Dorn: Environmental covanates of Merlucous productus growth 



261 



Methods 



The U.S. Foreign Fisheries Ob- 

 server Program at the Alaska 

 Fisheries Science Center (AFSC) 

 uses a two-phase sampling de- 

 sign to sample the catch of Pa- 

 cific whiting (French et al. 1981). 

 The first phase consists of obtain- 

 ing a large initial sample of fish 

 and recording the length and sex. 

 For the second phase of sam- 

 pling, a subsample of fixed size 

 is selected for each combination 

 of length category and sex. All 

 fish in these subsamples are aged 

 using otoliths. 



Typically, each observer sam- 

 ples 2-3 hauls or joint-venture 

 deliveries per day for length and 

 sex, and 150 otoliths (5 per centi- 

 meter-length category per sex) are collected 

 over a two-month cruise. The numbers of aged 

 and measured fish from 1978-88 are given in 

 Table 1. This information resides in a data base 

 maintained by the Resource Ecology and Fish- 

 eries Management Division (REFM) at the 

 Alaska Fisheries Science Center. 



Kimura and Chikuni (1987) point out that, 

 with a two-phase sampling design, estimates 

 of mean length-at-age are biased when ob- 

 tained simply by averaging the lengths of the 

 aged fish. To avoid this bias, stratified length- 

 at-age estimates were compiled from fishery 

 data for the years 1978-88 using separate age- 

 length keys for each stratum (see Appendix for 

 details). Three spatial strata were defined as: 



(1) the area from lat. 39°00'N to lat. 43°00N, 

 including part of the International North Pa- 

 cific Fishery Commission (INPFC) Monterey 

 region and the Eureka INPFC region (EUR); 



(2) the area from lat. 43°00N north to Cape 

 Falcon (lat. 46°45N) in the southern part of the 

 Columbia INPFC region (SCOL); and (3) the 

 area north of Cape Falcon to the U.S. -Canada 

 border including the northern part of the Co- 

 lumbia INPFC region and the U.S. portion of 

 the Vancouver INPFC region (VNC) (Fig. 1). 



Figure 1 



Spatial strata used to compile length-at-age 

 for midwater trawl fishery samples of Pacific 

 whiting Merluccius ■productus. 



British Columbia 



Nortlieasl Pacific Ocean 



50°00'N 



45°00' 



40°00' 



35°00' 



30°00' 



1 30°00'W 



125°00' 



1 20°00' 



