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

Abstract

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.

Effects of subsidies from spawning chum and pink salmon on juvenile coho salmon body size and migration timing

“…These resources are readily used by juvenile salmonids (Scheuerell et al 2007, Denton et al 2009. A concurrent study of many of the same streams that we used found spawning salmon biomass predicted primary productivity better than habitat characteristics, and that aquatic invertebrates used both nitrogen and carbon resources from spawning salmon .…”

“…Although the authors showed carry-over effects into the nonspawning season, we are the first to show an entirely indirect effect of spawning salmon on juvenile coho salmon body size by studying age 0 coho salmon. Another study looking at naturally occurring spawning salmon found increased growth rate in dolly varden in seven ponds increasing in spawning salmon biomass (Denton et al 2009). Our study is the first to separate effects by age class (including age 0 with no direct contact and age 1 with direct contact with spawning salmon), to examine effects on coho age composition, and to include the comparative influences of habitat characteristics.…”

“…Furthermore, older age classes of juvenile coho salmon can prey upon newly hatched pink and chum salmon fry (Hunter 1959), as well as invertebrates that have been stirred up by adult salmon digging nests an fighting for space and mates. They may also benefit from blowfly larvae on salmon carcasses in streams, which are a preferred food source for juvenile salmonids (Scheuerell et al 2007, Denton et al 2009.…”

Seasonal persistence of marine‐derived nutrients in south‐central Alaskan salmon streams

“…Predictions based on these models showed that (Denton et al 2009, 2010, Jaecks 2010. Maximum d 15 N in generalist macroinvertebrates was substantially less than that of salmon, implying incorporation of less MDN.…”

“…Substrate disturbance during salmon spawning, however, can offset these effects and reduce overall periphyton and macroinvertebrate biomass Schindler 2008, Verspoor et al 2010), especially in streams with fine sediments (Janetski et al 2009). Growth and energy storage among stream-dwelling fishes is increased by MDN (Bilby et al 1996, Wipfli et al 2003, Heintz et al 2004, Rinella et al 2012) and the direct consumption of eggs and carcass material appears to be more important than bottom-up pathways for conveying MDN benefits (Scheuerell et al 2007, Denton et al 2009, Armstong et al 2010. MDN is transferred to the riparian environment by hyporheic processes, flooding, and terrestrial scavengers (Cederholm et al 1989, Ben-David et al 1998, Hilderbrand et al 1999, O'Keefe and Edwards 2002), where it enriches riparian soils (Bartz andNaiman 2005, Drake et al 2005).…”