By Caitlin Birdsall
When out for authentic ‘west coast’ cuisine, diners around British Columbia often decide easily on one dish: salmon. Whether barbequed, smoked, baked or raw, BC salmon has a well-deserved reputation for being a delicious meal. However, it is not just humans that are after the iconic fish. Resident killer whales are also salmon specialists.
While studying killer whale diet, researchers Graeme Ellis and Dr. John Ford revealed that 96% of their prey is salmonids (Ellis and Ford 2006). More specifically, while a small amount of all six species of Pacific Salmon were found to be part of their diet, the whales seem to preferentially select just one species, the chinook salmon. Ellis and Ford found that 72% of the salmon consumed by resident killer whales is chinook, often refered to as ‘spring’ or ‘king’ salmon. Interestingly, chinook is one of the least abundant species in the north east Pacific, making them harder to find, but are probably favored by the whales due to their large size, high fat content and year round availability within the range of resident killer whales. The whales’ preference for chinook is so great that they continue to selectively forage for them in areas and at times of the year that other, smaller species are much more abundant. Unfortunately, the preference for chinook may be bad news for resident killer whales.
A study published this January in the journal Environmental Toxicology and Chemistry reveals that chinook salmon are highly polluted with persistent organic pollutants (POPs), toxins that are passed on to their predators, the resident killer whales (Cullon et al 2009).
The study, authored by Donna Cullon of Fisheries and Oceans Canada and the University of Victoria with several colleagues in both Canada and the US, described the abundance of POPs in chinook salmon and investigated where the salmon were being exposed to these contaminants. The goal was to explain the difference in POP accumulation in the two populations of resident killer whales in the north east Pacific. While both populations have the same preference for chinook salmon, the southern residents killer whales, who spend the summer months around the Puget Sound area of Washington and in the southern Strait of Georgia in British Columbia, have accumulated toxin levels four times higher than the northern resident population, found along the central and northern coasts of British Columbia during the summer and fall.
POPs include flame retardants (such as PCBs and PBDEs), organochlorine (OC) pesticides and industrial chemicals. These chemicals do not break down in the environment and instead accumulate up the food chain, concentrating further at each trophic level. By testing chinook smolts and adults the authors revealed that 97-99% of the POP load is acquired while the fish are at sea as older animals, as opposed to their riverine habitats early in life. They also tested adult chinook from various areas along the coastline to compare levels of toxin accumulation. Chinook salmon tested in the range of southern resident killer whales were considerably more contaminated than fish found in northern waters. As an example, polycholorinated biphenyls (PCBs), a toxin known to affect reproductive and immune function in mammals and just one of the many toxins considered in this study, was four times higher in adult chinook sampled in southern waters than those in northern areas.
The difference in contaminant levels may be a result of chinook migration patterns once they reach the oceanic environment. The majority of the fish sampled in the Johnstone Strait (northern Vancouver Island) are from stocks that migrate to the northern water of British Columbia and into the Gulf of Alaska during their oceanic years. Comparatively, Deschutes River chinook found in the south are known to reside year round in the Puget Sound, a more polluted environment.
Alarmingly, on top of being more toxic, the authors also discovered that southern resident killer whales likely need to eat many more of the polluted chinook than their northern counterparts. The chinook salmon sampled in the southern resident killer whales range was considerably less fatty than the salmon tested in the north. As salmon migrate towards their natal streams, the amount of lipids (fats) in their body decreases. Southern resident killer whales likely intercept and feed on chinook salmon closer to the end of their migration, when their lipid content is at it’s lowest. As a result, the whales need to consume more salmon to compensate for the lack of fat in their fish. This further increases the amount of PCBs consumed by southern resident killer whales; not only is the chinook in their range more contaminated, but they need to eat more of it. The authors estimate that through this ‘nutritionally adjusted scenario’ southern resident killer whales are exposed to 6.6 times more PCBs than their northern counterparts.
All of these findings are scary news for the southern resident killer whale population, already listed as ‘endangered’ by Canada and the United States. With only 83 animals left in the population, and multiple threats affecting them, the need to study toxins in the marine food web and protect our waters becomes obviously evident. These toxins aren’t just affecting whales, humans are consuming the same fish. For the health of the whales AND humans, citizens around the world can help reduce new toxins entering the ocean by:
– Engaging in ‘whale friendly’ gardening. Avoid chemical pesticides and fertilizers that run off gardens and into the water system.
– Choose PBDE free when purchasing furniture and electronics: find out which companies have banned these persistent organic pollutants here.
– Go organic and local! Choose foods and other products that have been produced without pesticides and haven’t travelled from miles away.
– Think before you buy: do you need it? High consumerism increases the spread of manufacturing contaminants.
The public can further help scientists learn which areas resident killer whales are frequenting and foraging in by reporting all sightings here.
Learn more:
Cullon, D.L., M.B. Yunker, C. Alleyne, N.J. Dangerfield, S. O’Neill, M.J. Whiticar, P.S. Ross (2009) Persistent organic pollutant in chinook salmon (Oncorhynchus tshawytscha): Implications for resident killer whales of British Columbia and adjacent waters. Environmental Toxicology and Chemistry 28(1): 148-161
Ford, J.K.B and G.M. Ellis (2006) Prey selection and food sharing by fish eating killer whales (Orcinus orca) in British Columbia. Marine Ecology Progress Series 319: 185-199. Abstract
Persistant Organic Pollutants
Discover PBDEs, a rising POP
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