YAMASHITA and BAILEY: BIOENERGETICS OF LARVAL WALLEYE POLLOCK 



declined to 29-32% at day 21 as a result of the 

 longer gut clearance times discussed previously. 

 The inverted U-shaped function observed here 

 was also noted by Houde and Schekter (1981) 

 while studying three species of subtropical 

 larvae. Daily rations for warm-water species are 

 considerably higher than those found in this 

 study, ranging fi-om 202 to 379%, 165 to 297%, 

 and 121 to 234% per day on a caloric basis for bay 

 anchovy, Anchoa mitchilli, hned sole, Achirus 

 lineatus, and sea bream, Archosargus rhom- 

 boidalis, respectively (Houde and Schekter 

 1983); values of 26-70% were found for northern 

 anchovy converted to a caloric basis from data in 

 Theilacker (1987); and values of 42-160% on a 

 dry weight basis for summer flounder, Para- 

 lichthys dentatus, (Buckley and Dillmann 1982). 



Our values of routine metabohsm at 6.2°C in 

 Table 4 correspond quite closely to values for 

 walleye pollock and cod found by other investiga- 

 tors. For example, our values of 2.24 and 2.06 

 |xL/h/mg for 11 and 14 d old larvae are similar to 

 values of 1.86 and 2.14 for 11 and 14 d old pollock 

 larvae at 4°C found by Clarke (1984). Adopting a 

 Qio value of 2.3 (Brett and Groves 1979), 

 Clarke's values are equivalent to 2.23 and 2.57 

 |xL/h/mg at 6.2°C. Routine metabolic rates for 

 young cod larvae at 5°C have been measured at 

 1.6 (Davenport and Lonning 1980) and at 1.8- 

 2.0 |xL/h/mg (Solberg and Tilseth 1984). 



We attempted to partition metabolism into its 

 component parts for estimating daily metabolic 

 cost. From the equation for total daily metabo- 

 lism, the four components — SDA, hghts-on gen- 

 erated nonfeeding activity, resting metabohsm, 

 and feeding activity — accounted for 13.3, 11.1, 

 45.7, and 29.9% of the total daily metabohc ex- 

 penditure. Because of the experimental nature 

 of these measurements, they should be con- 

 sidered a first approximation, subject to refine- 

 ment. For example, degradation of rotifers that 

 were defecated in the DO bottles could have 

 consumed some of the available oxygen and 

 should be controlled for in future studies. Our 

 values for resting metabolism may include some 

 cost for biosynthesis because a 24 h period of 

 nonfeeding acclimation time is probably not 

 enough to eliminate the effect of SDA (Brett and 

 Groves 1979). The value for active feeding 

 metabolism seems high compared with the rela- 

 tively inactive behavior of walleye pollock 

 larvae. The assumption that active metabohc 

 rate is twice the routine metabolic rate may have 

 resulted in an overestimate of this component. 



Net assimilation efficiency [(G + M)/I], ranged 



fi-om 24 to 64% in our study, as a U-shaped func- 

 tion related to age. These efficiencies are low 

 compared with generalized rates of 65-75% for 

 young fish given by Ware (1975) and 73% for 

 young carnivorous fish given by Brett and 

 Groves (1979). However, the assimilation rate 

 during larval life seems to change gi-eatly during 

 development, and rates are usually quite low for 

 young larvae. For example, net assimilation effi- 

 ciency for northern anchovy changed with in- 

 creasing larval size from 44.4 to 65.7% for well- 

 fed larvae (Theilacker 1987). Net assimilation 

 efficiency for bay anchovy, hned sole, and sea 

 bream ranged from 17.2 to 33.7%, 26.6 to 46.1%, 

 and 37.2 to 67.6%, respectively, for different 

 developmental stages of these fishes (Houde and 

 Schekter 1983). 



High assimilation efficiency during the first 

 few days of feeding may be due partly to residual 

 yolk contributing to "ingestion". Yolk is con- 

 verted into body tissue very efficiently (Lasker 

 1962). Assimilation efficiency decreased to a low 

 point at day 13 of our experiments. We suggest 

 that development of the digestive system lagged 

 behind that of behavioral feeding prowess, and 

 that low assimilation efficiencies were linked to 

 the growth lag observed during the transition 

 from endogenous to exogenous food. Assimila- 

 tion efficiency and growth rate increased when 

 ingestion reached a maximum and the ahmen- 

 tary canal developed midgut coiling, resulting in 

 longer gut clearance time. 



Gross growth efficiency (G//) ranged from 11 

 to 34% as a U-shaped function of age (and size). 

 Houde and Schekter (1983) reported similar 

 U-shaped functions with size ranging from 10.9 

 to 20.8% for bay anchovy, from 12.8 to 23.3% 

 for lined sole, and from 21.4 to 41.3% for sea 

 bream. Most values for larvae are suggested to 

 be in the 5-40% range (Houde and Schekter 

 1983) or 14-41% range (Theilacker and Dorsey 

 1980). The efficiencies of pollock larvae are con- 

 sistent with these ranges. Our gi'oss gi'owth ef- 

 ficiencies are lower than those of 30-47% found 

 by Theilacker (1987) for well-fed northern 

 anchovy larvae. 



Growth rates, ingestion rates, assimilation 

 efficiencies, and growth efficiencies determined 

 from this study differed surprizingly little be- 

 tween high and low rations. These results indi- 

 cate that larvae robust enough to successfully 

 initiate feeding at low prey densities were able 

 to maintain high rations, and furthermore that 

 growth responded very little to increased prey 

 density. Lower levels of ration used here may be 



533 



