FISHERY BULLETIN: VOL. 87, NO. 3, 1989 



necessary to assess the influence of marginal 

 feeding conditions on growth or assimilation effi- 

 ciency. Although we did not monitor feeding in- 

 cidence and survival closely, both were higher in 

 the high ration treatments. Consequently, in- 

 cluding nonfeeding larvae in our study probably 

 would have made differences appear in growth 

 and ingestion rates between ration levels. Paul 

 (1983) found fairly high incidences of pollock 

 larvae feeding at low densities on copepod 

 nauplii; however, the extreme smallness (250- 

 1,000 mL) of containers used in that study prob- 

 ably invahdates the results. 



According to Nishiyama and Hirano's (1985) 

 formula for estimating mean gut content as a 

 percent of wet body weight, guts of field-caught 

 larvae 6 mm in total length (TL) should contain 

 about 2% body weight. By contrast, larvae in our 

 experiments contained 10-12% dry body weight. 

 These results would indicate that either larvae in 

 the field are not consuming prey at maximum 

 rates, or there is a problem with collecting lar- 

 vae from the field. Pollock larvae could be def- 

 ecating when captured with nets; however, we 

 observed that walleye pollock larvae did not 

 defecate when probed with a dissecting needle, 

 in contrast to anchovy larvae (Yamashita in 

 press). 



We can approximate mean caloric consumption 

 of pollock larvae caught in the Bering Sea from 

 the data of Dagg et al. (1984), who estimated 

 that at an average temperature of 4.5°C, larvae 

 5.2 mm in length ingest 18.3 copepod nauplii/d. 

 The mean length of copepod nauplii eaten by 

 pollock larvae (5.2 mm TL) is 0.22 mm, as esti- 

 mated from equations given by Nishiyama et al. 

 (1986), and an equivalent wet weight is 1.38 jjig 

 (Nishiyama and Hirano 1985). Assuming 70% 

 water content (Ikeda 1970) and the caloric con- 

 tent of adult Pseudocalanus (Laurence 1976), 

 the mean caloric content of the average naupliar 

 prey would be 0.0021 calories. Daily ingestion of 

 larvae in the study of Dagg et al. (1984) would be 

 0.038 calories. Assuming 50% assimilation effi- 

 ciency, 0.019 calories are available for metabo- 

 lism and growth. This value, however, does not 

 meet even the daily caloric requirement of 0.023 

 calories for metabolism alone, at 4.5°C (con- 

 verted from 0.027 calories for metabolism of 13 d 

 old larvae, 5.1-5.2 mm SL, at 6.2°C with an 

 assumption of Qio = 2.3 from Brett and Groves 

 1979). A daily caloric ingestion of about 0.16 

 calories is required for growth and metabolism 

 for this size of larva from the results of our 

 study. This value would be equivalent to 76 



nauplii. Of course, prey size and metabolizable 

 energy content may vary significantly. 



The mean number of copepod nauplii at the 

 depth of their maximum abundance in the Bering 

 Sea during normal first-feeding of pollock larvae 

 is 10-20/L (Clarke 1984; Dagg et al. 1984). These 

 values are low compared with previously re- 

 ported ranges of nauphar densities in the sea 

 (e.g., Houde 1978; Hunter 1981). We believe 

 that the low prey density, low percentage of gut 

 contents to body weight of field-caught larvae, 

 and the energetic requirements of larvae com- 

 pared with estimated ingestion rates from field 

 studies indicate that pollock larvae, like anchovy 

 (Lasker 1975), are probably subject to food 

 shortages in the sea. Since the growth response 

 of walleye pollock larvae (at the low tempera- 

 tures used in this study and in the sea) is low, 

 one would not expect to see periodic episodes of 

 low ration expressed markedly in mean larval 

 growth rates (Yoklavich and Bailey 1989), but 

 episodes of low ration would be better assessed 

 on an individual basis, using chemical or histo- 

 logical methods. 



ACKNOWLEDGMENTS 



We thank T. Sibley, A. Kendall, G. Stauffer, 

 and R. Francis for facilitating the visit of Y. 

 Yamashita to Seattle. We thank M. Yoklavich, 

 A. Kendall, G. Theilacker, and H. Mulligan for 

 reviewing the manuscript and N. Merati, M. 

 Yoklavich, and N. Navaluna for help in larval 

 rearing activities. Much more than an acknowl- 

 edgment is owed to Dr. Reuben Lasker for his 

 influence on the authors. His early work on 

 energetics introduced us to this field; his later 

 work showed the importance of energetics to 

 understanding recruitment processes. Beyond 

 energetics, his enthusiasm for progress was in- 

 spirational. 



LITERATURE CITED 



Bailey, K. M., and C. L. Stehr. 



1986. Laboratory study on the early life history of the 

 walleye pollock, Theragra chakogramma (Pallas). J. 

 exp. mar. Biol. Ecol, 99:233-246. 



1988. The effects of feeding periodicity and ration on the 

 rate of increment formation in otoliths of larval 

 walleye pollock Theragra chalcogramma. J. exp. 

 mar. isiol. Ecol. 122:147-161. 

 Bakkala, R. G., T. Maeda, and G. McFarlane. 



1986. Distribution and stock structure of pollock 

 (Theragra chalcogramma) in the North Pacific 

 Ocean. Int. North Pac. Fish. Comm. Bull. 45:3-20. 



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