662 



Fishery Bulletin 98(4) 



Materials and methods 



Length-at-age samples 



-130'00' -128'00' -126»00- 



-124«00' -\2Z'O0- -120*00' -IIS'OO' 



-116'00- 



48*00' 



A total of 4780 Dover sole were col- 

 lected for age determination during 

 1984—93 from bottom trawl surveys of 

 the upper continental slope (Raymore 

 and Weinberg, 1990; Parks etal.. 1993; 

 Lauth et al., 1997). Biological sam- 

 ples were collected from three depth 

 strata (183-549 m, 550-914 m, and 

 915-1280 m) within the International 

 North Pacific Fisheries Commission 

 ( INPFC ) statistical areas ( Table 1 ; Fig. 

 1) of Monterey (38''24'N-40°30'N), 

 Eureka (40°30'N^3°00'N), Colum- 

 bia (43°00'N-17°30'N), and Vancou- 

 ver ( 47°30'N-48°29'33"N ). The survey 

 area was length- and depth-stratified 

 to ensure adequate coverage for oto- 

 lith sampling. For each fish collected, 

 sex was determined, total length (to 

 the nearest cm) was measured, and 

 otoliths were extracted for age deter- 

 mination. 



Maturity stage was determined 

 for 4490 Dover sole by visual inspec- 

 tion of gonads. Only samples from 

 surveys conducted by the RV Miller 

 Freeman (Table 1) were used for 

 analyzing maturity rates because 

 these samples were collected prior 

 to the spawning season during win- 

 ter when gross anatomical features 

 provide a reasonably accurate mea- 

 sure for determining whether a 

 fish is mature or not (Hagerman, 

 1952; Hunter et al., 1992). Total fish 

 weights (to the nearest gram) were 

 also recorded for 3019 samples from 

 surveys conducted by the MV Half 

 Moon Bay and the RV Miller Free- 

 man; these data were used for length-weight analyses. 



Fish ages were determined by the break and burn 

 method, which is the standard for Dover sole (Pikitch 

 and Demory, 1988), although the method has not been 

 validated. In our study, the initial otolith increment was 

 assumed to be an age-1 marker in accordance with the con- 

 vention for aging adult Dover sole (Chilton and Beamish, 

 1982). Increments in each otolith were counted two or 

 more times and assigned a single age by one of four read- 

 ers. In some cases, a mark within the otolith core was 

 counted as the initial increment because it met the identi- 

 fication criteria of an annual increment. A paired compari- 

 son of age readings between the most experienced reader 

 and the other readers indicated that there was a mean dif- 

 ference of ^1=0.22 years (<T^=0.13) based on a blind sample 

 of 474 Dover sole; this difference was almost significant at 

 the5%level(P=0.08). 



«'00' 



M-OO' 



42'00' 



40*00  



38"O0' 



36*0O' 



34*00- 



:o5 



U.S. VANCOUVER 



COLUMBIA 



Pacific Ocean 



EUREKA 



MONTEREY 



CONCEPTION 



48°0O'N 



46*00- 



44*00' 



42*00' 



40*00' 



38*00' 



36*00' 



34*00' 



-130-00- 



•12S*00' -126*00' -124*00' -122*00' -120*00' 



-118-00- -1 16*00' W 



Figure 1 



International North Pacific Fisheries Commission statistical areas in the 

 North Pacific Ocean. 



Empirical comparisons of mean ages, lengths, and 

 weights by sex within each INPFC area were performed. 

 The GT2 test is appropriate for unplanned comparisons of 

 populations with unequal sample sizes (Sokal and Rohlf, 

 1981) and it was used to test for differences in mean age, 

 length, and sex among INPFC areas. 



Growth curves 



Bartlett's test of homogeneity of variance was used to 

 test whether variance in length at age varied by sex for 

 Dover sole from ages 3 to 30 years. Test results were 

 used to select an appropriate error structure for growth 

 curve estimation. Growth curves for male, female, and 

 pooled-sex samples were estimated by maximum likeli- 

 hood (see Kimura, 1980). We used an alternative para- 

 meterization of the von Bertalanffy growth cui^ve (Schnute 



