Muter and Norcross: Distribution, abundance, and growth of Theragra chalcogramma 



581 



cause no replicate samples were taken, confidence 

 limits could not be calculated. 



A Student f-test ( two-sample comparison ) or a one- 

 way ANOVA followed by a Tukey multiple compari- 

 son test (multiple samples) was employed to detect 

 differences in mean standard length of larvae among 

 different depths at the same station and among dif- 

 ferent stations. Nonparametric tests were employed 

 in addition to parametric test procedures when the 

 assumptions for parametric tests were violated. The 

 nonparametric tests used were a Kruskal-Wallis test 

 (nonparametric analysis of variance), a Mann- 

 Whitney test (two-sample comparison), and aTukey- 

 type multiple comparison test (Zar, 1984). 



Differences in larval length among stations were 

 examined by using the most shallow samples from 

 each station (<22 m), thus minimizing potential er- 

 rors resulting from differences in size due to vertical 

 distribution. In addition, pollock lengths from all 

 depths were pooled for each station and the mean of 

 the pooled data was compared between stations. For 

 all between-station comparisons, larval mean SL was 

 corrected for the date of sampling by using growth 

 rates obtained during this study. 



Ages of larvae were estimated from the number 

 of otolith increments on sagittal otoliths as de- 

 scribed in Kendall et al. (1987). Increments were 

 independently counted a second time by the same 

 reader. Readings were confirmed for a subsample 

 of 20 otoliths by the Alaska Fisheries Science Cen- 

 ter laboratory in Seattle, Washington. Only those 

 independent readings that did not differ by more 

 than one increment (in which case the higher count 

 was used) were used for growth determination. 

 Random subsamples of larvae from two stations, 

 one in the inner basin ( RES 2 ) and one in the outer 

 basin (RES 4) were aged. Only larvae from these 

 stations could be aged because otoliths from all 

 other samples showed signs of erosion. 



Larval growth rates were determined by fitting 

 linear regression lines to length-at-age data. The 

 linear regression equations describing growth 

 were compared between stations to test for differ- 

 ences in regression coefficients. Slopes and eleva- 

 tions were compared by using Student's /-statis- 

 tic (Zar, 1984). Hatch dates were estimated after 

 correcting for mortality, because older fish in the 

 sample experienced a higher cumulative mortal- 

 ity than larvae hatched closer to the date of sam- 

 pling. Following Yoklavich and Bailey (1990), we 

 created a stepped, size-specific mortality function 

 with rates of 0.1, 0.08, 0.06, 0.03 per day for fish 

 <7, 7.01-10.0, 10.01-15.0 and 15.01-20.0 mm SL, 

 respectively. The range of ages corresponding to 

 each size range was calculated from the growth 



equation obtained in this study. The hatch date dis- 

 tribution was then estimated by backcalculating 

 numbers of larvae at hatching for each daily cohort 

 with the above mortality rates. 



Results 



Hydrography of the fjord 



On 6 April 1989, temperatures at RES 2.5 (inner 

 basin) and GAK 1 (mouth of fjord) increased with 

 depth from approximately 4 C in the surface layer 

 to almost 6°C below 200 m (Fig. 2). Between April 

 and May 1989 the properties of the water masses 

 inside and outside the fjord changed considerably. 

 In April the upper 100 m were nearly homogenous, 

 but a strong seasonal thermocline had developed 

 between 10 and 20 m in early May. The surface tem- 

 perature in May varied between 5.8 C C at RES 2.5 

 and T C at RES 3 (Fig. 2 ). Temperature profiles in 

 May showed a pronounced minimum of about 3.5°C 

 to 4.5°C near 80 m. While temperatures in April in 



6 April 1989 



3 4 6 6 



Temperature (°C) 



3 4 6 6 7 



Temperature (°C) 



RES 2.5 

 RES 1.2.3.4 1'GAK 1 



1-4 May 1989 



28 29 30 31 32 33 34 26 29 30 31 32 33 34 



Salinity (psu) Salinity (psu) 



Figure 2 



Temperature and salinity profiles at six stations in Res- 

 urrection Bay, Alaska, 6 April and 1-4 May 1989. 



