HUGHES and HIRSCHHORN BIOLOGY OF WALLEYE POLLOCK 



FIGL'RE 7. — Average of residuals from fit of observed mean 

 length-at-age, taken over all age-groups of the 1967 and 1970 

 year classes of walleye pollock sampled in the western Gulf of 

 Alaska by region and sex, 1973-75- 



tion of optimality in biomass relations. Since no 

 independent estimates of M are available for Gulf 

 of Alaska pollock, we shall assume M = K. Yield 

 relations based on this assumption have been 

 examined in detail by Alversonand Carney ( 1975) 

 for cases where the third Bertalanffy parameter 

 it^) is close to zero. In the preceding section and 

 Table 2, such growth estimates were provided. 



As indicated earlier, means of observed female 

 lengths at first maturity ranged from 30 to 35 cm, 

 comparable to 32-37 cm when length at this stage 

 is considered as two-thirds of the final length iLj 

 associated with the extreme growth completion 

 rates for females in Table 2 (0.386, 0.255). Use of 

 this constant was discussed and examined by Holt 

 ( 1962). Corresponding ages at first maturity i ?",„„) 

 ranged from 2.84 to 4.30 yr, and ages of maximum 

 (unfished) biomass per unit input (7",,,^) ranged 



from 3.60 to 5.45 yr, according to equation 5 of 

 Alverson and Carney (1975). The differences (T,,,,, 

 - T",,,,,,) at each extreme imply two annual pre-r,,,^ 

 spawnings m the low-A' cohort compared to one in 

 the h.\g\\-K cohort. The maximum production per 

 unit input from the low-R' (0.255, L, = 56.04) 

 cohort exceeds that for the Kigh-K (0.386, L, = 

 47.59) cohort by 64'^^r assuming a cubic length- 

 weight relation. Under the assumption M = K, 

 any environmental or fishing conditions tending 

 to reduce gi'owth completion rate (and increaseL^) 

 would be expected to increase cohort productivity 

 as well as delay the achievement of maximum 

 biomass per unit input. Such a delay also implies 

 an increased number of spawning opportunities 

 for females prior to the age at maximum biomass. 



DISCUSSION 



Pollock size and age composition, size and age at 

 first maturity, sex composition, length-weight re- 

 lationship, density distribution by depth and area, 

 estimate of exploitable biomass, in addition to 

 growth and yield relationships in the western Gulf 

 of Alaska have been described. The survey area. 

 Cape Cleare to the western end of Unalaska Is- 

 land, carries an exploitable pollock biomass that 

 we have calculated at 0.6-1.2 million t. Compared 

 with the 54,000 t of exploitable pollock in the cen- 

 tral and eastern portions of the Gulf of Alaska 

 (Ronholt et al.^), the resource described in this 

 report represents over 90*^* of pollock in the Gulf of 

 Alaska. 



Agreeability of size, age, and gi-owth data be- 

 tween surveys over the 3-yr study period indicated 

 that assessment techniques were reliable. 

 Weighting size and age composition data by catch 

 rate and square miles within each depth sampling 

 stratum is regarded as desirable when dealing 

 with a species which displays bathymetric prefer- 

 ence. Such weighting procedures coupled with the 

 collection of otoliths over all depths and areas 

 appear to provide accurate descriptive informa- 

 tion for a large region. It is possible that pollock 

 display bathymetric variations in growth rate as 

 Westrheim (1973) has shown for Pacific ocean 

 perch. However, our data did not allow such de- 



^Ronholt. L. L.H, H. Shippen. and E.S. Brown. 1976. An 

 assessment of demersal fish and invertebrate resources of the 

 northeastern Gulf of Alaska, Yakutat Bay to Cape Cleare, 

 May-August 1975: NEGOA Annual Report. Unpubl. man- 

 user., 184 p. Northwest and Alaska Fisheries Center, National 

 Marine Fisheries Service. NOAA, Seattle. WA 98112, 



273 



