Diet, Movement, and Growth of Dolly Varden in Response to Sockeye Salmon Subsidies

A large and growing body of literature has documented the transfer of marine‐derived nutrients from the ocean to freshwater and riparian systems by semelparous Pacific salmon Oncorhynchus spp. The pathways by which these nutrients reach resident fish are often indirect, and the evidence for direct benefits to the resident fish is not always conclusive. However, the consumption of salmon tissue (in one form or another) by resident fish would constitute a direct and efficient pathway for energy transfer. We studied a population of small‐bodied, nonanadromous Dolly Varden Salvelinus malma feeding on the fry and eggs of sockeye salmon O. nerka and blowfly (family Calliphoridae) larvae that had fed on salmon carcasses at a series of spring‐fed and otherwise unproductive ponds in southwestern Alaska. The Dolly Varden fed heavily on sockeye salmon fry when available, shifted their diet almost exclusively to eggs after salmon spawning commenced, and then shifted to blowfly larvae toward the end of the season. Dolly Varden large enough to eat eggs moved into ponds where sockeye salmon spawn synchronously with the arrival of the salmon, and Dolly Varden growth rates increased greatly once salmon eggs and blowfly maggots were available. Young‐of‐the‐year Dolly Varden, which were too small to eat eggs and fry, were concentrated in small streams between ponds where fewer sockeye salmon spawn, perhaps to minimize the risk of predation from larger conspecifics. These results indicate the importance of a pulse of salmon‐related food resources for this population of resident fish and their adaptations to take advantage of these resources. It is likely that similar dependence occurs in other systems where sockeye salmon produce a suite of temporally predictable energy resources; thus, resident fish may depend on large populations of salmon.