Picture a salmon steak, maybe marinated in olive oil and spices, grilled just long enough to crisp the edges, served with roasted asparagus and wild rice – that’s a dinner I would be happy to serve (and eat).
Salmon have long been a key source of protein for the people living in the Pacific Northwest and Alaska, in addition to being culturally and economically significant. Yearly runs of salmon returning from the ocean to reproduce in the freshwater rivers and streams where they were born represent a pulse of nutritiously rich food that supports river ecosystems as well as people – adult salmon die after they spawn, releasing nutrients incorporated from the ocean into the freshwater systems where the fish originated.
In rivers, the first recipients of this marine bounty are the macroinvertebrates that colonize and consume salmon after they’ve spawned (and which will eventually serve as food for the juvenile fish that hatch from the salmon eggs). As reported recently in the journal Aquatic Sciences, a team of scientists from the University of Notre Dame investigated how the precise location of salmon carcasses influences the macroinvertebrates that devour them.
The researchers, working in streams on Prince of Wales Island, Alaska, set out salmon steaks in four different areas of each stream – buried in the streambed, resting on top of it, lying out of the water on a gravel bar, and placed above the water among the trees next to it. They found that different types of insects were present in the four habitats – as they expected, terrestrial locations (the gravel bar and the riverbank) featured macroinvertebrates that specialize in consuming carrion, while underwater locations were dominated by generalist consumers, aquatic insects that were likely already living in those locations and opportunistically consumed the salmon steaks.
Despite high rates of decomposition, the scientists also found that, by the end of their experiment (two weeks in some locations, four weeks in others), the remaining salmon was still high in nutrients. They interpret this to mean “that salmon carcasses may provide a resource supporting a succession of consumers over an extended period of time.” In other words, the pulse of energy that a salmon run brings to a river may be less of a spike and more of a slow release, sustaining different types of organisms as it plays out.
Salmon are more than a source of fillets and steaks for human plates – they also nourish the river ecosystems where they’re born by returning there to spawn at the end of their lives.