SIZE AND DIET OF JUVENILE PACIFIC SALMON 

 DURING SEAWARD MIGRATION THROUGH A SMALL ESTUARY 



IN SOUTHEASTERN ALASKA 



Michael L. Murphy, John F. Thedinga, and K V. Koski' 



ABSTRACT 



To assess competition and predation among juvenile Pacific salmon iOncorhynchus spp.) migrating 

 through the estuary of Porcupine Creek, a small stream in southeastern Alaska, their size and diet 

 were determined in 1979 and 1981. Mean fork length (FL) during May and June increased from 32 

 to 73 mm (1.5 mm/day) for pink salmon, O. gorbuscha; from 39 to 51 mm (0.4 mm/day) for chum 

 salmon, O. keta; and during June and July, from 99 to 165 mm (1.6 mm/day) for coho salmon, O. 

 kisutch. Prey, in order of importance, included larval fish (mostly Gadidae), larval molluscs (Mesogas- 

 trofKxla), and calanoid copepods for pink salmon; larval molluscs, larvaceans, and hyperiid amphipods 

 for chum salmon; and fish (Clupea harengus pallasi, Ammodytes hexapterus, and Gadidae), insects, 

 and larval decapods (Brachyrhyncha) for coho salmon. No pink or chum salmon were found in the coho 

 salmon stomachs. Prey size for pink and chum salmon was similar (median, 0.4 mm long for both 

 species), and much smaller than that of coho salmon (median, 2.3 mm). Diet overlap was greater 

 between pink and chum salmon than between either species and coho salmon. Pink salmon, however, 

 ate almost exclusively (95%) pelagic prey, whereas chum salmon ate both pelagic (74%) and epiben- 

 thic (26%) prey. Rapid ecirly growth and differences in diet probably help minimize predation and 

 competition among salmon during seaward migration. 



The early marine life stage of juvenile Pacific 

 salmon iOncorhynchus spp.), during transition 

 from freshwater to seawater, is important in de- 

 termining brood-year survival and subsequent 

 adult returns (Manzer and Shepard 1962; Parker 

 1968); their survival rate is lowest during this 

 time (Parker 1968; Bax 1983). Salmon often 

 school in large concentrations in estuaries as they 

 migrate seaward, and are more likely to deplete 

 food supplies and compete for food than after they 

 disperse to the sea (Bailey et al. 1975; Feller and 

 Kaczynski 1975). Survival depends on size 

 (Parker 1971; Healey 1982), and competition for 

 food can depress early growth (Peterman 1984) 

 and prolong vulnerability to predators (Taylor 

 1977; Walters et al. 1978). Size and diet of juve- 

 nile salmon in an estuary, therefore, determine 

 the potential for predation and competition and 

 can greatly affect survival. 



As salmon aquaculture expands and more juve- 

 nile salmon are released into estuaries, competi- 

 tion and predation among salmon may increase 

 (Johnson 1974). To optimize hatchery production 

 and avoid adversely affecting wild stocks, an 



1 Northwest and Alaska Fisheries Center Auke Bay Labora- 

 tory, National Marine Fisheries Service, NOAA, P.O. Box 

 210155, Auke Bay, AK 99821. 



Manuscript accepted November 1987. 

 FISHERY BULLETIN: VOL. 86, NO. 2, 1988. 



understanding is needed of how different stocks of 

 salmon interact in estuaries. This paper compares 

 size and diet of juvenile pink, O. gorbuscha; 

 chum, O. keta; and coho, O. kisutch, salmon 

 to assess potential predation and competition 

 between the species during their seaward mi- 

 gration through the estuary of a small, pristine 

 stream. 



STUDY AREA 



This study was conducted in the estuary of Por- 

 cupine Creek, the only salmon stream flowing 

 into Steamer Bay in southeastern Alaska (Fig. 1). 

 The estuary (about 5.5 km long) consists of a 1.5 

 km stream reach that is periodically inundated 

 by tides, and a 4 km series of three estuarine 

 basins. At low tide, the inner and middle basins 

 are small (2 and 7 ha, respectively) and shallow 

 (14 and 16 m, respectively) compared with the 

 outer basin (120 ha and 42 m deep). The littoral 

 zone ranges from low-gradient mudflats to steep 

 cobble. Bottoms of the basins are level and com- 

 posed of shell, gravel, and mud. 



During low tide, the inner and middle basins 

 are partially isolated from the outer basin and the 

 main part of Steamer Bay by tidal rapids 1-3 m 

 deep. Salinity is lower in the inner and middle 



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