BIOLOGY OF WALLEYE POLLOCK, THERAGA CHALCOGRAMMA, 

 IN THE WESTERN GULF OF ALASKA, 1973-75 



Steven E. Hughes and George Hirschhorn' 



ABSTRACT 



Data on the stock composition, growth, mortality, and abundance of walleye or Alaska pollock, 

 Theragra chakogramma , in the western Gulf of Alaska were collected during six demersal trawl 

 surveys in 1973-75. Over 102,000 km^ of continental shelf and slope were surveyed; most of this area 

 was covered during spring and summer. 



Using the area-swept technique and catchability coefficients of 1.0 and 0.5, the e.\pIoitable pollock 

 biomass in the survey region was between 610,000 and 1,220,000 metric tons. The percentage of larger 

 and older fish increased to the west. Sexual maturity was reached at age 3, Growth completion rates 

 ranged from 0.2 to 0.4. Natural mortality was estimated (assuming natural mortality equals growth 

 completion ratei at 0.33 for males and 0.30 for females. Variations in growth completion rates within 

 year class and variable recruitment strength indicated a probable east-west separation of pollock 

 spawning populations near Kodiak. 



The National Marine Fisheries Service conducted 

 six trawl surveys of walleye or Alaska pollock, 

 Theragra chalcogramma , and other groundfish re- 

 sources in the western Gulf of Alaska from Cape 

 Cleare, Montague Island, west to Unalaska Island 

 during each spring and summer of 1973-75 (Fig- 

 ure 1). These surveys have provided information 

 on the geographic and bathymetric distribution 

 and densities of species within the groundfish 

 commmunity (Hughes and Parks 1975). 



An additional goal of these surveys and subject 

 of this report was the collection of pollock life his- 

 tory data for management purposes. 



METHODS 



Six cruises were completed. Five were con- 

 ducted from the 28-m NOAA RV John N. Cobb, 

 employing 400-mesh Eastern otter trawls with 

 30-m footropes. During these five surveys, fishing 

 was conducted following a predetermined, strat- 

 ified random survey pattern (Grosslein^l. Fishing 

 densities varied from one 30-min trawl/1,370 km'-^ 

 in strata of anticipated low densities (depths of 90 

 m or less) to one 30-min trawl/515 km^ in the 

 remaining depth strata of 91-180 m, 181-270 m. 



'Northwest and Alaska Fisheries Center, National Marine 

 Fisheries Service, NOAA, 2725 Montlake Boulevard East, Seat- 

 tle. WA 98112. 



^Grosslein, M. D. 1969. Some observations on accuracy of 

 abundance indices derived from research vessel surveys. Un- 

 publ. manuscr. Northeast Fisheries Center, National Marine 

 Fisheries Service, NOAA, Woods Hole. MA 02543. 



Manuscnpt accepted August 1978 

 FISHERY BULLETIN; VOL. 77, NO. 1, 1979 



271-360 m, and 361-450 m. The other cruise was 

 conducted from the 26-m chartered trawler A«/!a 

 Marie, with similar but larger modified Eastern 

 and Norwegian-style otter trawls with about 34-m 

 footropes. Because the purpose of the Anna Marie 

 survey was to determine commercial production 

 potentials (Hughes and Parks 1975), fishing was 

 concentrated where fish schools were detected by 

 echo sounding; no predetermined survey pattern 

 was followed. Consequently, the A;!na Mane data 

 (Sanak-Unalaska. May-June 1974) were not used 

 for pollock density or biomass studies. 



Stretch mesh measurements ( 1 knot included ) of 

 all trawls ranged from 10.2- to 14.0-cm mesh in the 

 intermediate and cod end sections. Trawls mea- 

 sured by scuba divers at depths of 15 m indicated 

 vertical heights of 2-3 m and horizontal spread of 

 11-13 m. 



Methods of selecting random samples of pollock 

 for collection of biological data were consistent 

 during all surveys (Hughes 1976a). Length- 

 frequency fork length (FL) measurements to the 

 nearest centimeter by sex were randomly collected 

 from each catch with the desired sample size being 

 300 pollock. While processing pollock for length- 

 frequency data, stratified subsamples of otoliths 

 ( 10/sex per cm) and individual fish weights (5/sex 

 per cm) were taken ( ±5 g). Otoliths were stored in 

 ethanol in plastic boxes (Hughes 1976b) and ages 

 were later determined as described by LaLanne 

 (in press). 



Length-frequency distributions determined 



263 



