Inferring salmon population dynamics and fishing impacts at multiple scales using sediment nitrogen isotopes

Background/Question/Methods

Salmon fisheries in Alaska and much of British Columbia are largely seen as examples of successful management for sustainability. Recent discussions on salmon management have nonetheless argued that the sustainability of salmon is fleeting because fisheries intercept large amounts of salmon-derived nutrients moving from the ocean to inland ecosystems.  Salmon-nutrients have been found in a wide variety of inland producers and consumers, from algae and trees to bears and insects, yet the importance of salmon-derived nutrients to ecosystem functioning remains unclear.  This is in part because few studies have put their results in a long-term context incorporating both natural population variability and climate dynamics.  Because salmon are highly enriched in ¹⁵N relative to ¹⁴N, it is possible to use paleolimnology tools and lake sediment nitrogen isotopes to infer salmon abundance before the onset of direct historical records.  I will present a comprehensive synthesis new and published research, based on lake sediment δ¹⁵N, fossil algal pigments, and recent fisheries records from coastal North Pacific coastal lakes, that frames salmon sustainability in the context of allochthonous nutrient loading and salmon population dynamics over the last 500 years.

Results/Conclusions

In 14 of 18 western Alaska lakes with long-term reconstructions of salmon abundance, there was a sharp reduction in the number of salmon returning to spawn since the onset of industrialized fishing (range 32-67%).  The broader survey of recent catch and escapement records shows that fisheries intercept on average 61±16% of salmon-derived nitrogen and phosphorus before they reach inland ecosystems. Coincident with the loss of salmon-derived nutrients there has also been a significant decrease in algal pigments indicating a reduction in primary productivity within these oligotrophic ecosystems.  Pre-fishing salmon reconstructions suggest natural salmon population fluctuations of similar or greater magnitude to the current fisheries-induced declines, and multiple periods of low spawning abundance.  Since the 1977 regime shift associated with the Pacific Decadal Oscillation, total salmon returns have generally exceeded average pre-fishing returns.  Therefore, despite the fact that the removal of most of the nutrients once imported by salmon has led to measureable ecosystem-level changes, there is no evidence this outside the range of normal salmon variability or impacting the long-term sustainability of the resource.