night. They were mostly captured near the 

 surface (table 10) where they fed on plankton, 

 especially in late summer (fig. 24). Presum- 

 ably sockeye salmon juveniles were attracted 

 to the surface waters by the nighttime con- 

 centration of plankton. Because our plankton 

 samples were taken in the day and our fish 

 samples at night, it is not surprising that the 

 depths of maximum density of sockeye salmon 

 and zooplankton did not coincide. It would be 

 enlightening to obtain a series of day and 

 night plankton samples at Brooks Lake to 

 compare abundance at different depths through 

 a series of 24-hour periods but we were unable 

 to undertake this in 1957. 



On the eight sampling dates weather ranged 

 from clear and calm to rough and windy. 

 There was no discernible relation of weather 

 conditions and horizontal or vertical plankton 

 distribution. This is not surprising since the 

 maximum concentrations occurred at depths 

 of 5 m. and more, below the influence of 

 surface water conditions. 



Of zooplankters eaten by sockeye salmon, 

 46 percent were the copepods, Cyclops and 

 Diapinmus, and 54 percent were the 

 cladocerans, Daphnia and liosmina. This im- 

 plies that plankton-feeding sockeye salmon 

 definitely prefer cladocerans to copepods, 

 because copepods were invariably more 

 abundant than cladocerans in samples (fig. 30). 

 Salmon seldom ate copepod nauplii, and there- 

 fore the significance of their distribution and 

 abundance to sockeye salmon is only indirect 

 in ultimate abundance of copepods. 



Figure 30 requires some explanation and 

 interpretation. The black shapes are spherical 

 curves (Birge and Juday, 1922). They repre- 

 sent three-dimensional columns of relative 

 plankton densities in a vertical water column 

 at each of the three stations. The water column 

 at each station was sampled at each indicated 

 depth. The bottom depth was 5 m. at station I; 

 35 m. at II; and 67 at III. 



The significant points in figure 30 are: 



I. Samples at station I are not repre- 

 sentative of the main body of the lake. Except 

 for copepod nauplii on July 4 and 14, zoo- 



plankters were less dense at station I than at 

 II and III. 



2. Zooplankters were usually less abund- 

 ant in the surface 5 m. of water at midday 

 than at lower depths, probably because of the 

 inhibiting effect of light. Usually the region of 

 highest concentrations was from 5 to 10 m. 



3. Plankton density was greatest during 

 the early summer and decreased gradually 

 through the summer and fall. 



4. Copepod juveniles and adults were 

 always more abundant than cladocerans or 

 copepod nauplii. 



5. Relative vertical distribution showed 

 little stratification, indicating that the lake 

 contained zooplankton throughout the season at 

 all depths. 



From 1926 to 1930, limnological studies 

 were conducted at Karluk Lake on Kodiak 

 Island (Juday, Rich, Kemmerer, and Mann, 

 1932). Karluk Lake was then a major pro- 

 ducer of sockeye salmon; conditions for sock- 

 eye salmon production were presumably satis- 

 factory. Since Karluk and Brooks Lakes are 

 in the same general geographic area and are 

 approximately the same size, general com- 

 parisons should be useful and valid, liosmina, 

 Daphnia, Cyclops, and Diaplomns were the 

 dominant forms in both lakes. Maximum abund- 

 ance of Daphnia was considerably greater in 

 Brooks Lake: 21 per liter compared with 9 

 per liter in Karluk Lake. The largest catch of 

 liosmina in Karluk Lake was 13 per liter 

 compared with 11 in Brooks Lake. Maximum 

 abundance of Diaptomns in Karluk Lake was 

 22 per liter compared with 53 per liter in 

 Brooks Lake, and for Cyclops, it was 145 per 

 liter compared with 31 in Brooks Lake. For 

 copepod nauplii, maximum abundance in Karluk 

 Lake was 100 per liter; in Brooks Lake it 

 was 56. The one striking difference between 

 Brooks and Karluk Lakes is that at Karluk 

 Lake rotifers reportedly were the most abund- 

 ant form of all zooplankters, while at Brooks 

 Lake rotifers were scarce and were never 

 found in net or centrifuge samples (they were 

 found in stomach samples). 



46 



