The Influence of Fall-Spawning Salmon on Growth and Production of Juvenile Coho Salmon Rearing in Beaver Ponds on the Copper River Delta, Alaska.
Introduction: The spawning and death of semelparous anadromous Pacific salmon (Oncorhynchus spp.) is recognized as an integral ecological process in freshwater ecosystems throughout the North Pacific Rim. Studies have revealed that salmon spawners play a role in the primary productivity of streams (Richey et al. 1975; Wipfli et al. 1998) and in population dynamics and growth of secondary consumers (Wipfli et al. 1999), including juvenile salmonids (Eastman 1996; Bilby et al. 1998; Wipfli et. al. 2003). Although studies have focused on the individual components involved in these processes, there is still much to be learned about the ecological relationships and consequences of spawning salmon (Gende et al. 2002).
Annual migrations of spawning salmon can bring a large amount of nutrients to freshwater ecosystems, and these salmon-derived nutrients are incorporated into freshwater food webs, including fishes (Kline et al. 1993; Piorkowski 1995; Bilby et al. 1996). During spawning, eggs that are not buried in the substrate can be consumed by juvenile salmonids. Carcasses can be consumed directly by juvenile fish and by organisms that serve as a food for juvenile salmonids (Wipfli et al. 1998, 2003). Salmon eggs can be a large proportion of the diet of juvenile salmonids (Bilby et al. 1998).
Salmon eggs appear to be an abundant and energy-rich food resource that is made available to stream-rearing fishes during spawning (Bilby et al. 1998; Foote and Brown 1998; Fukushima et al. 1998). Salmon eggs can be especially important for juveniles during the fall or winter months, when fish may be ingesting less food. High quality foods supply more energy per unit ingested. Since metabolic rates are depressed at lower water temperatures and some of the energy ingested above that required for maintenance can be stored, juveniles with access to eggs could experience positive growth during the winter (Weatherley 1972; Brett 1979). Excess energy can also be stored as lipid reserves to be utilized when less favorable conditions of food availability exist (Weatherley 1972), and lipid content of fish at the onset of winter is important in overwinter survival (Toneys and Coble 1980; Cunjak et al. 1998).
Increased quantity and quality of food resources can have direct effects on the growth rates of juvenile anadromous salmon rearing in habitats near spawning salmon. Bilby et al. (1998) observed an increase in weight, condition factor, and densiy of juvenile steelhead (O. mykiss) and coho salmon (O. kisutch) in response to the addition of carcasses to a small stream in Washington State. Juvenile salmonids rearing in two streams of the Wood Lake system in Alaska were estimated to increase in weight by 12% and 17% during the peak of sockeye salmon (O. nerka) spawning activity (Eastman 1996). Wipfli et al. (2003) recorded increased growth rates of juvenile coho salmon with increasing amounts of salmon tissue and eggs in a mesocosm experiment and found higher growth rates of resident cutthroat trout (O. clarki) and Dolly Varden (Salvelinus malma) in a stream in Alaska enriched with salmon carcasses.