Stokesbury et a\ Spatial distributions of Clupea pallasi and Themgra chalcogramma 



401 



their distributions varied season- 

 ally in relation to the stage of 

 life of fish, school structure, and 

 coastal structure. We then exam- 

 ined the degree of overlap between 

 juvenile Pacific herring and 

 juvenile walleye pollock distribu- 

 tions. Further we determined if 

 cohorts (age and size) of Pacific 

 heiTing were spatially segregated. 

 Adult Pacific herring and walleye 

 pollock distributions were also 

 observed; however, these fishes 

 appeared to emigi'ate from Prince 

 William Sound at certain times of 

 the year. 



Materials and methods 



148"W 



147W 



evN- 



60N 



Prince William Sound is a large body 

 of water separated from the Gulf of 

 Alaska by a series of mountainous 

 islands and deep passages ( Fig. 1 ). 

 The rocky coastline is highly iiTegu- 

 lar and has numerous islands, pas- 

 sages, bays, and deep Qords. The 

 Soimd had a semidiurnal tide with 

 a maximum range of 4.4 m during 

 the period of our study. 



Prince William Sound coastal 

 wateis were acoustically surveyed in October 1995, 

 and in March and July 1996 (Fig. 1). Five vessels 

 were used during each 10-day survey ( 12 hours per 

 day): three commercial seiners ( = 16.8 m) deployed 

 the acoustic and oceanographic equipment and fished 

 the seines; a trawler (Alaska Department of Fish and 

 Game RV Pandalus, =20 m) was used for midwater 

 sampling; and a tour vessel (=25 m) was used for pi'o- 

 cessing samples. Surveys were conducted during the 

 night (2000 to 0800 h) in October 1995 and March 

 1996, and durnig the day ( 0800 to 2000 h ) m July 1996. 

 Northern latitude conditions dictated this sampling 

 design because there was little daylight in March and 

 little darkness in July. Further, the summer cruises 

 had to be coordinated with aerial surveys for forage 

 fish that could only be flown during daylight (E. Brown, 

 unpubl. data). We observed and collected both Pacific 

 herring and walleye pollock in surface waters and near 

 the sea floor, a range of area that coincides with the 

 known diel distributions for these two species. 



The acoustic vessel followed a zigzag transect pat- 

 tern along the shore (MacLennan and Simmonds, 

 1992; Gunderson, 1993) to a distance of =1 km at a 

 speed of 14 to 17 km/h. We attempted to survey the 



10 km 



Figure 1 



Location of the October 1995, March 1996. and .July 1996 acoustic survey tran- 

 sects in Prince William Sound, Alaska. The dark areas represent the locations 

 within which we conducted our zigzag transect survey. 



same bays, passages, and open coast during all three 

 sui'veys. The vessel's sonar (50 kHz. 46) was used 

 to locate fish schools along these transects. When 

 a fish school was encountered, the acoustic vessel 

 slowed to 9 to 11 km/li and completed a series of 

 parallel transects perpendicular to shore by using a 

 120-kHz BioSonics 101 echosounder with a preampli- 

 fied dual-beam transducer (6x15"). The transducer 

 was mounted on a BioSonics 1.2-m BioFin and towed 

 at =1 m depth. The acoustic signals were processed 

 in real-time with BioSonics ESP 221 Echo square 

 integration software and ESP 281 Dual beam soft- 

 ware, and the raw signal was stored on digital audio 

 tape (Thome. 1983a; 1983b; MacLennan and Sim- 

 monds, 1992). The acoustic system was calibrated 

 before each cruise with a standard target and the 

 source level (SL) was 225.07 dB pPa. Acoustic noise 

 was well below our lowest density of interest. 



Fish schools observed with the acoustic equip- 

 ment were sampled to determine species composi- 

 tion and size structure. Fish were captured with a 

 modified l^ottom trawl in deep water (1.52 x 2.13 m 

 Nor 'Eastern Astoria V trawl doors, 21.3-m head rope, 

 29.0-m, foot rope, 3 x 20.0-m mouth, 10.2-cm mesh 



