wastes can be estimated by difference. Also, the 

 efficiencies we used— G//, Mil, {G + M)II, and 

 GI{G + M) — are not dependent on excretion 

 measurements. 



Average daily ingestion in dry weight was cal- 

 culated as (mean larval weight) x (weight- 

 specific daily ingestion). Dry weight of rotifers 

 was converted to caloric equivalents using a fac- 

 tor of 4.4 cal/mg (Theilacker and Kimball 1984). 



Daily growth of larvae at (<,) was calculated 

 from the daily growth rate (K) for the interval U 

 - t, + i. Dry weight was converted to calories 

 using a factor of 5.077 cal/mg estimated for larval 

 walleye pollock (Fukuchi 1976; Harris et al. 

 1986). Fukuchi (1976) also provided constants for 

 converting dry weight to net weight, nitrogen, 

 and carbon (8.59, 0.092, and 0.131, respectively). 



Daily O2 consumption rate (M24) |xL/d/individ- 

 ual (14 h light and 10 h dark photoperiod) was 

 calculated as 



M24 = 2 X {M,o) X 14 + (Mfn) X 10. 



Oxygen consumption was converted to calories 

 using an oxycaloric equivalent of 4.63 cal/mL O2 

 (Brett and Groves 1979). We attempted to 

 measure active metabolic rates as described 

 above, but larvae did not actively feed. There- 

 fore, active metabolic rate was approximated as 

 twice the routine metabolic rate (Houde and 

 Schekter 1983; see also Lasker 1970). 



RESULTS 

 Growth rates 



Hatching began 7-8 days after fertilization, 

 and by 3-5 days later more than 80% of the eggs 

 had hatched. The day of 50% hatching was desig- 

 nated as the day of hatching for all larvae. 



By day 6 after hatching, yolk-sac dry weight 

 was less than 5% of the initial weight; the yolk 

 was absorbed completely by days 11-15. A small 

 percentage of larvae started feeding on day 5 

 and most larvae started on day 6. Therefore, day 

 6 was designated as the onset of feeding. Newly 

 hatched yolk-sac larvae averaged 3.80 mm (SL) 

 and 55.5 \i.g (dry weight), and by day 4 (existing 

 only on yolk) averaged 4.59 mm and 47.5 (xg. By 

 day 21, high ration larvae grew to 6.02 mm 

 (mean SL) and 120.9 |xg (mean dry weight) and 

 low ration larvae grew to 5.94 mm and 116.2 [ig. 

 The linear growth rate in SL during yolk-sac 

 period (from days to 4) was 0.20 mm/d; it de- 

 creased to 0.07 (high ration) and 0.06 (low ration) 

 mm/d during the transition from endogenous to 



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



exogenous energy (from days 4 to 16), and then 

 increased to 0.13 (high ration) and 0.12 (low ra- 

 tion) mm/d (from days 16 to 21). Specific growth 

 rate in weight from days 7 to 21 was estimated as 

 6.71%/d for high ration and 6.04%/d for low ra- 

 tion larvae. Standard length (SL) and dry weight 

 (W) were related by a power function: 



high ration: W 



low ration: W 



- 0.1261 X SL^*^^ 

 (r = 0.980 N = 45) 



0.1754 X SL^"*^ 

 (r = 0.979 N = 45). 



Although the length-specific weight gain of high 

 ration larvae tended to be greater than that of 

 low ration larvae, the difference between the 

 two food levels was not statistically significant 

 (i^-test for difference between two linear regres- 

 sion slopes, P > 0.1). 



Evacuation Rates 



The weight of dyed rotifers in larval guts de- 

 creased exponentially for the first 5-7 hours, 

 then the rate of larval digestion slowed. Instan- 

 taneous rates of gastric evacuation (R) were esti- 

 mated from the exponential phase for 8-9 d old 

 larvae and 17-21 d old larvae. Rates for younger 

 larvae were higher than those for older larvae, 

 and high ration larvae had higher evacuation 

 rates than low ration larvae (Table 1). The same 

 tendencies were found in gut clearance times. 

 Coiling of the midgut is initiated at about 5.2 mm 

 SL (day 14) and is completed at about 5.8 mm 

 (day 20), effecting the differentiation of foregut 

 and intestine. The relationship between R and 

 the percentage of larvae having coiled midgut 

 (>5.5 mm SL) showed higher evacuation rates 

 for larvae without the coiled gut. 



Ingestion Rate 



By our observation, larval walleye pollock are 

 continuous feeders. There was no significant dif- 

 ference (Chi-square test: P > 0.1) between 

 widths of rotifers in larval guts and widths of 

 rotifers in cultures provided to larvae as food. 



The ratio of gut contents to larval dry weight 

 generally increased asymptotically with time 

 (Fig. 1). Maximum mean percent of gut contents 

 measured after 14 h feeding were 6.9% and 5.1% 

 for 7 d old high ration and low ration larvae, 

 respectively. These values increased to about 

 10% by day 13 and thereafter remained fairly 



528 



