Sturdevant et al.: Feeding habits, prey fields, and potential competition of Theragra cha/cogrammci and Clupea pallasi 
483 
148° W 147° W 
Sampling regions and stations for YOY walleye pollock and Pacific herring diet sam- 
ples collected in Prince William Sound, Alaska. Circles: July-August 1995; squares: 
October 1995; triangles: November 1994. 
1997), but researchers believe that long-term changes be- 
gan with a climate regime shift prior to the oil spill (Bailey 
et al., 1995; Piatt and Anderson, 1996; Anderson and Pi- 
att, 1999). For example, the number of walleye pollock and 
other demersal fish increased at the same time that the 
other taxa decreased (Anderson and Piatt, 1999), the PWS 
spawning population of Pacific herring declined by 75% by 
1993 (Brown et al., 1996b), and fish biomass of PWS de- 
creased by 50% (Piatt and Anderson, 1996). Concomitant 
decreases in populations of marine birds and mammals in 
PWS may be related to these shifts in the composition and 
abundance of forage fish prey (Oakley and Kuletz, 1996; 
Piatt and Anderson, 1996; Iverson et al., 1997). Apparent- 
ly, fewer high-quality forage fish have been available, and 
the species composition has changed to one in which the 
predominant fish taxa are less energetically valuable to 
marine piscivores (Piatt and Anderson, 1996; Anthony and 
Roby, 1997; Anderson and Piatt, 1999; Payne et al., 1999). 
The potential for such shifts to cascade throughout ma- 
rine food webs (Livingston, 1993; Springer and Speckman, 
1997) is an important reason to understand the trophic in- 
teractions of forage fish. 
This reports stems from the Alaska Predator Ecosystem 
Experiment (APEX), a multidisciplinary study that at- 
tempted to link current knowledge about the forage fish of 
PWS with their seabird predator populations. We describe 
differences in the feeding habits of YOY walleye pollock 
and YOY Pacific herring caught in summer and autumn in 
PWS and compare feeding attributes of fish caught in al- 
lopatric (single species) and sympatric (co-occurring, mul- 
tispecies) aggregations in autumn to support the hypoth- 
esis that the presence of potential competitors may induce 
changes in feeding habits. We compare fish size, zooplank- 
ton prey fields, fish feeding habits, prey selection, food 
quantity, and diet overlap of these species. 
Materials and methods 
Field methods 
Fish stomach and zooplankton samples were collected 
during APEX forage fish population surveys in central, 
northeastern, and southwestern PWS 3,4 5 (Fig. 1). In a pilot 
study in 1994, we sampled from 7 to 13 November aboard 
the Alaska Department of Fish and Game RV Medeia\ 
in 1995, we sampled from 20 July to 12 August aboard 
the charter FV Caravel le, and from 5 to 14 October 
aboard the RV Medeia. Surveys were conducted offshore 
along a grid of parallel transects spaced at two-mile 
