Kendall and Picquelle Egg and larval distributions of Ther^gra chalcogramma 



149 



plankton, including walleye pollock in the Gulf of Alas- 

 ka based on sampling prior to 1980. Very little sam- 

 pling had been done before 1977, when a series of 

 cruises was conducted mainly southeast of Kodiak 

 Island. Eggs were found mainly nearshore in spring, 

 and to a lesser extent in fall. Larvae were found in 

 spring and summer throughout the shelf and slope area 

 sampled. The spawning concentration in the Shelikof 

 Strait was discovered inadvertantly in 1980. The mean 

 number of eggs and larvae per 10 m- in cruises re- 

 ported by Kendall and Dunn (1985) did not exceed 40 

 and 228 respectively, whereas we found mean numbers 

 per cruise as high as 177,000 for eggs and 13,700 lar- 

 vae in Shelikof Strait (Table 2). 



Based on the patterns of distribution elucidated here, 

 several topics for future work are indicated. By com- 

 paring distributions of cohorts of eggs and larvae, we 

 inferred advection rates and directions. The reason- 

 ableness of these drift patterns needs to be investigated 

 based on the physical oceanography of the area. 



Although these results show that most spawning oc- 

 curs in Shelikof Strait, evidence of some spawning 

 south of Chirikof Island was observed. Surveys of 

 adults producing the eggs in this area need to be con- 

 ducted so that the importance of the area can be 

 evaluated. The source of the eggs and larvae found in 

 the Amatuli area and the northern part of Shelikof 

 Strait in 1985 also needs to be established. 



The cruises available for this study were made only 

 through early June. By this time the larvae had drifted 

 a considerable distance southwest of the spawning area 

 and grown to about 8-11 mm. The distribution of lar- 

 vae later in spring and summer is virtually unknown. 

 To understand recruitment in this population, the lar- 

 vae need to be followed through summer to find the 

 location of their nursery area. Such information is a 

 necessary prerequisite to studies of factors contribut- 

 ing to survival throughout the planktonic stages of 

 walleye pollock. 



During the years of study here, the adult spawning 

 biomass of walleye pollock in Shelikof Strait declined 

 from 3.77- 10*^ mt in 1981 to 0.62- 10'^ mt in 1986 

 (Nelson and Nunnallee 1987). The planktonic egg data 

 from these years can be analyzed to see how closely 

 they reflect this decrease in spawning biomass. Based 

 on the analysis here, a model of seasonal and 

 geographic egg production could be developed that 

 would allow extrapolation of expected egg catches 

 when sampling did not cover the entire area of their 

 occurrences. 



Given the interannually consistent patterns of egg 

 distribution in Shelikof Strait described here, there is 

 a reasonable opportunity to establish egg and larval 

 mortality rates based on changes in population abun- 

 dance during April and May. The timing of changes in 



mortality could be determined, and differences in mor- 

 tality rates among years could be investigated. This can 

 lead to studies of the environmental and biological 

 causes of mortality and how these vary during the 

 season and interannually. Knowledge of these varia- 

 tions in mortality rates is critical to an understanding 

 of recruitment patterns in this population. 



Summary 



Based on analysis of walleye pollock egg and larval 

 distributions in 1,929 MARMAP bongo tows taken on 

 32 cruises in the northern Gulf of Alaska from 1972 

 to 1986, we made the following conclusions: 



1 Most eggs occur in April, and larvae occur in late 

 April through May. 



2 Most eggs and larvae result from spawning in 

 Shelikof Strait near Cape Kekurnoi. 



3 Little difference in geographic distribution of eggs 

 was seen through the season or among the years 

 sampled. 



4 Larvae occurred progressively to the southwest of 

 the area of egg occurrence, as the larvae developed 

 through the season. 



5 Larval population averaged from 4.2 to 4.9 mm in 

 mid April, and reached 7.5 to 9.1 mm by the end 

 of May, except in 1983 when the population averag- 

 ed 10.5 mm at the end of May. 



6 Considerable variation in distribution of larvae was 

 seen by late May among the years sampled, in- 

 dicating differences in amount of advection. 



Acknowledgments 



Sample collection and processing, as well as the data 

 entry and editing that went into producing this publica- 

 tion, required contributions of persons too numerous 

 to mention; however, without them we could not have 

 considered undertaking this project. Suffice it to say 

 that the environment that walleye pollock find suitable 

 for spawning is often extremely inhospitable to those 

 on ships collecting their eggs and larvae, and we ap- 

 preciate the efforts of all involved. Most samples were 

 sorted and fish eggs and larvae identified, counted, and 

 staged (eggs) or measured (larvae) at the Polish Plank- 

 ton Sorting Center under the direction of Dr. Leonard 

 Ejsymont, whose diligence in overcoming the many 

 problems associated with such demanding work, as well 

 as the challenges of a cooperative program between 

 two countries, is gratefully acknowledged. Many peo- 

 ple at NWAFC assisted in processing egg and larval 

 samples, but we particularly want to thank Ann 

 Matarese, Beverly Vinter, and Deborah Blood. Jay 

 Clark and Richard Bates at NWAFC made the data 



