and 1973-1975 NMFS research trawl surveys. Catch rates are only from those stations that were located at bottom depths between 100 and 300 m Region
Kodiak (151°-154 ° W. long)
Chirikof (154°-159 ° W. long)
Shumagin (159°-167 ° W. long)
Period
Number of stations
Catch per standard haul (kg) a
F (df}
July-Sept. 1961 July-Sep. 1973
16 21
123 494
4.7* (1,35}
June-July 1961 June-July 1975
46 27
37 417
4.3* (1,71)
July-Aug. 1961 July-Aug. 1974
44 36
19 454
10.7" {1,78)
aFor the 1961 surveys a standard tow was 1.0 h in duration and for surveys in 1973-1975 the duration was 0.5 h. *Significant at the 95% level.
catch for 1977 was conservatively set at the lower value. In that year, the catch rose sharply to 118 000 t, as foreign trawlers intensified their efforts on pollock ( Table I ). In 1981, the pollock catch approached the allowable catch of 152 000 t. Although there were relatively few years of catch-at-age data (1976-1981), Alton and Deriso (1983) performed a catch-at-age analysis using a modified version of the method described by Doubleday (1976). The model provided a good fit between observed and predicted annual catch at age and indicated that exploitable biomass and surplus production had increased sharply between 1978 and 1979 and continued to increase in 1980 and 1981. Average annual exploitable biomass for the period 1976-1981 was estimated at 1.040 million t. When 1982 data became available and included in the catch-at-age analysis, poor correspondence between observed and predicted catch at age resulted. This was attributed to changes that had occurred in age-specific selectivity during 1982. Changes in selectivity were also found by Megrey (1986) when 1983 data were included in a later analysis. Megrey (1986) applied three different catch-at-age models to 1976-1983 catch-at-age data. Results were similar for all models and showed the same sharp 1979-1981 rise in pollock abundance found by Alton and Deriso (1983). However, abundance declined in 1983. This decline is also reflected in independent biomass estimates taken from acoustic surveys of pre-spawning and spawning fish in Shelikof Strait in
192 TABLE III Changes in biomass (million t, age 3 and older) of western Gulf of Alaskapollockas estimated by (1) catch-at-age analysis using two values of natural mortality (M) (Megrey, 1986) and (ii) acoustic trawl surveys (Nelson and Nunnallee, 1986) Year
1976 1977 1978 1979 1980 1981 1982 1983 1984
Estimates from catch-at-age analysis with 95% confidenceinterval M = 0.32
M = 0.40
0.74 _ 0.07 0.64 ± 0.07 0.96 ± 0.14 1.65~+0.30 2.08 ± 0.39 2.46 ± 0.50 2.63 ± 0.54 2.23 ± 0.49
0.95 ± 0.09 0.80 ___0.09 1.12 ± 0.17 1.85 ± 0.35 2.17 ± 0.43 2.29 ± 0.48 2.23 ± 0.47 1.71 _ 0.39
Acoustic trawl survey estimate with 95% confidence interval
3.77_+0.91 No survey 2.43 ± 0.74 1.84 ± 0.63
1983 and 1984 (Table III). The decline did not appear to result from a change in distribution, because surveys outside the strait during the same period in 1983 and 1984 did not detect any substantial concentrations of pollock. The decline of pollock is attributed, in part, to the fact t h a t the 1980 year class was below average in abundance. As 4-year-olds, this year class was poorly represented in samples from both the commercial catch and the acoustic survey in 1984 (Alton, 1985; Nelson and Nunnallee, 1985). This was the first time t h a t 4-year-old fish were a relatively minor component in the Shelikofpollock fisheries ( Fig. 3 ). In addition, findings from the 1985 acoustic survey ( Nelson and Nunnallee, 1986), indicated t h a t the 1981 and 1982 year classes were poorly represented in the population. The results of catch-at-age analysis (Megrey, 1986) and acoustic survey estimates (Nelson and Nunnallee, 1985) show t h a t pollock biomass had increased substantially, starting in 1979, peaked in 1981 or 1982 and is currently declining (Table III). The increase in abundance is attributed to a succession of five a b u n d a n t year classes (1975-1979) t h a t followed the weak year classes of 1973 and 1974. The population estimates of age 3 fish for these exceptional year classes range from 1.2 to 2.1 billion, 4-10 times the estimated number of 3-year-olds from the 1973 and 1974 year classes (Megrey, 1986). The 1980 year class is considered comparable in size to the weak 1973 and 1974 year classes. In 1984, the pollock biomass in the western Gulf of Alaska was comprised of mainly fish of the 1978 and 1979 year classes. The rise in pollock abundance since 1978 has coincided with changes in the geographical distribution of the foreign pollock catch. Before 1977, catches were primarily from the Fox Island and Chirikof-Kodiak regions. During
193 30F
1980
I
10
APRIL MAY
P
30
2 0 10
52
~
"30 20
1981
IL
1982
JANUARY--APRIL
10
~
30~ 2A P 0 R
1983 I~ L
30
1
0
1984
2M A0R C~H 1 0 2
4
6 Age
8
10+
Fig. 3. Age composition of pollock in joint venture catches from the Shelikof Strait region, 1980-1984. 1977-1980, catches south and east of Shumagin Islands increased and by 1981-1983, ~ 75% of the total foreign pollock catch came from there (Fig. 2). Preliminary fisheries information for 1984 suggests that the Chirikof-Kodiak grounds are resuming their importance in the foreign pollock fisheries. Most of the foreign pollock catch is taken in the late summer and autumn. Coincident with this geographical shift of the foreign pollock harvest, has been the development of joint venture fisheries targetting mainly on pollock. Such fisheries have been centered in Shelikof Strait on pre-spawning and spawning pollock during January-April. In 1983 and 1984, the joint venture fisheries catch exceeded that of the foreign fisheries (Table I). DISCUSSION It is apparent that since 1961, five important changes have occurred with respect to the western Gulf of Alaska pollock resource. These are: (i) the substantial increase in stock size between the early 1960s and 1973-1975, which resulted largely from the success of the 1967, 1970 and 1972 year classes, (ii) the succession of five abundant year classes (1975-1979) that caused an exceptional increase in pollock abundance during 1978-1982, i.e., to a level about four times that of the mid-1970s. (iii) the abundance decline observed since
194 1981-1982, (iv) a regional shift in pollock catches from the Chirikof-Kodiak region to the Shumagin area that began in 1977, and (v) the development of a fishery on spawning fish in Shelikof Strait. The lack of complete and comparable time-series data on the abundance of pollock and other species and on the physical and biological environment, make it difficult to examine the extent to which different biotic and abiotic factors have contributed to the major changes in pollock abundance that have been observed since the early 1960s. However, two processes, species replacement (Cushing, 1980; Daan, 1980) and density-dependent control of recruitment, may have been important. The evidence for replacement is circumstantial and relates to the decline of Pacific ocean perch. The pronounced increase in the size of the pollock population that was detected by the 1973-1975 surveys occurred during and after a period in which exploitation caused a major reduction in Gulf of Alaska stocks of Pacific ocean perch. In the early 1960s, before there were intensive fisheries for Pacific ocean perch, the adult biomass of this species was estimated at 1.1 million t (Kimura et al., 1984). Then, during a relatively short period (1963-1969), its biomass was reduced by ~ 70% ( Ito, 1982 ). Pollock biomass, in turn, increased to a 1973-1975 level comparable to that of the original biomass of Pacific ocean perch. Pacific ocean perch, as well as some other species of rockfish, are potential competitors of pollock because they and pollock feed on similar prey (euphausiids and other nekton) and overlap in distribution. If, however, replacement involving competition was operating alone, the second increase in pollock biomass to > 2 million t in the late 1970s and early 1980s appears excessive. If replacement did not occur between the 1960s and 1970s, then the abundance of Pacific ocean perch and that of pollock fluctuated independently of one another. The fisheries may have begun in the Gulf of Alaska during a period of high abundance of Pacific ocean perch. Because it is such a long-lived species with low fecundity, one would expect the Pacific ocean perch population to be composed of many sub-dominant age groups. However, Westrheim (1970) found from trawl surveys in the early 1960s, that three age groups comprised >~69% of the population. He raises the possibility that ocean perch abundance in the early 1960s was at an unusually high level because of the recruitment of a small number of abundant cohorts over a relatively brief period ( S.J. Westrheim, personal communication, 1986). Pollock is currently in a period of decline because of the emergence of a series of poor year classes (1980, 1981, 1982 and possibly, 1983). Since these weak year classes were spawned when pollock were at high levels of abundance, density-dependent control of recruitment may be operating on this stock. This would agree with Megrey (1986), who suggests that Western Gulf of Alaska pollock may have a Ricker type spawner-recruit relationship. The recent emergence of a strong year class (1984) when population was low in abundance
195 further suggests a Ricker type relationship. This year class was found in exceptional numbers as 1-year-old fish in Shelikof Strait during an acoustic survey in 1985 ( Nelson and Nunnallee, 1986). We can only speculate as to the mechanisms involved in density-dependent recruitment. For eastern Bering Sea pollock, cannibalism of young fish has been proposed as a factor controlling recruitment (Laevastu and Larkins, 1981, p. 106; Francis and Bailey, 1983), but limited studies of the diet of Gulf of Alaska pollock (Simenstad, 1979; Blackburn et al., 1983; Clausen, 1983 ) show no evidence of this phenomenon. The increase since 1977 in the pollock catch by the foreign trawl fisheries in the Shumagin area and the corresponding decrease in catch in the previously more important Chirikof-Kodiak region can be intepreted as a regional shift in availability. There is no evidence that this regional change in catch was caused by quotas or other regulations. In recent years (1981-1983), ~ 75% of the foreign trawl catch of pollock came from the Shumagin area and 37% from only two statistical blocks in that area (Fig. 2). This localization of large removals indicates that pollock abundance was centered in this region in those years when pollock biomass was high. These removals took place principally during July-November, a period of intense feeding, indicating that pollock availability was probably related to the abundance and behavior of prey. This is further supported by the fact that the catch of other species (Atka mackerel, Pleurogrammus monopterygius and Pacific ocean perch) having similar prey preferences (euphausiids and other nektonic organisms) was also disproportionately higher in the Shumagin area than in other regions. Furthermore, the Japanese setline fishery for Pacific cod ( Gadus macrocephalus) has been centered in the Shumagin area since 1977. Cod feed on both benthic and nektonic organisms, including pollock (Jewett, 1978). The concentration offish removals from the Shumagin area suggests that this area contained a large food base for fish populations. ACKNOWLEDGMENTS The authors thank George Hirschhorn and other members of the Ageing Unit of the Northwest and Alaska Fisheries Center in Seattle for providing timely information on the ages of pollock. They also thank Russell Nelson, Jr. and his colleagues in the U.S. Foreign Fisheries Observer Program in Seattle for their advice and information on the foreign and joint venture fisheries of the Gulf of Alaska.
