Cetacean food supply is a complex issue, influenced by multiple factors including overfishing, climate change, underwater noise, and human activity.
The interconnected nature of the marine ecosystem means that even small changes can have pronounced effects on cetacean health, prey availability, and foraging success.
Prey sampling studies by Dr. John Ford and Graeme Ellis have determined that resident killer whale diets are made up of at least 96% salmonids (Ford and Ellis 2006). Of that, 72% was Chinook salmon. The whales’ preference for chinook salmon is likely due to this species’ relatively large size, high fat content, and unlike other types of salmon, they are present year-round in the whales’ range (Ford & Ellis 2006). A further study in 2009 by Dr. John Ford and colleagues found evidence that resident killer whales not only prefer Chinook salmon, but depend on them. They showed that periods of killer whale population decline were primarily due to unusually high mortality rates in all ages of northern and southern resident killer whales. This mortality across age classes was particularly evident in the mid-1990s, when Chinook abundance remained well below the average until 2002-2003. During this time, the southern resident population dropped by 17% and the northern residents by 8%.
The link between resident killer whales and chinook salmon is strong, but took many years of research to understand. In contrast, the importance of other prey species to the survival of other cetaceans is not well known. Suboptimal prey availability may work in tandem with other issues. For example, grey whale population in the eastern Pacific showed a marked decline in the early 2000s. This may have resulted from the population reaching a carrying capacity combined with a decline in their prey in the Bering Sea feeding grounds.
Prey reduction for at-risk cetaceans either through competition with fisheries, climate change or other factors is a serious concern. While many animals may not die specifically from ‘starvation’, low prey availability may result in ‘nutritional stress’ – and this can make them prone to illness, infection, and the affects of contaminants.
The diet of northern resident killer whales. Chinook salmon is consistently preferred by all three clans.
Scale samples collected after a feeding event. From theses scales, scientists can identify which species of fish was being preyed upon.
What can you do to help?
To help protect healthy fish populations, ensure your seafood is sustainably caught. Sustainable seafood can be defined as species that are caught or farmed in a way that ensures the long-term health and stability of that species, as well as the greater marine ecosystem.
Use the Oceanwise Program
Follow Fishing Regulations at All Times
Follow the SeaChoice Program
Become A Streamkeeper
References
- Ford, J.K.B., G.M. Ellis. 2006. Selective foraging by fish-eating killer whales (Orcinus orca) in British Columbia. Marine Ecology Progress Series 316:185-199.
- Ford, J.K.B., G.M. Ellis, P.F. Olesiuk. 2005. Linking prey and population dynamics: did food limitation cause recent declines of ‘resident’ killer whales (Orcinus orca) in British Columbia? Canadian Science Advisory Secretariat Research Document 2005/042. 27pp. 2005.
- Ford, J.K.B., G.M. Ellis, P.F. Olesiuk, K.C. Balcomb. 2009. Linking killer whale survival and prey abundance: food limitation in the oceans’ apex predator? Biology Letters. 6(1): 139–142.
