FISHERY BULLETIN: VOL. 70, NO. 2 



Scale m kilometers 



Figure 1. — Sampling areas of Iliamna Lake. 



The following species of zooplankton occur 

 commonly in the limnetic area of the Lake: Di- 

 aptomus gracilis, Erytemora yukonensis, Cy- 

 clops scutifer, Bosmina coregoni, Daphnia long- 

 iremis, and Holopedium gibherum (Lenarz, 

 1966). 



Juvenile sockeye salmon and zooplankton ex- 

 hibit a similar dial vertical migration. Echo- 

 grams have shown a movement toward the sur- 

 face at dusk, a concentration usually at less than 

 10 m at night and a downward movement of fish 

 at dawn. Pella (1968) described a similar diel 

 vertical migration for juvenile sockeye salmon 

 in Lake Aleknagik, Alaska. About half of the 

 zooplankton population occurs above 15 m at 

 midnight and above 50 m at noon in Iliamna 

 Lake (Fowler and Lenarz, 1965). 



MATERIALS AND METHODS 



Fry (age 0) and yearlings (age I) were col- 

 lected with a tow net, having an opening of 

 2.7 m' and a cod end mesh size of 0.2 cm, and 

 suspended between two boats spaced 15 m apart 

 with 30.5 m of tow line. Each tow was 20 min 

 in duration at a speed of about 1.5 m/sec. Al- 

 ternate tows were made at 1.5 and 6 m, and occa- 

 sionally deeper tows were made when fish were 

 observed at greater depths on the echo sounder. 

 Most samples were taken during darkness (nor- 

 mally between 2100 and 0300 hr). During the 

 day catches were very small at depths of 30 m, 

 and major fish concentrations usually were not 



seen on the echo sounder. Fishing near the bot- 

 tom with monofilament, small-meshed gill nets 

 was tried without success. 



Fry and yearlings were preserved separately 

 in 10% Formalin within 15 min after capture. 

 No regurgitation of the stomach contents was 

 observed. The fish were measured and the 

 stomachs removed several months later. 



The stomach was divided at the major bend, 

 and only the contents of the foregut were exam- 

 ined so that bias from unequal digestive rates 

 among different food items would be minimized. 

 The foregut contents from all the fish in a sample 

 were combined and mixed with water of a known 

 volume. Two subsamples were taken with a 

 1-ml bulb pipette, and the organisms identified 

 and counted under a low-powered dissecting 

 microscope. Each organism was identified as 

 either Cyclops, Daphnia, Bosmina, Holopedium, 

 calanoid copepods, or insects. Nauplii and roti- 

 fers were seldom observed and were not counted. 

 The total number of each food item in the sam- 

 ple was estimated by multiplying the number of 

 each food item in the two subsamples by the 

 appropriate factor. The variance between sub- 

 samples was less than 1% of the total variance 

 among samples within date and area and was 

 ignored. 



The samples ranged from 1 to 45 fish but 

 usually contained between 15 and 25 fish. The 

 results from each sample were weighted by the 

 number of fish per sample and grouped by area 

 and sampling period. The sampling periods 

 were: /' 



1. Late summer 1966 (August 15-September 



10). 



2. Early summer 1967 (June 20-July 20) . 



3. Late summer 1967 (August 10-September 



5). 



The zooplankton sampling (described by 

 Lenarz, 1966) was undertaken for a separate 

 study and diff"ered spatially and temporally from 

 the young fish sampling. Samples were taken 

 with a conical, nylon net of No. 6 mesh attached 

 to a 0.5-m ring by vertical hauls, either from 

 100 m to the surface or from the bottom to the 

 surface if the depth was less than 100 m. The 

 zooplankters were identified and counted simi- 



356 



