Grizzly bear with salmon in mouth in river A\J AlternativesJournal.ca Photo © Galyna Andrushko - Fotolia.com

THROUGHOUT THE MAJESTIC COASTAL FORESTS of the Pacific Northwest, spawning salmon and bears form one of the ecosystem’s most vital, nourishing relationships. Yet their keystone connection faces enormous threats. Wild salmon must survive overzealous fisheries and sea lice-infested farms. Brown and black bears suffer from habitat loss, sport hunting and other conflicts with humans. Both face the consequences of pollution and, increasingly, global warming. When salmon return to freshwater streams to spawn, their bodies carry precious nutrients from the sea, which bears unwittingly share with countless other species. Ultimately, as the relationship between salmon and bears degrades, so too does their shared ecosystem.

Salmon = Bears

Bear populations near the coast grow with salmon availability, and can be 80 per cent greater than in interior regions. For female grizzlies, body mass and the number of cubs they have also increase with the amount of salmon they eat. A healthy condition prior to hibernation is made possible because salmon arrive in late summer and fall, when fat accumulation becomes crucial. In addition to fasting through the winter, female bears need enormous amounts of energy to undergo gestation and feed their young. Recent research also shows that when salmon-dependent male grizzlies eat less they have higher levels of cortisol, a stress hormone known to suppress immune system function in mammals.

Bioaccumulation

Salmon are responsible for 70 per cent of organochlorine pesticides (like DDT), 85 per cent of brominated flame retardants and 90 per cent of PCBs found in coastal BC’s grizzlies. After migrating to the North Pacific Ocean, salmon become vulnerable to various pollution sources. For example, DDT has been widely banned for decades, but its discovery in bears indicates that salmon absorb it in the ocean, likely from mainland Asia. During hibernation, bears don’t excrete and their fat gradually depletes, which aids bioaccumulation of persistent organic pollutants. For example, PCBs – which cause cancer and disrupt immune, reproductive, nervous and endocrine systems – can become 2.21 times more concentrated during winter, and are likely passed on to cubs through milk.

Symbiosis in the Bank

Vegetation nearest to spawning streams receives the most nitrogen from salmon captured by bears. One Alaska-based study found that 22 to 24 per cent of nitrogen in leaves of trees and shrubs near such streams were salmon-derived, which significantly increased Sitka spruce growth. Riparian forests are vital for maintaining water quality because they filter sediment, moderate temperature with shade, provide woody debris and stabilize banks. In turn, these services also benefit salmon. Erosion control, for example, reduces silt that can threaten the survival of salmon embryos, and woody debris feeds aquatic insects consumed by juvenile salmon.

Ecosystem Engineers

Bears disperse thousands of kilograms of salmon-derived nutrients in riparian and forest ecosystems. They remove up to 90 per cent of spawning salmon, often transporting them hundreds of meters away from streams. The higher the densities of fish, the more grizzlies eat and dispose; the healthier their diets, the more energy they have to carry nutrients further inland. Only half of caught salmon are generally consumed, and the remains nourish insects, amphibians, small mammals, vegetation, soil microbes and a range of birds, including eagles, crows, ravens and gulls. Along with bear urine and scat, salmon carcasses create a substantial influx of nitrogen, one of life’s building blocks.

Sport hunters have killed nearly 10,000 grizzlies in BC since 1977 – including 638 in protected areas.

Last fall, the Cohen Commission concluded that pollution, shoreline development and climate change were all factors in the Fraser River’s sockeye salmon collapse, and recommended that salmon farms around the Discovery Islands be closed because of their potential to foster devastating diseases.

Check this article out in print for additional insights and a visual representation of the entire eco-system: bears, salmon, scavengers, trees, plants and bugs! Or continue your learning by exploring the resources below.

Resources for Stressed-Out Symbiosis

Academic Research

News & Reports

 

A\J Covers 2014Like our stuff? 

A\J is a not-for-profit media organization that relies on support from readers like you to keep our content flowing.

Subscribe now or donate today to keep Canada's Environmental Voice talking.

174

Author Information

Leeza Shabekov

Leeza Shabekova is a graduate from the University of Waterloo. She is most passionate about the environment, animals, and music.