BODY SIZE AND LEARNED AVOIDANCE AS 



FACTORS AFFECTING PREDATION ON COHO 



SALMON, ONCORHYNCHUS KISUTCH, FRY BY 



TORRENT SCULPIN, COTTUS RHOTHEUS 



Wild coho salmon juveniles, Oncorhynchus 

 kisutch, in Washington streams range in fork 

 length (FL) from about 30 mm at the time of 

 emergence from the gravel to 120 mm on migra- 

 tion to the sea. Predation by sculpins, Cottus spp., 

 is limited to the smaller salmon; few salmon >45 

 mm FL have been recovered from the stomachs of 

 sculpins (Patten 1962, 1971a, 1972). Yet, sculpins 

 are capable of eating hatchery reared fall chinook 

 salmon, O. tshawytscha, of 60 mm FL (Patten 

 1971a). Apparently, the reason sculpins do not 

 normally prey on wild coho salmon >45 mm FL is 

 not entirely dependent on the relative sizes of prey 

 and predator. 



The present study is on the ability of torrent 

 sculpin, C. rhotheus, to prey on coho salmon >45 

 mm FL, as well as the predator avoidance be- 

 havior of coho salmon to torrent sculpins in stream 

 aquaria adjacent to the Cedar River near 

 Ravensdale, Wash., during 1965 and 1966. One 

 experiment indicates the absolute size of coho 

 salmon that can be caught, subdued, and swal- 

 lowed by a torrent sculpin of a given length. The 

 other suggests that coho salmon previously ex- 

 posed to torrent sculpins become less susceptible 

 to these predators in future interactions. 



Facilities and Procedures 



Two related studies — one on predator-prey size 

 relations and the other on the learned predator 

 avoidance ability of coho salmon prey — were con- 

 ducted in stream and holding aquaria that re- 

 ceived water from the Cedar River. The two 

 stream aquaria were 2.4 m long, 0.6 m wide, and 

 0.6 m high; water depth ranged from 2 to 18 cm. 

 The eight holding aquaria were 34 cm wide, 41 cm 

 long, and 36 cm high; water depth was 18 cm (a 

 more complete description of the experimental 

 facilities is given by Patten 1971b). 



Water was gravity fed from a low level dam on 

 the Cedar River to a head box through a flume and 

 then to the aquaria. Each aquarium received a 

 continuous supply of clear water; temperatures in 

 the morning during the study ranged from 4.4° to 

 12°C. 



Torrent sculpins were collected by electro- 

 fishing in Soos Creek, King County, Wash., and 



coho salmon were seined in upper Rock Creek of 

 the Cedar River drainage. It was assumed that 

 the state of hunger of all torrent sculpins was 

 similar, that the coho salmon had little experience 

 with fish predators, and that this experience was 

 similar for all subjects. The assumption for the 

 coho salmon was probably valid because the only 

 other common species of fish at the seining site 

 was the shorthead sculpin, C. confusus — a rela- 

 tively nonpredaceous species of fish (unpubl. 

 studies of author). Furthermore, the few individu- 

 als of the shorthead sculpin observed were small. 

 The effect of predator-prey length relations on 

 predation was determined from 23 tests where six 

 coho salmon of a given length group were avail- 

 able to four torrent sculpins of a given length 

 group (Table 1) for 4 days. The test procedure was 

 to collect torrent sculpins the first day and place 

 them in a holding aquarium without food; on the 

 second day, coho salmon were collected and six 

 individuals within 5 mm of a given length were 

 placed in a holding aquarium; on the third day, 

 four torrent sculpins within 5 mm of a given 

 length were introduced into the holding aquarium 

 containing the coho salmon; 4 days later, the 

 number of coho salmon eaten was recorded and the 

 experimental fish were discarded. The largest 

 available size group of torrent sculpins used was 

 120 mm total length (TL). 



TABLE 1. — Results of 23 tests where six coho salmon of a length 

 group were subjected to predation by four torrent sculpins of a 

 length group. Predation on one or more coho salmon is denoted 

 by P and no predation by N. 



The ability of coho salmon to learn to evade 

 predation was tested by comparing the relative 

 survival of naive coho salmon ( those which had not 

 been exposed to torrent sculpin predators) with 

 coho salmon conditioned to predation by the tor- 

 rent sculpin. Coho salmon were conditioned by 

 placing 20 individuals into a stream aquarium 

 with eight torrent sculpins. Some of those that had 

 survived a 48-h association with torrent sculpins 



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