Patterns in marine trophic ecology of sockeye salmon over time and within/between populations

Broad-scale shifts in climate during the 20th century, most notably a shift in the Pacific Decadal Oscillation in 1977, had large effects on the ecology of the North Pacific Ocean, including a substantial change in the composition of the dominant food web. Production of Alaskan salmon stocks increased with this shift and has since been consistently high through recent years. 

The biological mechanisms for this increase in production remain unclear, but one potential cause might be a change in marine trophic ecology or foraging patterns of sockeye.  To test this theory, we analyzed carbon and nitrogen stable isotopes of archived sockeye salmon scales collected from 8 rivers in Bristol Bay between 1964 and 2003.  Isotope values for all Bristol Bay sockeye combined were remarkably stable over the study despite substantial changes in salmon production and oceanographic conditions in this region, indicating no major shifts in the types of prey preferred by salmon. Our results also suggest river-specific patterns in the variation of stable isotopes through time; stable isotope changes were related to stock identity and showed some geographic organization. Larger salmon tended to have depleted δ15N and δ13C, indicating foraging at lower trophic levels.

The result that larger salmon foraged at lower trophic levels was interesting and unexpected. We built upon this result by again using carbon and nitrogen stable isotopes to examine whether marine foraging tactics in sockeye are linked to their ultimate freshwater life history as adults (i.e. habitat or age). Our results indicate that adults from large-bodied populations spawning in deep freshwater habitats forage at higher trophic levels (or in different habitats) than small-bodied populations from shallow streams.  Within populations, younger individuals had higher δ15N than older fish, indicating that younger salmon eat at higher trophic levels (or in different habitats) than older fish. These results further indicate that the trophic diversity of sockeye salmon in the ocean may be linked to trade-offs in ecological and evolutionary constraints they eventually experience as adults in freshwater ecosystems.

Johnson and Schindler. 2012. Four decades of foraging history: Stock specific variation in the carbon and nitrogen stable isotope signatures of Alaskan sockeye salmon. Marine Ecology Progress Series 460: 155-167. DOI:10.3354/meps09772.

Johnson and Schindler. 2013. Marine trophic diversity in an anadromous fish is linked to its life-history variation in fresh water. Biology Letters. 9(1). Article Number: 20120824.

Trophic ecology of trout and char

It is well established that in salmon-bearing ecosystems, salmon are a crucial component of the food web.  Our program has devoted substantial effort to expanding upon the paradigm that resident fishes, such as Arctic char, dolly varden, rainbow trout, and Arctic grayling preferentially consume salmon eggs and tissue when available. 

In Lake Iliamna specifically, there are numerous small populations of Arctic char, Dolly varden, and rainbow trout that exhibit various life history strategies (anadromy vs. non-anadromy, lake vs. stream residents, etc). In this system, or research has generally focused on comparing and contrasting the trophic ecology of these different ecotypes.  Using both stomach content and stable isotope analysis, we know that both stream and lake-type char species and rainbow trout consume similar diets prior to arrival of salmon (in general, stream fish eat aquatic insects, while lake fish eat benthic snails and amphipods).  Once salmon arrive, the diet of both ecotypes of both species shift to predominantly salmon eggs and tissue.  Additionally, small populations of resident dolly varden and char will move between locations in search of salmon resources.  And while consumption rates of salmon resources is similar between among-species resident fish populations, growth rates vary, indicating the underlying importance of the role of environmental and genetic variation in determining the ultimate growth benefit to consumers.

Denton, K.P., H.B. Rich, Jr., and T.P. Quinn. 2009. Diet, movement, and growth of non-anadromous Dolly Varden Salvelinus malma in response to sockeye salmon Oncorhynchus nerka subsidies. Transactions of the American Fisheries Society 138: 1207-1219. DOI: 10.1577/T09-006.1

Denton, K.P., H.B. Rich, Jr., J.W. Moore, and T.P. Quinn. 2010. The utilization of a Pacific salmon Oncorhynchus nerka subsidy by three populations of charr Salvelinus spp. Journal of Fish Biology 77: 1006-1023. doi:10.1111/j.1095-8649.2010.02746.x

Arostegui, M.C. and T.P. Quinn. 2018. Trophic ecology of nonanadromous rainbow trout in a postglacial lake system: partial convergence of adfluvial and fluvial forms. Canadian Journal of Zoology 96: 818-827.

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