Southern Resident killer whales have a special relationship with Chinook salmon; it’s their favourite food, and Chinook abundance is correlated with their health and survival. Chinook salmon can comprise between 50% and almost 100% of a Southern Resident’s diet, depending on the season.
The evolution of Resident killer whales to develop such a focussed dependency on Chinook salmon (locally called spring and king salmon) likely has some basis. One reason is its near year-round availability. Southern Residents benefit from the large size and high fat content of these fish. The large size also facilitates their meal sharing culture, as well as providing a greater caloric return from energy spent in the capture process. The seasonal migrations of Chinook returning to their spawning rivers mean they can be caught by killer whales en route in the spring, summer, fall, and even winter. With their penchant for larger Chinook, Southern Resident selectivity extends to older salmon with lengths generally greater than 74 centimetres.
Factoring the age and size of both predator and prey, and a diet consisting of 90% Chinook salmon, an individual whale would consume roughly 16-20 Chinook per day, the majority of which are greater than four years old.
The declining trend and endangered status of Southern Resident killer whales required the Canadian federal government to identify and legally designate their critical habitat in 2008. The southern parts of the Salish Sea and Swiftsure Bank are legally protected critical habitat in Canada. The presence of Southern Residents here overlaps with the seasonal migration routes of their prey, Chinook salmon, to the Fraser River and Puget Sound. The presence of Chinook is a feature of critical habitat that must be protected. Additionally, acoustic quality, and water quality are also protected features of their critical habitat.
So more than just hatched lines on a map delineating an area in the Salish Sea, critical habitat must provide adequate food, be free from pollutants, and be quiet enough to support successful foraging.
The case for including the Fraser River, with its Chinook salmon, in critical habitat
Canada’s most important source of Chinook salmon for Southern Resident killer whales lies within British Columbia’s Fraser River. In 1964, Dr. D.J. Milne at Fisheries and Oceans Canada estimated that one-third of British Columbia’s Chinook catch spawned in the Fraser watershed. Assessments over the last few decades documented 114 Chinook spawning populations within 65 tributaries that are today grouped into 17 distinct Conservation Units (19 including two transplants).
Studies on the diet of Southern Residents show they specialize on Chinook salmon from the Upper Fraser, Middle Fraser, South Thompson, and Lower Fraser regions of the watershed. These sub-regions correspond to the seasonal waves of Chinook that return to the Fraser from spring through fall. Chinook from these regions of the Fraser comprised an incredible 80%-90% of the Chinook consumed by Southern Residents from May to September between 2004-2008.
The health of Southern Resident killer whales is linked to the abundance of Fraser Chinook
Further evidence from studies that use aerial drone images (photogrammetry) to evaluate body condition of individual killer whales have linked the nutritional state of whales, specifically J pod, to the abundance of Chinook returning to the Fraser River. Poor body condition and a declining nutritional state associated with low abundance are often precursors to mortality, reinforcing the importance of strong populations of Fraser Chinook as a prey base for Southern Resident killer whales. The river is a cornerstone for the future survival of Southern Residents.
The decline in Chinook salmon abundance that killer whales face is the result of a suite of cumulative effects that are all rooted in our human footprint on the land and in the ocean. Freshwater habitat loss within Chinook rivers is a strong factor in their decline (as are fishing pressure, hatcheries, and climate change).
The loss of freshwater habitat
In the US Pacific Northwest, the consequences for salmon from freshwater habitat loss have been well documented. However, this is not the case in BC, where high-quality studies on the biological consequences of land use activities on salmon have been few, thus contributing to the narrative that changes in the marine environment are the dominant driver of the low salmon survival observed today.
But this is beginning to change. In 2021, a study by Kyle Wilson, and his colleagues at Simon Fraser University, found that logging strongly influenced the survival of three salmon species that required year-round freshwater habitat. Increases in the cumulative area logged were associated with a 97% or greater decline in freshwater productivity.
Logging is just step one in a series of sequential land use conversions that constitute ‘freshwater habitat loss’. Subsequent bulldozing, paving of the lands in a river basin, and putting watercourses in underground pipes, trigger a cascade of effects that sever the structure, function, and processes of rivers that support salmon. They unravel the water quantity, quality, and physical attributes that nurture salmon through their most vulnerable life stages.
Land use activities such as these have transformed the Lower Fraser watershed over the last 150 years and are now transforming the Fraser interior. In addition to the impacts on salmon, these changes intensify the severity of floods and droughts from climate change, having the potential to create significant societal damage and hardship. The degradation of freshwater habitat in the Fraser watershed, including water extraction for agriculture, has contributed to the threatened and endangered status of most of the Fraser River’s Chinook populations. Fourteen of the Fraser River watershed’s 17 ‘Conservation Units’ (unique Chinook populations) are considered by COSEWIC to be at risk of extinction. These fish cannot recover without functioning freshwater habitat. Salmon need rivers.
Little progress has been made to recover Fraser Chinook
Despite a decade of evidence on the importance of Chinook salmon to the health and recovery of Southern Residents, little, if any, progress has been made on managing the Fraser, or other Chinook rivers, with such importance in mind. In Canada, consideration of rivers as habitat or providing ecosystem services is rarely factored into any land use decisions. Would a legal mandate for better protection and management of the Fraser River change that?
Mapping the watersheds that feed into the critical habitat of endangered Southern Residents
This spring, Raincoast began work with a UBC sustainability scholar to examine the case for expanding SRKW critical habitat to include the Fraser River and its tributaries. The map above, created by Raincoast GIS technician Brooke Gerle, shows the Chinook rivers that drain into the range and critical habitat of Southern Residents. The three most important watersheds that once provided almost year round availability of large, old Chinook salmon were the Fraser (BC), the Columbia (BC, Washington and Oregon), and the Sacramento – San Joaquin (California).
With their designation as a component of Southern Resident killer whale critical habitat, we could be one step closer to management, conservation, and protection of these rivers that is in keeping with their critical role in maintaining the prey base and survival of critically endangered whales.
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