NOTE Graver: Between-year-class trophic relationship of Theragra chalcogramma in southeastern Bering Sea 



721 



tion of the diet for both age groups, however age-0 fish 

 primarily ingested small (<2mm in length) Pseudo- 

 calanus sp., while age-1 fish primarily ingested larger 

 (>3mm) Calanns marshallae (Table 1). 



Fish scales were observed in the stomachs of six 

 age-1 fish. Only one scale was found in each stomach, 

 and no other evidence of piscivory was observed. The 

 scales were most likely ingested incidentally as a result 

 of the collection process. 



The mean number of prey ingested by age-0 fish was 

 134.8 (90.6 excluding copepod eggs), while age-1 fish 

 ingested an average of 156.3 prey items. The incidence 

 of feeding was 100% for both age groups. 



Discussion 



Based on two age and growth relationships (Nishimura 

 and Yamada 1984, Yoklavich and Bailey 1989) and the 

 timing of spawning in the southeast shelf region of the 

 Bering Sea in 1985 (Mulligan et al. 1989), the age of 

 age-0 pollock in this study was estimated to be 2-4 

 months. Assuming that the age-1 fish in this study were 

 spawned as well as collected in the southeast shelf 

 region, and then based on when spawning occurred in 

 this region in 1984 (Hinckley 1987), the age of age-1 

 pollock was estimated to be 13-16 months. 



At the time and place of this collection, diets of the 

 two year-classes were divergent in terms of both prey 

 size and taxa. Older, larger fish ingested larger prey. 

 The importance of small copepods, notably Pseudo- 

 calanus sp., in the diet of age-0 pollock is consistent 

 with previous studies (Kamba 1977, Cooney et al. 1980, 

 Lee 1985, Grover 1990); however, one suborder of 

 copepods, Cyclopoida, that was included in earlier 

 dietary accounts was noticeably absent in the present 

 study. Although, by most accounts, cyclopoids were not 

 primary prey, Lee (1985) reported that for pollock 

 between 20.0 and 24.9mm TL and 45.0 and 49.9mm 

 TL, these copepods were either the foremost or second- 

 most important prey in the southeastern Bering Sea. 

 Differences between studies may reflect temporal or 

 spatial patchiness of prey. 



Large numbers of copepod eggs have previously been 

 reported in the diet of juvenile pollock (Cooney et al. 

 1980, Lee 1985, Grover 1990). The eggs were probably 

 ingested incidentally, as the dominant copepod in the 

 diet of age-0 pollock, Pseudocalanus sp., carries its eggs 

 (Corkett and McLaren 1978). 



The diet of age-1 pollock was dominated by a large 

 copepod, C. marshallae, and euphausiid furcilia and 

 juveniles. While previous dietary accounts have also 

 illustrated the importance of large copepods, other 

 specifics of diet apparently vary seasonally and spatial- 

 ly. Bailey and Dunn (1979) found that in the eastern 



Bering Sea, amphipods replaced euphausiids in the diet 

 of pollock <24cm in the summer of 1974. Dwyer et al. 

 (1987) reported that copepods were the only prey in- 

 gested by pollock <30cm in the southeastern Bering 

 Sea during the summer, although in the northwestern 

 Bering Sea euphausiids and larvaceans were also a part 

 of the summer diet. The absence of copepod eggs from 

 the diet of age-1 fish is consistent with the fact that 

 C. marshallae females broadcast rather than carry eggs 

 (Peterson 1980). 



Another trophic relationship reported for these two 

 year-classes is cannibalism (Livingston et al. 1986, 

 Dwyer et al. 1987). While adult pollock cannibalize 

 age-0 pollock to some extent during all seasons in the 

 eastern and southeastern Bering Sea, age-1 pollock 

 (<30cm) have been observed to be cannibalistic only 

 during autumn (Livingston et al. 1986, Dwyer et al. 

 1987). In the present study, based on a collection in 

 summer, no conclusive evidence of piscivory or can- 

 nibalism was observed. Cannibalism has been shown 

 to be highest when the vertical distribution patterns 

 of juveniles and adults overlap (Bailey 1989). As this 

 collection was not vertically discrete, the extent to 

 which vertical distributions of age-0 and age-1 pollock 

 overlapped at station 95 is unclear. However, based on 

 the short duration and nature of this collection, i.e., 

 that it targeted a hydroacoustic trace at a specific depth 

 (Jim Traynor, NMFS Alaska Fish. Sci. Cent., Seattle, 

 pers. commun. May 1991), then, minimally, vertical 

 juxtaposition of the two year-classes is suggested. In 

 which case, the present data indicate that when ap- 

 propriate alternate prey are available, the juxtaposi- 

 tion of two juvenile year-classes can occur without can- 

 nibalism resulting. At station 95 in the eastern Bering 

 Sea in August 1985, age-0 and age-1 pollock had a 

 trophically neutral relationship: the food habits of each 

 year-class did not impinge upon nor imperil the other. 



Acknowledgments 



I would like to thank Bori Olla for his comments on 

 several drafts of this manuscript, Art Kendall for his 

 interest in this project, and AFC personnel who col- 

 lected and provided the specimens. 



This work was supported by the Alaska Fisheries 

 Science Center, National Marine Fisheries Service, 

 NOAA Contract Nos. NA-85-ABH-00025 and NA-89- 

 ABH-00039. 



