Eggs from pink salmon, O. gorbuscha, chum 

 salmon, 0. keta, sockeye salmon, 0. nerka, fall 

 Chinook salmon, O. tsliairijfscha, coho salmon, O. 

 kisutch, and from steelhead trout of Puget Sound 

 stocks were placed in holding aquaria and covered 

 with coarse gravel. 



The salmonid fry were subjected to predation 

 tests as soon as the yolk sacs were absorbed. Since 

 the time of emergence from the gravel by the fry 

 of these six species varies, the tests extended from 

 March to June, during which period water 

 temperatures (Table 1) and day lengths differed. 

 The salmon and trout fry were not fed but could be 

 seen mouthing particles entering the holding 

 aquaria. I assume grow^th of fry negligible and size 

 differences to be fixed by the species and race 

 used. Observations of viability and vigor of fry in 

 the holding aquaria were made before, during, and 

 after testing as a standard of comparison for test 

 fish. Samples of the salmon and trout fry were 

 measured in millimeters from snout to fork of tail 

 (Table 1); their volumes were determined by 

 displacement in a graduated tube. Sculpins were 

 measured in millimeters from snout to end of tail 

 (Table 1). 



Twenty sculpins were placed in holding aquaria 

 without food the first day of the experiment. On 

 the second day 10 fry of one species were placed in 

 each stream aquarium and on the third day, 10 

 sculpins were quietly introduced at the down- 

 stream end of each stream aquarium. On the fifth 

 day the fry surviving after 48 h were counted; then 

 both predators and prey were removed. These 

 subjects were not used again. Two to seven 

 replicate tests were made for each species of 

 salmon or trout (Table 1). 



Comparative Survival of Salmon and 

 Trout Against Predation 



The positions and activities of the salmon, trout, 

 and sculpins in the stream aquaria are first 



described because these varied between species, 

 affecting predator-prey interrelations. The 

 following sections report on the viability and vigor 

 of fry and on the survival rates of the species of 

 salmon and trout. 



The positions and activities of a species of salm- 

 on or trout during daylight tests varied. Fish in 

 the stream aquaria maintained positions and ap- 

 parently fed; Chinook and coho salmon displayed 

 intraspecific aggression, indicating accommoda- 

 tion to the enclosure. All species were observed in 

 the deepest areas of the stream aquaria where 

 they distributed themselves vertically 1 cm from 

 the bottom to the water surface. Most of the 

 steelhead trout fry and some pink and chum salm- 

 on fry hid under rocks, but this behavior was 

 seldom exhibited by the other salmon species ex- 

 cept for short periods when they were frightened. 



Torrent sculpin typically spaced themselves 

 through the deeper parts of the stream aquaria. 

 They were distributed through its length with the 

 greatest number at the upstream end. They were 

 inactive and curled around large rocks or partially 

 buried themselves in areas with soft bottoms. The 

 concealment of the sculpins was so complete that I 

 often had to search for as long as 20 min to remove 

 all of them after an experiment. 



After the sculpins were placed in the stream 

 aquaria, the salmon and trout fry, on recovering 

 from the disturbance, modified their vertical dis- 

 tribution. Salmon fry reacted to an active sculpin 

 by moving away laterally and upward. In the 

 presence of sculpins all salmon fry increased their 

 distance from the bottom to about 5 cm. Steelhead 

 trout fry, that usually hid under rocks when un- 

 disturbed, moved off the bottom and maintained 

 positions near the water surface when sculpins 

 were present. Behavior of the steelhead trout fry 

 was apparently more disturbed by sculpins than 

 was that of the salmon fry. 



Sculpins rarely stalked the fry in bright 

 daylight but waited immobile for them to come 



Table 1.— Survival of salmon and trout fry subjected to predation in 1966 by torrent sculpin, Cottus rhotheus. 



'Average temperature (°C) at 0800 for 2 days of test. 



932 



