Varying effects of anadromous sockeye salmon on the trophic ecology of two species of resident salmonids in southwest Alaska

Abstract

1. Anadromous salmon transport marine-derived nutrients and carbon to freshwater and riparian ecosystems upon their return to natal spawning systems. The ecological implications of these subsidies on the trophic ecology of resident fish remain poorly understood despite broad recognition of their potential importance. 2. We studied the within-year changes in the ration size, composition and stable isotope signature of the diets of two resident salmonids (rainbow trout, Oncorhynchus mykiss; Arctic grayling, Thymallus arcticus) before and after the arrival of sockeye salmon (Oncorhynchus nerka) to their spawning grounds in the Bristol Bay region of southwest Alaska. 3. Ration size and energy intake increased by 480-620% for both species after salmon arrived. However, the cause of the increases differed between species such that rainbow trout switched to consuming salmon eggs, salmon flesh and blowflies that colonized salmon carcasses, whereas grayling primarily ate more benthic invertebrates that were presumably made available because of physical disturbances by spawning salmon. 4. We also observed an increase in the d 15 N of rainbow trout diets post-salmon, but not for grayling. This presumably led to the observed increase in the d 15 N of rainbow trout with increasing body mass, but not for grayling. 5. Using a bioenergetics model, we predicted that salmon-derived resources contributed a large majority of the energy necessary for growth in this resident fish community. Furthermore, the bioenergetics model also showed how seasonal changes in diet affected the stable isotope ratios of both species. These results expand upon a growing body of literature that highlights the different pathways whereby anadromous salmon influence coastal ecosystems, particularly resident fish.

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

“…Our results suggest that body size in age 1 coho salmon had stronger relationships with adult chum salmon than body size in young-ofthe-year, which may reflect greater benefits when salmon nutrients are available directly to the juvenile fish, through preferential diet switching to eggs and tissue (Hicks et al 2005, Scheuerell et al 2007). These diets can have dramatically improved energy rations compared to diets not containing eggs (Armstrong et al 2010).…”

“…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 .…”

“…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.…”

“…This can come from direct consumption of adult salmon tissue and eggs (Kline et al 1990) and indirectly through increased aquatic (Wipfli et al 1998, Verspoor et al 2011) and terrestrial invertebrates in the presence of spawning salmon (Hocking et al 2013), which provide potential prey for juvenile salmonids. The presence of spawning salmon increased energy intake of juvenile rainbow trout (Scheuerell et al 2007), and coho salmon (Heintz et al 2004, Armstrong et al 2010. Furthermore, nutrients from spawning salmon have been linked to improved condition and growth rate of juvenile coho in a number of carcass addition experiments (e.g., Bilby et al 1998.…”

Spawning salmon and the phenology of emergence in stream insects

“…(iv) Direct mortality from salmon nest-digging could drive earlier emergence if late emerging individuals are killed by salmon nest-digging. Salmon nestdigging dislodges and kills benthic invertebrates (Peterson & Foote 2000;Monaghan & Milner 2009) and may make them more vulnerable to predation by stream-dwelling fishes (Scheuerell et al 2007). This nest-digging can directly control insect abundance; in a previous study that was performed in the subset of the same streams (Moore et al 2004), we found that excluding salmon from small plots in spawning areas led to a three times increase in densities of Chironomidae relative to reference plots.…”