stomach was three times as great at midday as at 

 midnight (fig. 10). The most intense feeding 

 period during this 24-hour period apparently was 

 during daylight hours and not during darkness. 



COMPARISONS OF GROWTH AND DIET 



By comparing the dramatically different 

 growth rates and sizes that pygmy whitefish at- 



100 



7b 



< _ 



O z 

 I- HJ 



en o 



o% 



w " 50 



2 </> 



=> K 



_I Z 



O UJ 



> K 



*s 



< o 25 



 insects 



r~l ZOO PLANKTON 

 I I OTHER 



JULY II, 1963 I SEPTEMBER 

 26, 1962 

 SOUTH BAY 



AUGUST 



14, 1962 



BROOKS 



LAKE 



AUGUST 19,1963 



GROSVENOR 

 LAKE 



Figure 9. — Percent of total volume of stomach contents 

 of age 0+ pygmy whitefish by major food categories. 



0.8 



z 

 o 



0.7 



0.6 



05 



0.4 



uj 0.3 



S 



02 



0.1 



0.0 



1800 2400 



AUGUST 24 



0600 1200 



AUGUST 25 



Figure 10. — Variation by time of day in mean volumes of 

 stomach contents of 63 pygmy whitefish caught at 6- 

 hour intervals from the same area in Brooks River, 

 August 24 and 25, 1963. 



tain in the Naknek system (figs. 2 and 6) with the 

 differences in diet, a strong positive correlation 

 between size of fish and utilization of insects be- 

 comes apparent. An analysis was made by first 

 grouping all contents from each sample into three 

 categories (insects, zooplankton, and other), then 

 grouping the samples according to the relative im- 

 portance of insects and zooplankton. Three rather 

 distinct groups of samples resulted: one with 

 heavy, one with moderate, and one with almost no 

 insect utilization. These grouped samples com- 

 pared with the largest known pygmy whitefish 

 from the same grouped areas (fig. 11) illustrate 

 the correlation between insect utilization and size. 

 Maximum size is not the only index, because the 

 general ranges of length frequencies from different 

 parts of the system (table 7) fall into the same 

 groupings. If this correlation is biologically 

 valid, it raises a question as to why Brooks Lake 

 pygmy whitefish do not eat insects. Merrell 

 (1964) has shown that other Brooks Lake fishes 

 utilize insects, which indicates their general avail- 

 ability. No comparative data are available, how- 

 ever, on differences in insect populations in the 

 Naknek system. 



Preference for, access to, or utilization of spe- 

 cific foods may not directly account for differences 

 in growth rates of pygmy whitefish populations 

 in Brooks Lake and South Bay-Iliuk Arm. Basic 

 differences in ecological characteristics of the areas 

 such as morphometry, limnology, productivity, 



Figure 11. — Relative importance of insects, zooplankton, 

 and other foods in diet of pygmy whitefish from three 

 areas of Xaknek system. Data based on percent of 

 sample volumes shown in table 8. Number beneath 

 each figure represents largest pygmy whitefish collected 

 from the grouped area. 



PYGMY WHITEFISH OF SOUTHWEST ALASKA 



571 



