Science Advisory Report 2009/075

Chinook salmon abundance levels and survival of resident killer whales

Summary

• Resident killer whales (Orcinus orca) are salmonid specialists that feed preferentially on Chinook salmon. This prey species is larger and more energy-rich than other salmonids, and is also available to the whales throughout the year.
• Chinook salmon (Oncorhynchus tshawytscha) availability appears to be important to the survival and recovery of northern and southern resident killer whale populations. An analysis of the relationship between mortality rates in resident killer whales and the Pacific Salmon Commission’s Chinook salmon abundance index over the period 1979 to 2008 revealed a strong negative correlation.
• This analysis suggests that a total PSC Chinook abundance index value above 1.1 is necessary to permit the survival and recovery of resident killer whales.
• Assuming that about 70% of the annual nutritional needs of resident killer whales is obtained through Chinook salmon, total consumption by the two populations at their current size may exceed 1,000,000 fish annually.
• Chinook salmon comprise about 90% of resident killer whale diet in Critical Habitat areas during the summer months of July-August. Based on estimated prey energy requirements and the caloric value of Chinook salmon prey, approximately 1200-1400 Chinook per day, or 67,000-81,000 Chinook over the July-August period, would be required by southern resident killer whales in their Critical Habitat (in both Canadian and U.S. waters) based on their 2008 population size. For northern residents, about 420-500 Chinook per day would be required in their Critical Habitat, or 26,000-31,000 Chinook over the two month period.
• Genetic stock identification of Chinook salmon consumed by resident killer whales indicates that stocks originating from the Fraser River system are of the greatest overall importance in their diet, particularly in Critical Habitat areas off northeastern and southeastern Vancouver Island. Local Chinook stocks from other river systems may also be of importance in certain coastal areas.
• Different Chinook salmon stocks undertake their return migrations at different times and some may be of particular seasonal importance to the whales. It is important to ensure sufficient geographic and seasonal availability of Chinook salmon, as well as adequate overall abundance, to meet the whales’ needs.

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