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



lightrlO h dark cycle, illuminated the tanks at an 

 intensity of 5-8 fjiE/s/m^ just below the water 

 surface. Water was replaced with filtered sea- 

 water at 10-15% per day. Larvae were fed roti- 

 fers, Brachionus plicatilis, that had been cul- 

 tured on a mixture of Isochrysis spp., Chaeto- 

 ceros spp. , and yeast. 



Three rearing experiments were done using 

 larvae hatched from eggs collected on different 

 days. In the first experiment, gi'owth, rate of 

 gastric evacuation, and daily ration were 

 measured in larvae reared at high (11.2 ± 1.4 SD 

 rotifers/mL) and low (2.0 ± 1.4 SD rotifers/mL) 

 rations. Mean temperature in both tanks was 

 6.4°C (±0.2 SD). In the second and third experi- 

 ments, oxygen consumption rates for larvae 

 were determined. Rotifer densities were main- 

 tained at about 10 individuals/mL. Average tem- 

 perature was 6.3° ± O.rC (SD). Temperatures 

 of 6.0°-6.5°C are common in Shehkof Strait (Gulf 

 of Alaska) in May when early-stage larval 

 walleye pollock are most abundant (Kendall et 

 al. 1987). 



Growth Rates 



The standard lengths (SL) of Hve larvae were 

 measured to the nearest 0.03 mm using a dissect- 

 ing microscope, the gut contents were removed, 

 and the larvae were dipped in distilled water and 

 then dried for 24 hours at 60°C. Dry weights 

 were measured to the nearest 0. 1 |jLg on a Cahn 

 25 electrobalance. Yolk-sac dry weight was esti- 

 mated as the difference between the mean dry 

 weight of larvae with yolk sacs and the mean dry 

 weight of larvae whose yolk sacs had been 

 excised. Instantaneous rate of growth (G) and 

 relative rate of growth (K) were estimated from 

 the following equations (Ricker 1975): 



Wt = Wo e^', where 



G = (In Wt - In Wo)/t, and 



W, = Wod + K)', where 



K = e^- 1. 



Wo is the initial dry weight (in \xg) and Wt is the 

 dry weight at time t (in days). Daily specific 

 growth rate is defined as G ( x 100%) (Laurence 

 1975). 



In order to compare subarctic walleye pollock 

 with the subtropical larvae used in other studies, 

 we used methods and analyses to parallel those 



of Houde and Schekter (1981, 1983) and Thei- 

 lacker (1987). One major difference was that we 

 did not use nonfeeding larvae, but sampled ran- 

 domly from feeding larvae. Theilacker (1987) 

 used all randomly sampled larvae including non- 

 feeding larvae to estimate ingestion and growth; 

 Houde and Schekter (1981, 1983) sampled only 

 feeding larvae to measure ingestion, and feeding 

 and nonfeeding larvae to measure growth. In 

 some species, relatively few larvae may feed on 

 cultured prey at low densities in laboratory con- 

 ditions. This may not have been a problem in the 

 Theilacker (1987) and Houde and Schekter (1981, 



1983) studies. In our studies we considered feed- 

 ing incidence as a behavioral problem not to be 

 included as a factor in a study of energetic effi- 

 ciency. Since our objective was to compare effi- 

 ciencies of larvae feeding at high and low rations, 

 we opted to exclude nonfeeding larvae from our 

 samples. 



Evacuation Rates 



Instantaneous rates of gastric evacuation 

 were measured for actively feeding larvae. 

 Larvae were fed rotifers, dyed with Alcian Blue, 

 at prey densities of 2.2-4. 0/mL. After 1.2-1.5 

 hours larvae were transferred to a tank contain- 

 ing undyed rotifers either at 11.1-15.6 roti- 

 fers/mL for the high ration treatment or at 

 0.8-1.8 rotiffrs/mL for the low ration treatment. 

 Larvae were sampled at intervals from 20 to 60 

 minutes, and widths of rotifers and their degree 

 of digestion determined. Mean gut-clearance 

 times were estimated for the duration between 

 ingestion and defecation of dyed rotifers. 



Theilacker and Kimball's (1984) method was 

 used to determine that 54 %> of rotifer dry weight 

 was lost after 4 hours of digestion. Based on that 

 loss, three correction factors for the degi-ee of 

 rotifer digestion were used to calculate inges- 

 tion: 0.9 for recently ingested rotifers; 0.7 for 

 moderately digested rotifers, which still had 

 abundant chlorophyll from ingested phytoplank- 

 ton; and 0.5 for well-digested rotifers with al- 

 most no chlorophyll. The total dry weight of 

 dyed rotifers in the gut was determined by sum- 

 ming the product of the width-specific dry 

 weight for each rotifer (Theilacker and Kimball 



1984) and the appropriate digestion factor. Data 

 were fitted to the model: 



At = Ao €-''■' 



where Aq and At are the ratios of the dry weight 



526 



