Stokesbury et al : Spatial distributions of C/upea pallasi and Theragra chalcogramma 



403 



moving 90° on either side of the first Hne, measur- 

 ing the distance to shore and selecting the shorter 

 of the two hnes. The third distance was determined 

 by moving 90° on either side of the first and second 

 hnes, measuring the distance to shore, and selecting 

 the shorter of the two lines. These three distances 

 were summed. This measurement was calculated at 

 specific points inside 26 bays and outside 17 bays 

 to verify that it accurately distinguished between 

 bays, passages, and open coast. The Z3NSD for each 

 fish school was compared with the same measure- 

 ments from randomly selected points along the same 

 survey transect by using a chi-squared test grouped 

 according to Cochran's rule (Sokal and Rohlf, 1981). 

 This technique removed sampling and shore struc- 

 ture discrepancies. 



Differences in water conditions within bays com- 

 pared with conditions in passages or along open 

 coastline were examined. Vertical water profiles of 

 temperature and salinity at 1-m intervals, mea- 

 sured with a SeaBird CTD instrument (SEACAT 

 SBE19), were collected inside and outside bays. 

 Differences between means were examined with a 

 Mann-Whitney rank sum test (U). 



Results 



Ninety-seven species of fish and macroinverte- 

 brates were collected during the October 1995, and 

 the March and July 1996 surveys. Pacific herring 

 (65.0% of subsample collections! dominated the ich- 

 thyofauna followed by walleye pollock (19.2%). 



Pacific herring and walleye pollock size frequencies 

 differed. The Pacific herring population consisted of 

 three size modes representing age-0, 1-2 year old, and 

 adult fishes ( Stokesbury et al. 1999 ) ( Fig. 2 ). The wall- 

 eye pollock population size distribution was bimodal, 

 representing age-0 and adult fishes in October 1995 

 and March 1996, whereas only age-0 fish were col- 

 lected during July 1996 (Fig. 3). Age-0 and adult pol- 

 lock were always collected separately. 



The proportion of Pacific herring schools consist- 

 ing of a single size cohort varied among seasons. In 

 October 1995, 49.0% of the Pacific herring schools 

 sampled consisted of a single size class, mostly age 0. 

 In March 1996, 38.9% of the Pacific herring schools 

 sampled consisted of a single size, primarily 1-2 

 year olds. In July 1996, 83.3% of the Pacific herring 

 schools sampled consisted of a single size, primarily 

 1-2 year olds. 



Juvenile Pacific herring and walleye pollock were 

 contagiously distributed in the east-northeast and 

 the west-southwest of Prince William Sound (Fig. 4). 

 Adult Pacific herring aggregated in the southwest 



October 95; n = 7626 



15 



10 



5 







15 



10 



r^lll[lH^Tfp|T>TTTqmTmp^ 



March 96; n = 33655 



. ,ji|llv4MVn 



July 96; n = 22700 



15 



10 



5 



4- 



^y^^T"* I 



1 1 1— I r 



30 60 90 120 150 180 210 240 270 300 



Fork length (mm) 



Figure 2 



Percent size-frequency distributions of Pacific herring, Clupea 

 pallasi. (fork length mm I collected during three acoustic sur- 

 veys of Prince William Sound, Alaska, in 1995 and 1996 

 (n=count). 



(210°-240°) in October 1995, in the south (180°) and 

 east (90°) in March 1996, and west (240°-270°) in 

 July 1996 (Fig. 4). 



Pacific herring schools had the lowest densities in 

 March 1996 and the highest densities in July 1996 

 (Table 1). Walleye pollock schools had low densities 

 in October 1995 and March 1996 compared with very 

 high densities in July 1996 (Table 1). 



Pacific herring were deeper in the water column 

 in March 1996 (27.0-28.9 ml than in October 1995 

 (15.0-20.2 m) and July 1996 (14.1-16.7 m) (Fig, 5). 

 Walleye pollock were distributed near the bottom 

 during all three surveys (Fig. 5). 



The 26 locations within bays had a mean S3NSD 

 value of 3.8 km (SD=2.45), significantly smaller than 

 9.8 km (SD=6.69) for the 17 locations within pas- 

 sages and along the open coast (^=-3.61, df=41, 

 P<0.001). The water conditions within these bays 

 generally differed from conditions in passages and 

 open coast (Table 2). The surface water temperatures 

 (0-30 m) were cooler inside than outside the bays by 

 0.32°C and 0.20°C in October 1995 and July 1996, 

 respectively. The surface water inside the bays was 

 also less saline in October 95 but was similar inside 



