MURPHY ET AL.: SIZE AND DIET OF JUVENILE PACIFIC SALMON 



100 



80 



60- 



40- 



20 



-A) 



. Pink 

 ' Chum 

 Coho 



^100^ 



Q. 

 O 



■5. 80 

 « 



<D 

 Q. 



^ 60 



c 



0) 



(J 



« 40- 

 Q. 



20 



26May 



9June 



7July 



B 



I 



100-f 

 80 

 60 

 40 

 20 



Inner Basin 



Middle Basin 



Outer Basin 



O-l 



/ 



/ 

 / 

 / 



^< 



/ 



50-59 60-79 80-99 



Salmon Length (mm) 



> 100 



Figure 4. — Number of pelagic prey as percent of total prey 

 eaten by individual salmon compared between sampling periods 

 (A), estuary basins IB), and salmon fork length classes (C) in the 

 Porcupine Creek estuary in 1981. Symbols are means; bars are 

 ±2 SE of the means. Symbols in B and C are the same as in A . 

 Pelagic prey are defined in the text. 



salmon's size mainly influences its vulnerability 

 to predators, whereas its diet determines poten- 

 tial competition for food. Size and diet, however, 

 are not independent. Salmon change their diet as 

 they grow, which helps relieve competition be- 

 tween salmon of different size, and a poor diet 

 slows their growth, which prolongs vulnerability 

 to predation. 



Table 6. — Analysis of variance of percentage pelagic 

 prey of coho salmon, with sampling period and estuary 

 basin as factors and salmon fork length as covariate. 

 Factors, covariate, and interactions were adjusted 

 simultaneously before assessing significant (Kim and 

 Kohout 1975). 



Because of similar diets, pink and chum salmon 

 are potential competitors. Although diets of pink 

 and chum salmon in the Porcupine Creek estuary 

 were similar in prey size and some prey taxa, 

 however, pink salmon fed almost solely on pelagic 

 prey , whereas chum salmon foraged both pelagi- 

 cally and epibenthically; such differences may 

 help reduce competition. Competition probably 

 also was reduced because, as the salmon grew 

 larger, they switched to larger prey. Coho salmon 

 probably did not compete for food with the other 

 two species because the coho fed on larger, differ- 

 ent prey. 



Rapid early growth of salmon is important in 

 reducing vulnerability to predators (Parker 1971; 

 Taylor 1977). For example, hatchery pink salmon 

 fry raised for 60 days (to 40 mm FL) before release 

 into an estuary in southeastern Alaska survived 

 68% better at sea than did fry released immedi- 

 ately after emergence (Martin et al. 1981). 

 Marine survival also is higher for year classes of 

 larger (9-11 cm FL) than for smaller (6-8 cm FL) 

 sockeye salmon, O. nerka , smolts (Foerster 1954). 

 Coho salmon smolts from Porcupine Creek in 

 1978 averaged 99 mm FL and their survival was 

 twice that of the 1979 smolts, which averaged 

 only 91 mm FL (Thedinga 1985). Smolt size and 

 migration timing, however, interact complexly to 

 influence marine survival of coho salmon (Bilton 

 1978). 



Growth of juvenile salmon in estuaries usually 

 inferred from changes in mean size 



IS 



(LeBrasseur and Parker 1964; Healey 1978), but 

 these estimates are subject to bias. In this study, 

 changes in mean size of fish in the catches on 

 successive dates could underestimate real growth 

 for two reasons: 1) small individuals may have 

 migrated continuously into the estuary from 



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