Moving marine-derived nutrients from the sea to the land

Salmon nutrients in terrestrial insects and plants, and the role that eagles play as nutrient vectors in coastal ecosystems.

A post-spawning chum salmon is scavenged by terrestrial predators and consumers. Photo by Kristen Walters

Across the Pacific Northwest, if you look around the estuaries of any salmon spawning rivers you will see little white heads peppering the green canvas of trees that surrounds the river’s mouth. These little specks are bald eagles, which have migrated hundreds of kilometers to opportunistically scavenge post-spawning carcasses that accumulate on riverbanks during the salmon spawning season.

An adult bald eagle sits in a tree in the sunshine.

An adult bald eagle keeps watch from a large conifer. Photo by Kristen Walters.

This mass congregation of juvenile and adult bald eagles results in competitive behaviour over available salmon carcasses. Often individuals will fight over a head, spine or chunk of a salmon, with the winner flying to a nearby conifer to consume their bounty. After eating the remaining tissue, eagles will drop the remnants on the ground, which typically includes bones and cartilage. Once on the ground, it is likely that terrestrial macroinvertebrates directly consume the remaining salmon available. As the bones and connective tissue decompose, marine-derived nutrients like carbon and nitrogen that accumulate in salmon during their lifetime may then be absorbed by the surrounding vegetation. While other studies have shown that predators like bears and wolves are critical in transporting salmon carcasses into the forest, we are uncertain on the role that bald eagles, and their foraging behaviour, play as nutrient vectors in this cross-ecosystem nutrient cycle. Determining how this cycle effects coastal food web dynamics and community processes is my current research focus, and the reason for my return to Vancouver Island.

The aim of this research is to inform salmon management strategies given the importance of allowing adults to return to their natal streams to spawn. Considering this life cycle is imperative for management agencies when setting allocations for commercial and recreational harvesting.

Last fall and winter, I spent hours and days observing this nutrient cycle first hand; starting with the competitive foraging behaviour among eagles in the estuaries, followed by collecting invertebrate samples from the base of eagle perch trees where salmon subsidies accumulated. To determine if these nutrients were moving further through the food web, I needed to return to the rivers this summer when the vegetation under the perch trees and the reference trees would be flourishing.

The rivers in June are a juxtaposition to their state in December; instead of the constant splashing that is heard as thousands of spawning salmon swim upriver, the water is quiet without a fish seen or heard. The bare-boned trees of winter are now exploding with greenery, and vegetation in the understory fills every available nook and cranny. Under the perch trees, salmon bones dropped by scavenging eagles last fall are still visible amongst the leaf litter.

I set out to conduct vegetation surveys and collect samples from nitrogen-loving species like salmonberry from both the perch trees and reference trees in all of my field sites. The vegetation surveys indicate if the species diversity within the forest varies between the perch trees and reference trees due to the presence of marine-derived nutrients. By determining if eagles act as nutrient vectors for transporting salmon subsidies from the sea to the land, we can ascertain if they contribute to larger ecosystem processes like productivity and species diversity.

Vancouver Island University students Jack and Sam assist in conducting vegetation surveys along the river bank.

Vancouver Island University students Jack and Sam assist in conducting vegetation surveys.

The aim of this research is to inform salmon management strategies given the importance of allowing adults to return to their natal streams to spawn. Considering this life cycle is imperative for management agencies when setting allocations for commercial and recreational harvesting. Due to the current decline in salmon populations, it is important that we shift our focus away from traditional fisheries practices and instead implement policies that consider the importance of ecosystem-based management and providing salmon for wildlife. Such a shift would increase the number of wild adult salmon returning to the rivers to spawn, enabling predators to transport their carcasses inland and facilitate this critical nutrient cycle.

As there are very few large predators in urban or developed areas, eagles fill this niche and may act as the primary nutrient vector. Considering this nutrient cycle and its implications in coastal food webs, this research will help inform salmon management and calls for a transition towards ecosystem-based management strategies for our Pacific salmon species. I’ll keep you posted.

Kristen Walters working with a net in the stream bed.

Kristen Walters

Kristen Walters is a Raincoast Research Associate and M.Sc candidate at Simon Fraser University. She’s here for the rain, the salmon and the eagles. klwalter@sfu.ca

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