Trophic Relationship of Age-0 

 and Age-1 Walleye Pollock 

 Theragra chalcogramma Collected 

 Together in the Eastern Bering Sea 



Jill J. Grover 



College of Oceanography, Oregon State University 

 Hatfield Marine Science Center, Newport, Oregon 97365 



Walleye pollock Theragra chalco- 

 gramma a gadid endemic to the 

 north Pacific, is one of the most im- 

 portant components of the Bering 

 Sea food web (Smith 1981) and sup- 

 ports the largest single-species com- 

 mercial fishery in the world (Me- 

 grey 1989). Although a number of 

 studies have documented the food 

 habits of larval (e.g., Kamba 1977, 

 Cooney et al. 1980, Clarke 1978 and 

 1984, Kendall et al. 1987, Grover 

 1990), juvenile (e.g., Kamba 1977, 

 Bailey and Dunn 1979, Cooney et al. 

 1980, Lee 1985, Grover 1990), and 

 adult walleye pollock (e.g., Takaha- 

 shi and Yamaguchi 1971, Bailey and 

 Dunn 1979, Livingston et al. 1986, 

 Dwyer et al. 1987, Bailey 1989), 

 previous studies have generally 

 pooled samples either over time or 

 space. While pooled data adequate- 

 ly characterize large-scale patterns 

 of prey utilization, small-scale pat- 

 terns of between-year-class trophic 

 relationships are best defined by 

 fish that were collected together. 

 This study examined the trophic 

 relationship of age-0 and age-1 pol- 

 lock that were collected in a single 

 haul in the southeastern Bering 

 Sea. As pollock are known to be 

 highly cannibalistic (Takahashi and 

 Yamaguchi 1971, Bailey and Dunn 

 1979, Livingston et al. 1986, Dwyer 

 et al. 1987, Bailey 1989), this study 

 looked for evidence of cannibalism, 



•Contribution FOCI-0106 to Fisheries- 

 Oceanography Coordinated Investigations, 

 NOAA. 



as well as defined the size and taxa 

 of prey that were ingested by the 

 two year-classes. 



Methods 



Juvenile pollock were collected as 

 one aspect of a survey of groundfish 

 in the eastern Bering Sea in 1985 

 by the Morning Star, using a Mari- 

 novich midwater trawl (Walters et 

 al. 1988). An examination of length- 

 frequency distributions from these 

 trawls, which principally targeted 

 age-0 fish, revealed that age-0 

 (zeros) and age-1 fish (ones) were 

 rarely collected together (Jim Tray- 

 nor, NMFS Alaska Fish. Sci. Cent., 

 Seattle, unpubl. data). Among sta- 

 tions where zeros and ones occurred 

 in the same haul, comparable num- 

 bers of both year-classes were ob- 

 tained at only one station. Coin- 

 cidentally, although not all of the 

 ones were saved, this was the only 

 station where both ones and zeros 

 were preserved. This collection was 

 made from 0900 to 0920 hours on 

 3 August 1985, at station 95 (56° 

 11.17'N, 162° 18.36' W), in the south- 

 east shelf region. Gear depth aver- 

 aged 20 m, and water temperature 

 was 6.5°C. The fish were frozen at 

 sea. In the laboratory, they were 

 defrosted in 10% formalin to facil- 

 itate the simultaneous separation 

 and fixation of the specimens. All 

 the preserved specimens from this 

 collection were analyzed. 



This study examined the diet of 

 52 age-0 and 22 age-1 fish. Age 



groups were determined based 

 on length, using the categories of 

 Dwyer et al. (1987)-age 0, 1-130 

 mm; age 1, 131-220 mm, which 

 were based on Smith's (1981) Ber- 

 ing Sea data. For each fish, total 

 length was recorded and the stom- 

 ach was removed. Stomach con- 

 tents were dissected out and iden- 

 tified. Due to prey condition, only 

 copepods could be consistently iden- 

 tified to species. Extremely well- 

 digested copepods were identified 

 largely by their size and shape. No 

 distinction was made between cope- 

 podite and adult stages. Copepods 

 eggs, Calanus marshallae, Pseudo- 

 calanus sp., Centropages abdomina- 

 lis, Metridia sp., Acartia sp., and 

 Tortanus discaudatus were identi- 

 fied. Calanus marshallae and Me- 

 tridia sp. were >2mm in length. All 

 other copepod species were <2mm. 

 Other prey were identified as am- 

 phipods, euphausiid furcilia/juve- 

 niles, mysids, decapod larvae, and 

 fish scales. Well-digested euphau- 

 siids were enumerated based on eye 

 counts. 



Food particle-size selection was 

 examined by measuring widths of 

 all prey items that were not severe- 

 ly digested or broken. Widths were 

 recorded from a total of 7291 prey 

 items. 



Diet was analyzed in terms of nu- 

 merical percent composition (%N), 

 volumetric percent composition 

 (%VOL), frequency of occurrence 

 (%FO), and the index of relative im- 

 portance (IRI = [%N + % VOL] x %FO) 

 (Pinkas et al. 1971). Volumes were 

 calculated from prey dimensions for 

 zeros (Grover and Olla 1987), and 

 were measured directly for ones. 



Results 



Little overlap was seen in the size 

 of prey that were ingested by age-0 

 pollock, which ranged from 26 to 64 

 mm TL (x 49.4mm, SD 9.6), and 



Manuscript accepted 24 June 1991. 

 Fishery Bulletin, U.S. 89:719-722 (1991). 



719 