REFERENCES Allen, K.R., 1966. Determination of age distribution from age-length distributions, IBM 7090, 7094, FORTRAN IV. Trans. Am. Fish. Soc.,95: 230-231.
196 Alton, M., 1985. Walleye pollock fisheries of the Gulf of Alaska, 1983-1984. In: R.L. Major (Editor), Condition of Groundfish Resources of the Gulf of Alaska Region as Assessed in 1984. U.S. Dep. Commer., NOAA Tech. Memo. NMFS F/NWC-80, pp. 1-10. Alton, M. and Deriso, R., 1983. Pollock. In: D. Ito and J. Balsiger (Editors), Condition of Groundfish Resources of the Gulf of Alaska in 1982. U.S. Dep. Commer., NOAA Tech. Memo. NMFS F/NWC-52, pp. 1-62. Alverson, D.L., 1968. Fisheries resources in the northeastern Pacific Ocean. In: The Future of the Fishing Industry of the United States. Univ. Wash. Publ. Fish. New Ser., 4, pp. 86-101. Bakkala, R., Maeda, T. and McFarlane, G., 1986. Distribution and stock structure of pollock ( Theragra chalcogramma) in the North Pacific Ocean. Int. North Pac. Fish. Comm. Bull., 45: 3-20. Berger, J.D., Smoker, J.E. and King, K.A., 1986. Foreign and joint venture catches and allocations in the Pacific Northwest and Alaska fishing area under the Magnuson Fishery Management and Conservation Act, 1977-1984. U.S. Dep. Commer., NOAA Tech. Memo. NMFS F/NWC99, 53 pp. Blackburn, J.E., Anderson, K., Hamilton, C.I. and Starr, S.J., 1983. Pelagic and demersal fish assessment in the lower Cook Inlet estuary system . In: Environmental Assessment of the Alaskan Continental Shelf, Final Reports, Biological Studies 17: 107-450. U.S. Dep. Commer., Natl. Oceanic Atmos. Admin., Natl. Ocean Serv., Off. Oceanography Mar. Sci., Juneau, Alaska. Chikuni, S., 1970. Data on the Pacific ocean perch in the northeast Pacific. Development and history of the Japanese trawl fishery through 1969. Fish. Agency Japan, Shimizu, INPFC doc. 1354 (unpublished,), 34 pp. Clausen, D.M., 1983. Food of walleye pollock, Theragra chalcograrama, in an embayment of southeast Alaska. U.S. Natl. Mar. Fish Serv. Fish. Bull., 81: 637-642. Cushing, D.H., 1980. The decline of the herring stocks and the gadoid outburst. J. Cons. Int. Explor. Mer, 39: 70-81. Daan, N., 1980, A revieW of replacement of depleted stocks by other species and the mechanisms underlying such replacement. Rapp. P.-V. R~un. Cons. Int. Explor. Met, 177:405-421. Doubleday; W.G., 1976. A least squares approach to analyzing catch at age data. Int. Comm. Northwest Atl. Fish. Res. Bull., 12: 69-81. Dunn, J.R., Kendall, A.W., Jr. and Bates, R.D., 1984. Distribution and abundance patterns of eggs and larvae of walleye pollock ( Theragra chalcogramma) in the western Gulf of Alaska. NWAFC Processed Rep. 84-10. Northwest and Alaska Fish. Cent., Natl. Mar. Fish. Serv., NOAA, Seattle, Wash. (unpublished), 66 pp. Francis, R.C. and Bailey, K.M., 1983. Factors affecting recruitment of selected gadoids in the northeast Pacific and east Bering Sea. In: W.S. Wooster (Editor), From Year to Year. Univ. Wash., Seattle, Coll. Ocean Fish. Sci., Sea Grant Program, Sea Grant Publ. WSG-WO 83-3, pp. 35-60. Grant, W.S. and Utter, F.M., 1980. Biochemical genetic variation in walleye pollock, Theragra chalcogramraa: population structure in the southeastern Bering Sea and the Gulf of Alaska. Can. J. Fish. Aquat. Sci., 37: 1093-1100. Hughes, S.E. and Hirschhorn, G., 1979. Biology of walleye pollock, Theragra chalcogramma, in the western Gulf of Alaska. U.S. Natl. Mar. Fish. Serv., Fish. Bull., 77: 263-274. International Pacific Halibut Commission, 1964. Catch records of a trawl survey conducted by the International Pacific Halibut Commission between Unimak Pass and Cape Spencer, Alaska, from May 1961 to April 1965. Rep. Int. Pac. Halibut Comm., 36:524 pp. Ito, D.H., 1982. A cohort analysis of Pacific ocean perch stocks from the Gulf of Alaska and Bering Sea regions. NWAFC Processed Rep. 82-15.Northwest and Alaska Fish. Cent., Natl Mar. Fish. Serv., NOAA, Seattle, Wash. (unpublished), 157 pp. Jewett, S.C., 1978. Summer food of the Pacific cod, Gadus macrocephalus, near Kodiak Island, Alaska. Fish. Bull., 76: 700-706.
