ARCTIC CHAR PREDATION ON SOCKEYE SALMON SMOLTS AT 

 LITTLE TOGIAK RIVER, ALASKA 1 



Gregory T. Ruggerone and Donald E. Rogers 2 



ABSTRACT 



Observations of Arctic char feeding on migrating sockeye salmon smolts at Little Togiak River, Alaska, 

 indicate a Type II functional response where the number of smolts consumed increased with smolt 

 abundance. The number of smolts migrating was usually low <■ 20,000 smolts/24 hours) and the 

 corresponding consumption of smolts averaged 0.8 smolts/char per 24 hours. When large smolt migra- 

 tions occurred ( -80,000 smolts/24 hours), char consumption of smolts generally increased to 5.6 

 smolts/char per 24 hours. In addition to smolt abundance, smaller smolts and longer char were 

 correlated with an increase in the number of smolts consumed. Estimates of percent smolt mortality, 

 based on two hypothetical char numerical responses to varying smolt abundances, indicate that smolts 

 were migrating at densities most susceptible to predation. 



A comparison of length of smolts consumed by char with those in the migration shows that char 

 consumed larger than average smolts when their stomachs were not full and smaller than average 

 smolts when char approached stomach fullness. This may be explained by the migration of larger 

 smolts during the feeding period of char and the possibility of char feeding less effectively when 

 approaching fullness. Although major hatchery releases often exceed 100,000 smolts per day, these 

 data suggest that hatchery-released smolts may be less susceptible to predation in small rivers when 

 released during the night in large numbers (> 20,000 smolts/24 hours). 



The relationship between predation on juvenile 

 salmon and relevant biological and environmen- 

 tal factors is important to the understanding of 

 salmon population dynamics. Development of 

 these relationships may be useful for establish- 

 ment of "optimal" escapement levels and for 

 maximum production from salmon enhancement 

 projects. A few investigations have related preda- 

 tion rates to juvenile salmon abundance and have 

 reported up to 85% juvenile mortality (Neave 1953; 

 Hunter 1959; Parker 1968; Peterman and Gatto 

 1978). Other investigations have examined the ef- 

 fect of biological or environmental variables such 

 as juvenile salmon size ( Parker 1971 ), predator size 

 (Ricker 1941; Hunter 1959; Rogers et al. 1972), 

 infection by parasites (Burke 1978), river velocity 

 and turbidity (Ginetz and Larkin 1976), thermal 

 stress (Sylvester 1972; Coutant 1973), or several 

 variables independent of juvenile salmon density 

 (Fresh et al. 1980). No investigation has analyzed 

 predation while concurrently assessing the par- 

 tial effect of prey density along with the partial 

 effect of biological and environmental factors. 



This investigation represents a 5-yr study of 

 predation by Arctic char, Saluelinus alpinus, on 



'Contribution No. 646, School of Fisheries, University of 

 Washington, Seattle, Wash. 



2 Fisheries Research Institute, WH-10, University of Washing- 

 ton, Seattle, WA 98195. 



emigrating sockeye salmon, Oncorhynchus nerka, 

 smolts at Little Togiak River, Alaska (Fig. 1). 

 Predator-prey interaction appears to be especially 

 refined in this river. With the onset of the smolt 

 emigration each spring, char migrate to the inter- 

 connecting rivers in the lake system where mi- 

 grating smolts are most vulnerable (McBride 

 1979). After the smolt migration ends, char return 

 to their spawning streams in the fall. The objec- 

 tives of this investigation, which were tested dur- 

 ing this brief period of predator-prey interaction, 

 were 1) to empirically model the daily functional 

 response of char (i.e., the relationship between 

 smolt abundance and number of smolts 

 consumed/char per 24-h period (Fig. 2)) while con- 

 currently measuring the effect of biological and 

 environmental variables; 2) to estimate percent 

 smolt mortality in relation to smolt abundance; 

 and 3) to test for disproportional consumption of 

 large or small smolts by char that differ in 

 stomach fullness and fork length. 



Numerous biological and environmental vari- 

 ables are likely to influence char predation on 

 salmon smolts. The variables concurrently tested 

 in the functional response model were 1) the num- 

 ber of migrating smolts during the 24-h period 

 prior to sampling the char; 2) the number of mi- 

 grating smolts during the 24-48 h period prior to 

 sampling; 3) smolt weight; 4) char length; 5) 



Manuscript accepted November 1983. 

 FISHERY BULLETIN: VOL. 82, NO. 2, 1984. 



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