New research shows the influence of salmon abundance on the movement of bald eagles

The effect of salmon carcass availability on eagle distribution across multiple rivers was previously not well understood.

Bald eagles are mobile and opportunistic scavengers that are often viewed as ubiquitous across the coast of British Columbia where spawning salmon occur. Yet, their movements on a finer scale, and the effect that salmon carcass abundance has on this distribution, was previously not well understood. 

Lead author of the paper, Kristen Walters, Raincoast’s Lower Fraser Salmon Conservation Program Coordinator, and researchers from Simon Fraser University examined the distribution of bald eagles in relation to salmon carcass availability across four rivers on Vancouver Island. The team found that river or estuarine areas with few salmon carcasses attracted slightly more eagles than expected compared to areas with more carcasses. This result varies from that predicted by ‘Ideal Free Distribution’, which is a foundational theory of behavioural ecology that predicts individuals distribute on a 1:1 basis with prey availability. 

K.E. Walters, J.D. Reynolds, and R.C. Ydenberg. Ideal free eagles: Bald Eagle (Haliaeetus leucocephalus) distribution in relation to Pacific salmon (Oncorhynchus spp.) availability on four spawning rivers. Canadian Journal of Zoology99(9): 792-800. https://doi.org/10.1139/cjz-2020-0191

“Ideal free eagles: Bald eagle (Haliaeetus leucocephalus) distribution in relation to Pacific salmon (Oncorhynchus spp.) availability on four spawning rivers” was published recently in the Canadian Journal of Zoology. Ron Ydenberg and John Reynolds contributed to this work. 

This result could be explained by differences in eagle competitive ability, which occurs between sexes and age classes, so that some individuals are able to monopolize carcasses or sites, forcing others to poorer field sites and skewing the distribution away from the 1:1 ratio. Alternatively, despite having excellent eyesight, it is possible eagles are not able to make adequate comparisons of prey across the field sites. 

The researchers also determined that there were more eagles in river estuaries compared to forested field sites located in upper reaches of rivers. It is possible that the physical characteristics of rivers are important for explaining variation within and among rivers in terms of abundance of eagles relative to salmon. As river estuaries often feature mudflats and sandbars, which act to strand salmon carcasses when they wash downriver, they provide an excellent foraging opportunity for eagles when the sandbars and carcasses are exposed during low tide.

“As Pacific salmon abundance continues to fluctuate, it is important for those with decision-making influence to consider the important role spawning salmon play in supporting a diverse range of wildlife species, including bald eagles. We will use the results of this research to inform Raincoast’s work on salmon for wildlife, which is identifying pathways to shift towards ecosystem-based management of our salmon populations”. 

Abstract

The movement of individuals according to the availability of resources has a fundamental effect on animal distributions. In the Pacific Northwest, Bald Eagles (Haliaeetus leucocephalus (Linnaeus, 1766)) rely heavily on scavenging opportunities during the non-breeding period, and their distribution and movements are thought to be strongly influenced by the availability of post-spawning Pacific salmon (genus Oncorhynchus Suckley, 1861) carcasses. We surveyed the abundance of eagles and salmon on four adjacent rivers on Vancouver Island, British Columbia, Canada, during the 2017 fall spawning season. Salmon began to arrive in late September, peaked in abundance in mid-November, and were absent after early December. The seasonal progression of Bald Eagle abundance matched that of salmon carcass availability. The slope of proportional eagle–salmon relationship was significantly positive, though lower than the 1:1 match predicted by Ideal Free Distribution theory. The numerical response of Bald Eagles to salmon abundance was elevated on one of the rivers, potentially due to physical features such as sandbars and mudflats that increased the availability of carcasses and provided access points for eagles.

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Figure 2

Figure 2 shows abundance of eagles and salmon by observations cycles.
Abundance of Bald Eagles (Haliaeetus leucocephalus) and Pacific salmon (Oncorhynchus spp.) over the 13 observation cycles, with 95% CI indicated by shading. Each point is the summed count from the upper and lower field sites on each of the four rivers. N = 52 observations (4 rivers  1 estimate for salmon and eagles  13 observation cycles). Points are jittered for clarity.

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