FISHERY BULLETIN VOL 77, NO 1 



Figure l.— Western Gulf of Alaska 

 regions where trawl surveys were 

 completed, 1973-75. 



from the five standard resource assessment sur- 

 veys were weighted by catch magnitude and area 

 within sampling strata, whereas data collected 

 during the commercial fishing (Sanak-Unalaska, 

 May-June 1974) trials were weighted only by 

 catch magnitudes. Length-age data from the strat- 

 ified otolith collections of sexed pollock in each 

 survey region (Figure 1) were compiled into age- 

 length keys. 



The proportions of observed ages on each length 

 interval above 19 cm were applied to the weighted 

 length frequencies. For this we used a computer 

 program by Allen (1966! modified to exclude ex- 

 trapolations beyond the aged length range and to 

 include the calculation of mean length at age, as 

 well as numbers at age. Thus numbers and size of 

 pollock in the fishable population were estimated 

 by region, age, and sex. 



Resulting analysis provided weighted age com- 

 position data and mean length-at-age data for 

 growth studies. Von Bertalanffy growth-in-length 

 parameters and length-weight data were deter- 

 mined for each region. 



An area-swept technique ( Al verson and Pereyra 

 1969) was employed to estimate the pollock 

 exploitable biomass, using the relation Pw = 

 (CPUE) ( A)/(f ) (a) where Pu' is equal to the aver- 

 age standing stock, in weight, of the catchable 

 population. A is the total area; a is the average 

 bottom area covered by the trawl per standard 

 tow; and c is a coefficient related to the effective- 

 ness of the trawl in capturing pollock. 



Whereas earlier studies of Alaskan pollock as- 

 sumed c = 1.0 (Alverson and Pereyra 1969), pol- 

 lock were often acoustically detected ofT the sea 

 bottom and above the trawl's headrope. Estimates 

 ofc given for some gadoid species of the northeast- 

 ern Atlantic Ocean indicated c may not exceed 



0.51 (Edwards 1968). In this report, values of both 

 0.5 and 1.0 provide a conservative range of 

 biomass estimates. 



RESULTS 



The surveys resulted m 144 fishing days on the 

 grounds and 368 successful trawl hauls. Over 

 455,000 kg of groundfish were sampled, including 

 49,912 pollock which were processed for biological 

 data. 



Size and Age Composition 



In the three regions where spring and summer 

 surveys were conducted during the same year 

 (Shelikof Strait 197.3; southeast Kodiak 1973; and 

 Sanak-Unalaska 1974), seasonal variations in 

 size and age composition were attributed to fish 

 measuring <28 cm which represented the 1- and 

 2-yr-old juvenile segment of the population (Fig- 

 ures 2, 3). However, substantial differences be- 

 tween regions indicate that size and age of adult 

 pollock consistently increased when moving from 

 the southeast Kodiak and Shelikof Strait regions 

 westward through the Chirikof region and into the 

 Sanak-Unalaska region. 



The age composition data also indicated that 

 Gulf of Alaska pollock display strong variations in 

 year-class strength. Both 1967 and 1970 year class- 

 es showed unusually strong recruitment. Indica- 

 tion of a strong 1967 year class, sampled as 6-yr- 

 olds, was noted during the May-June 1973 surveys 

 of the southeast Kodiak and Shelikof regions. The 

 relative strength of this year class was again noted 

 3 mo later during the August-September survey of 

 southeast Kodiak and. particularly, of Shelikof 

 Strait. Farther west, in the October 1973 Chirikof 



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