First rains bring spawners and bald eagles back to Vancouver Island rivers

Last fall, while the dry start to autumn put the brilliant golds, reds and oranges of tree leaves on display, my mind continuously returned to the rain. Not the rain that many dread, but the rain that floods small watersheds and raises the water levels in the rivers, bringing the salmon back en masse and with them the bears, bald eagles and biologists. In this case, I’m the biologist.

Salmon are the ecological foundation of the BC coast. They are the primary food source for many species, from apex predators like the Salish Sea’s Southern Resident orca population to tiny invertebrates like terrestrial insects. The dynamic salmon life cycle begins and ends with adult salmon migrating hundreds of kilometers to return to their natal streams to spawn.

However, the real magic happens when the post-spawning carcasses begin to decompose. Marine-derived nutrients like nitrogen and phosphorus that have accumulated in salmon tissue are deposited into watersheds, or transferred into forests by predators like bears, wolves and eagles when they drag or fly carcasses inland.

As a biologist who studies bald eagle foraging behaviour on the BC coast, my daily life, in fact my entire field season, revolves around the fall rain.

I study the role that bald eagle foraging behaviour, specifically their competition and kleptoparasitism levels (the rate that they steal things from each other), plays in this nutrient cycle. When salmon carcasses accumulate on river banks or in estuaries, eagles fight with each other over available food, which often results in winners flying their salmon to nearby ‘perch trees’. These large conifers act as their dinner table, where they consume their meal and then drop their scraps onto the ground. Over the spawning season, salmon bones and tissue accumulate at the base of the trees, which provides food for small mammals, bugs and even vegetation.¹

This transportation of nutrients from the river to the land is critical to food web dynamics and forest health. However, due to the current decline in salmon populations and corresponding decrease in carcass abundance, this cycle could be disrupted.

Therefore, this research will help determine if bald eagle foraging behaviour and the amount of nutrients they transfer into forests will diminish in the future, and if this could ultimately impact the stability of complex salmon watershed ecosystems.

As a biologist who studies bald eagle foraging behaviour on the BC coast, my daily life, in fact my entire field season, revolves around the fall rain. With the rivers and streams still dry in early October with no sign of rain, spawning salmon or bald eagles in the immediate future, I anxiously counted the weeks left in my field season. Luckily, the second week of October brought gray skies that promised precipitation. The first few days of rain encouraged the salmon that had been holding in the mouth of the river to vigorously swim upstream. With their movement came bald eagle foraging activity, and I gleefully put on my rain gear and hiked up the rivers to start my data collection.

A typical day in the field includes waking up before dawn to be at the river when eagles are indulging in their fishy breakfasts. After counting the number of eagles, I observed their kleptoparasitism rate, that’s their thievery remember, and documented their competition levels over a several hour period. Additional observations included noting the salmon carcass abundance, which varies depending on spawner abundance (rain), water flow (rain), and weather (rain).

To measure the amount of salmon nutrients being transferred into the forest by eagles, I took soil, macroinvertebrate (bugs) and vegetation samples at the base of perch trees. By analyzing the samples, the amount of marine-derived nutrients that salmon acquire while at sea can be traced all the way through the food web to terrestrial bugs and vegetation, all thanks to competitive eagles flying their bounty into the forest.

Ultimately, the data collected during last fall’s spawning events will allow us to determine how the continued decline in salmon populations will affect apex predators like bald eagles, and the role they play in food webs and nutrient cycling. The results of this study will help inform ecosystem-based management strategies for sustainably managing our salmon populations. Most importantly, further understanding the role salmon play in sustaining coastal ecosystems, from megafauna to microorganisms, will enable us to be better stewards of a keystone species.

While the fall may have had a dry and beautifully sunny start, the salmon, bald eagles and I will always welcome the gray, rainy days with excitement.

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