Changing trophic structure and energy dynamics in the Northwest Atlantic: implications for Atlantic salmon feeding at West Greenland
Despite diverse population structures and management regimes, concurrent abundance declines in disparate North American and European Atlantic salmon populations suggest that conditions experienced at common marine areas may be causative. While abundance estimates of 1SW population segments (which are primarily males) have declined slightly and stabilized from peak abundance estimates in the late-1970’s/early-1980’s, 2SW population segments (typically females) have experienced dramatic declines in abundance and remain low throughout the much of the species range (approximately 48-75% of historic estimates).
In the early 1990’s, bottom-up ecosystem processes changed as a result of a region-wide regime-shift in the Northwest Atlantic where North American and southern European Atlantic salmon stocks congregate. By comparing historical (pre-1990’s regime-shift) and contemporary (post- 1990’s regime-shift) stomach content data from the West Greenland foraging grounds, we determined that Atlantic salmon are generally consuming slightly less capelin by weight, but more lower quality prey (amphipods and squid) than they were in the late 1960’s. Additionally, we show that the energy density of capelin, their primary prey item, decreased by almost 34% after the regime shift. This coincides with an approximate 66% reduction in Atlantic salmon marine productivity and suggests that destined 2SW spawners may not be able to satisfy energy demands due to changes in regional trophic dynamics compared to 1SW conspecifics with a shorter marine phase and migration. The inability of eventual 2SW females to meet energetic requirements resulting from their longer marine phase and migration may negatively affect survival over the winter, maturation, migration, and ultimately population abundance.
The influence of declining resource quality does not appear unique to Atlantic salmon, as Atlantic cod, Bluefin tuna, seabirds, marine mammals and even polar bears are either of lower body condition and/or not as productive as they once were in the region. This may partially be a response to reduced prey quality caused by changes in bottom-up processes that altered energy dynamics in the marine food-web after the 1990’s regime-shift. Determining and understanding the mechanisms that influence marine food-webs is necessary to fully evaluate survival and productivity trends, and to establish realistic management targets for commercial, recreational, and protected species.
For more information please contact Mark Renkawitz at 508-495-2122 or Mark.Renkawitz@noaa.gov.
Renkawitz, M.D., Sheehan, T.F., Dixon, H. J. and Nygaard, R. 2015. Changing trophic structure and energy flow in the Northwest Atlantic: implications for Atlantic salmon feeding at West Greenland. Marine Ecology Progress Series. Marine Ecology Progress Series. 538: 197–211. doi: 10.3354/meps11470.