FISHERY BULLETIN: VOL. 74, NO. 1 



studies on C. aleuticus and C asper, which are 

 widely distributed and commonly abundant in 

 coastal streams from California to Alaska, have 

 emphasized their potentially destructive role as 

 predators on the eggs and fry of salmon and trout 

 (Shapovalov and Taft 1954; Hunter 1959; McLar- 

 ney 1967). Conversely, it has been generally 

 shown that sculpins in streams of the North 

 Temperate Zone prey incidentally on salmon and 

 trout, but sculpins do share a common source of 

 food — the benthic invertebrate community. 



The probable importance of interspecific com- 

 petition in general, and for food in particular, in 

 such streams where the several species of fishes 

 consume in common a wide variety of food or- 

 ganisms has been readily acknowledged (Hartley 

 1948; Maitland 1965; Mann and Orr 1969) but 

 continues to defy quantitative analysis. The over- 

 lapping summer foods of juvenile coho salmon, 

 cutthroat trout, and coastrange sculpins in Cabin 

 Creek (Table 7) clearly show the possibility of 

 competition for food in the present study streams. 

 Numerically, Ephemeroptera and Diptera were 

 important in all three diets but most important in 

 the coho salmon diet, while Trichoptera were 

 most important in the trout and sculpin diets. 

 The sculpins showed the least varied diet as 

 Ephemeroptera, Diptera, and Trichoptera com- 

 posed nearly 95% of the food items consumed. 

 Dietary differences can be related to behavioral 

 differences in feeding and habitat response. The 



Table 7. — The percentage composition by frequency of 

 occurrence (0) and number (N) of the midsummer (June-July) 

 foods eaten by juvenile coho salmon, cutthroat trout, and 

 coastrange sculpins in Cabin Creek. Based on 30 fish of each 

 species collected simultaneously. 



'Refers to adult stage, all categories of Insecta are larval stages unless 

 noted otherwise. 



sculpins were abundant in all habitats but ate 

 few foods of surface origin, being crepuscular 

 grazers on the benthos. The trout were princi- 

 pally riffle-dwellers and grazed the benthos (both 

 trout and sculpins ate large numbers of Trichop- 

 tera larvae) but exploited the invertebrate drift to 

 a lesser extent than did the coho salmon, which 

 preferred the pool and glide habitats of low cur- 

 rent velocity. Despite this behavioral diversity, 

 niche differentiation remains poorly developed in 

 the Eltonian sense discussed by Weatherley 

 (1963) who proposed that the niche be defined as 

 "...the nutritional role of the animal in its 

 ecosystem... ." 



Recent experiments have clearly illustrated 

 that populations of juvenile coho salmon in these 

 streams are limited by their food supply during 

 the summer months (Mason 1974b, 1974c). Rates 

 of growrth, survival and emigration were amena- 

 ble to manipulation by varying population den- 

 sity and food availability. Thus, in that young 

 coho salmon share a common food supply with 

 both trout and sculpins, the likelihood of food 

 competition is strongly suspected. 



Since direct documentation of competition 

 among stream fishes in natural environments 

 continues to elude us, the inferential definition of 

 competition proposed by Maitland (1965) appears 

 to have greater utility than the modus operandi 

 definition of Larkin (1956), ". . .the demand, typi- 

 cally at the same time, of more than one or- 

 ganism, for the same resources of the environ- 

 ment in excess of immediate supply." Maitland 

 (1965) suggested that competition occurs "... 

 when the presence of more than one species 

 causes the average total biomass (standing crop) 

 of one of them to be less than it would be if that 

 species were existing alone — species which are 

 directly parasitic or predatory on one another 

 being excepted." 



Fish biomass in small coastal streams of Van- 

 couver Island usually ranges between 7 and 10 

 g/m^ in midsummer (unpubl. data). Of this 3-6 

 g/m^ (50-80%) consists of sculpins (C. asper and 

 C. aleuticus) in the first several kilometers above 

 the estuarine zone. Studies by Brocksen et al. 

 (1968) have shown that, within the carrying 

 capacity of laboratory streams producing natural 

 drift foods, production of cutthroat trout was de- 

 termined by the biomass ratio of trout and scul- 

 pin, C. perplexus, at time of stocking, whereas 

 sculpin production remained independent of trout 

 biomass. These results were obtained over a 



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