FISHERY BULLETIN: VOL. 71, NO. 3 



of western Oregon produced up to 128 kg/ 

 hectare/yr. The maximum first-year production 

 of 170 kg/hectare/yr and the total 1962 pro- 

 duction of 206 kg/hectare/yr by salmon in 

 Happy Valley rank this reservoir as high among 

 impoundments studied and apparently com- 

 parable to some of the more productive streams 

 studied. 



The yield of juvenile salmonids from 10 im- 

 poundments has also been summarized by 

 Coche (1967). The lowest value (2.3 kg/hectare/ 

 yr) was for sockeye salmon in a large oligo- 

 trophic lake in Canada. Most values ranged 

 between 20 and 100 kg/hectare/yr; the highest 

 was 143 kg/hectare/yr. Ellis, Pressey, and 

 Smith (1958), citing Washington Department 

 of Game figures, stated that Washington lakes 

 yield 45 to 209 kg/hectare/yr (average 112) 

 in rainbow trout. Although covering a wide 

 range, these figures suggest that yields of 100 

 kg/hectare/yr may be expected of fertile im- 

 poundments. This was true for Happy Valley 

 which, despite high mortality rates, yielded 98 

 and 73.5 kg/hectare/yr of first-year salmon in 

 the 2-yr of this study. Both net production and 

 yield estimates corroborate chemical evidence 

 that the reservoir is highly fertile. 



FOOD HABITS 



Studies such as those by Johnson and Hasler 

 (1954) and Ricker and Foerster (1948) have 

 shown Entomostraca to be the primary food 

 type eaten by many lake-resident juvenile salm- 

 onids. The coho salmon of the small lake in 

 Kamchatka Penninsula studied by Dvinin (1949), 

 however, fed on insects (midges and mosquitoes), 

 although older fish took some zooplankton. In 

 food-habit studies of stream-inhabiting coho and 

 Chinook salmon juveniles, Brueser (1961) and 

 Chapman et al. (1961) found mainly Diptera 

 and Trichoptera larvae, Ephemeroptera nymphs, 

 and terrestrial insects. Availability very likely 

 played a major role in these differences of food 

 habits; for example, planktonic forms in a strat- 

 ified lake and "drift" organisms in a stream 

 would be available forms, with salmonids ca- 

 pable of eating a variety of foods. 



Food habits of the 1961 year class in Happy 

 Valley Reservoir were determined by dry-weight 

 analyses of stomach contents from 17 samples 

 offish taken between April 1961 and November 

 1962. The results (Table 3) indicated a heavy 

 utilization of Entomostraca (mostly Cladocera) 

 and chaoborid larvae in both summer and winter 

 until the 1962 year class was released. These 

 foods averaged 97% of the stomach contents for 

 the samples taken before the 1962 salmon were 

 released, and only about 6% of the sample con- 

 tents taken thereafter. The planktonic Ento- 

 mostraca and vertically migrating chaoborid 

 larvae were apparently both abundant and 

 available to the salmon, which probably fed in 

 the upper 3 m of water during the summer. 

 Competition from the 1962 year class, however, 

 evidently forced a shift in food habits and led 

 to the slow growth already mentioned. A variety 

 of benthic forms (crayfish, snails, damsel fly 

 naiads, and amphipods) as well as chironomid 

 pupae, terrestrial insects, and members of the 

 1962 year class were then eaten. 



Meanwhile, the 1962 year class was utilizing 

 Entomostraca, as shown by field checks of 

 stomach contents. In a 23-member sample of 

 these fish taken on 21 August 1962, Entomos- 

 traca were found in all stomachs and made up 

 roughly 80% of the total contents identified. In 

 another sample of 10 fish taken on 2 September 

 1962, 9 stomachs contained Entomostraca, 

 which formed 45% of the total contents. Most of 

 the remaining food in both samples was 

 chironomids. 



Increased size does not seem to have been 

 the direct cause of the change in food preference 

 by the 1961 salmon because their size (Figures 

 12 and 13) did not appreciably increase from fall 

 1961 to spring 1962, when the apparent food 

 shift occurred. That fish of their size are capable 

 of feeding upon the Entomostraca was shown 

 by an earlier analysis of the stomach contents 

 from 25 salmon of the 1959 year class. These 

 fish averaged 193 g when collected on 8 April 

 1961. Entomostraca were found in 13 of the 

 stomachs and constituted 90% of the combined 

 sample on a settle volume basis. The high pro- 

 duction by first-year salmon in Happy Valley 

 Reservoir apparently was supported chiefly by 

 entomostracan foods. 



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