197 Kimura, D.K., Balsiger, J.W. and Ito, D.H., 1984. Generalized Stock Reduction Analysis. Can. J. Fish. Aquat. Sci., 41: 1325-1333. Laevastu, T. and Larkins, H.A., 1981. Marine Fisheries Ecosystem. Fishery News Book Ltd., Farnham, Surrey, England, 162 pp. La•anne, J.J., 1975. The validity and consistency of age determinations from otoliths of Pacific pollock (Theragra chalcogramma). Int. North Pac. Fish. Comm. Annu. Rep., 1977: 99-107. Megrey, B., 1986. Application of three Catch-at-Age Models to stocks of walleye pollock ( Ther agra chalcogramma) in the Western Gulf of Alaska. In: M. Alton {Compiler), Proceedings of a Workshop on Comparative Biology, Assessment, and Management of Gadoids from the North Pacific and Atlantic Oceans. U.S. Natl. Mar. Fish. Serv., Northwest and Alaska Fish. Cent., Seattle, Wash. (unpublished), 29 pp. Nelson, M.O. and Nunallee, E.P., 1985. Acoustic-midwater trawl surveys of spawning walleye pollock in the Shelikof Strait region, 1980-1981 and 1983-84. In: R.L. Major (Editor), Condition of Groundfish Resources of the Gulf of Alaska Region as Assessed in 1984. U.S. Dep. Commer., NOAA Tech. Memo. NMFS F/NWC-80, pp. 179-206. Nelson, M.O. and Nunnallee, E.P., 1986. Results of acoustic-midwater trawl surveys for walleye pollock in Shelikof Strait, 1985. In: R.L. Major (Editor), Condition of Groundfish Resources of the Gulf of Alaska Region as Assessed in 1985. U.S. Dep. Commer., NOAA Tech. Memo., NMFS, in press. Nelson, M.O., Williamson, N.J. and Nunnallee, E.P., 1981a. A preliminary report on 1980 and 1981 acoustic-trawl surveys of spawning walleye pollock (Theragra chalcogramma) in the Shelikof Strait-Chirikof Island region of the Gulf of Alaska. Northwest and Alaska Fish. Cent., Natl. Mar. Fish. Serv., NOAA, Seattle, Wash. (unpublished), 33 pp. Nelson, R., Jr., French, R. and Wall, J., 1981b. Sampling by U.S. observers on foreign fishing vessels in the eastern Bering Sea and Aleutian Island region, 1977-1978. U.S. Natl. Mar. Fish. Serv. Mar. Fish. Rev., 43: 1-19. Nunnallee, E.P., Williamson, N.J. and Nelson, M.O., 1982. Acoustic-trawl surveys of spawning walleye pollock (Theragra chalcogramma) in the Shelikof Strait-Chirikof region of the Gulf of Alaska in 1980 and 1981. Northwest and Alaska Fish. Cent., Natl. Mar. Fish. Serv., NOAA, 2725 Montlake Blvd. E., Seattle, Wash. (unpublished), 31 pp. Rigby, P.W., 1984. Alaskan domestic groundfish fishery for the years 1970 through 1980 with a review of two historic fisheries - - Pacific cod (Gadus macrocephalus) and Sablefish (Anoplopoma fimbria). Alaska Dep. Fish Game, Tech. Data Rep. 108, 446 pp. Ronholt, L.L., Shippen, H.H. and Brown, E.S., 1978. Demersal fish and shellfish resources of the Gulf of Alaska from Cape Spencer to Unimak Pass, 1948-1976. (A Historical Review). Processed Rep., Northwest and Alaska Fish. Cent., Natl. Mar. Fish. Serv., NOAA, Seattle, Wash. {unpubl.), 4 Vols., 955 pp. Simenstad, C.A., 1979. ADF & G-OCS fish food habits analysis. In: Environmental Assessment of the Alaskan Continental Shelf, Annual Reports of Principal Investigators 4: 411-446. U.S. Dep. Commer., Natl. Oceanic Atmos. Admin., Environ. Res. Lab., Boulder, Colo. Traynor, J.J. and Williamson, N.J., 1983. Target strength measurements of walleye pollock (Theragra chalcogramma) and a simulation study of the dual beam method. In: O. Nakken and S.C. Venema (Editors), Symposium on Fisheries Acoustic, 21-24 June 1982, at Bergen, Norway. Food Agric. Org. U.N., Rome, FAO Fish Rep. 300, pp. 112-124. Wall, J., French, R. and Nelson, R., Jr., 1981. Foreign Fisheries in the Gulf of Alaska, 1977-78. U.S. Natl. Mar. Fish. Serv. Mar. Fish. Rev., 45: 20-35. Westrheim, S.J., 1970. Survey of rockfishes, especially Pacific ocean perch, in the Northeast Pacific Ocean, 1963-66. J. Fish Res. Board Can., 25: 1781-1809.