Stokesbury et al Spatial (distributions of Clupea pallasi and Theragra chalcogramma 



405 



When larval Pacific herring meta- 

 morphose into juveniles, they develop 

 the ability to form and maintain schools 

 (Blaxter, 1985; Gallego and Heath, 

 1994). Schooling behavior is the juve- 

 nile herring's primary defense against 

 predation (Blaxter and Hunter, 1982). 

 School structure results from the dy- 

 namic interaction of several variables 

 causing opposing behaviors, primarily 

 the availability of food, the physical 

 size and condition of the fish, and the 

 threat of predation (for a review see 

 Pitcher, 1986). Newly recruited age-0 

 Pacific herring were tightly aggregated 

 in shallow water at the heads of bays 

 in July. These herring formed a few 

 dense schools of a single-size cohort 

 near the surface. This distribution pos- 

 sibly resulted from a high threat of pre- 

 dation ( Stokesbury et al. M, coupled with 

 an abundance of food as indicated by 

 the fullness of herring stomachs (Foy 

 and Norcross, 1999) and from zooplank- 

 ton samples (Foy-). 



Age-0 Pacific herring were aggregated 

 within bays near the surface, in Octo- 

 ber. However, the number of schools 

 increased, schools became less cohesive, 

 and size cohorts began to mix in con- 

 trast to the highly aggregated unimodal 

 schools observed in July. The distribution 

 observed in October may have resulted 

 from a decrease in food abundance, a 

 decrease in the threat of predation, or 

 both (Foy and Norcross, 1999, Stokesbury et al.^). 



Age-0 Pacific herring school structure and distri- 

 bution completely changed in March. These herring 

 had just survived the winter when prey abundance is 

 minimal and the risk of starvation is great (Paul and 

 Paul, 1998; Paul et al., 1998; Foy and Paul, 1999), 

 Herring moved away from the shores into deeper 

 water and spread out forming sparse (<1 fish/m-^) 

 shoals of mixed-size cohorts. Distance between fish 

 within schools increases with hunger, reducing cohe- 

 sion, and causing lower mean densities (Pitcher and 

 Partridge, 1979; Robinson, 1995). This independent 

 segregating behavior reduces competition for food 

 and increases the chance of encountering food by 



October 1995 



80 

 60 

 40 -I 

 20 

 



-P^ 



J 



Dage tiernng 

 Hage 1+ tiernng 

  adult tiernng 

 D pollock 



Marcti 1996 



80 -I 

 60 

 40 - 

 20 

 



rw|W|#i , [i Wrt m, 



July 1996 



80 -I 

 60 

 40 

 20 

 



ry-fai 



L*- 



30 60 90 120 150 180 210 240 270 300 330 

 Nortti East Soutti West 



Degrees 



Figure 4 



Angle percent-frequency distributions of Pacific herring, Clupea pallasi. 

 and walleye pollock, Theragra chalcogramma. observed during three acous- 

 tic surveys of Prince William Sound, Alaska, in 1995 and 1996. 



' Stokesbury, K.D.E., J. Kirsch, and B.L. Norcross. 1999. Mor- 

 tality estimates of juvenile Pacific herring (Clupea pallasi) in 

 Prince William Sound, Alaska. Unpubl. manuscript. 



- Foy, W. 1996-1998. Unpubl. data. Institute of Marine Sci- 

 ence, Industrial Technology Center, University of Alaska, Fair- 

 banks, 118 Trident Way, Kodiak, AK 99614-7401. 



