Scientists have gathered new insights into the dietary habits of two groups of killer whales, commonly known as orcas: the Alaska residents and the southern residents, which primarily inhabit the Salish Sea and the coasts of Washington, British Columbia, Oregon, and northern California. Both populations generally favor salmon, particularly Chinook, chum, and coho species. However, they differ in their timing of switching to different salmon species and the other types of fish they seek to add to their diets.
Researchers from the University of Washington and the National Oceanic and Atmospheric Administration (NOAA) have revealed significant information about the eating habits of resident killer whales. It has long been known that these whales prefer hunting fish, especially salmon. However, some populations are thriving while others are having a harder time surviving. Scientists are keen to understand how dietary choices influence these varying outcomes.
Amy Van Cise, an assistant professor at the University of Washington’s aquatic and fishery sciences department, emphasized the intelligence of killer whales. She noted that these orcas learn foraging techniques from their matriarchs, who are aware of the richest prey spots in their areas. “Our goal was to discover if social learning influences dietary preferences in various resident killer whale populations or among different pods,” she explained.
In a study published on September 18 in the journal Royal Society Open Science, Van Cise and her team detailed the food preferences of the Alaska residents and southern residents. Both groups mainly prefer salmon, particularly Chinook, chum, and coho, but they differ in the timing of when they hunt and consume various salmon types, as well as the other fish they catch for dietary variety.
Southern resident killer whales are critically endangered, while other populations are increasing. This study aims to support conservation strategies for these orcas along the coast from northern California to the Gulf of Alaska.
“Food scarcity is a major threat to the endangered southern resident killer whales,” said Van Cise. “By comparing their diet with that of a thriving population, we hope to gain insights into how we can protect this vulnerable group.”
Historically, salmon levels in the rivers of Alaska, British Columbia, and the Pacific Northwest have provided ample food for resident killer whales. Unfortunately, human activities have disrupted this food source through pollution, dam construction that reduces salmon runs, and noise pollution that hampers their hunting abilities. In addition, during the latter part of the 20th century, many resident killer whales, especially the southern residents, were captured for marine parks, disrupting their social structures and further diminishing their populations.
The negative impacts of human activities are evident. While the Alaska resident killer whale population is in the thousands and the northern residents are steadily increasing, the southern resident population has stagnated at about 75 individuals, with recent research linking noise pollution from cargo vessels and higher pregnancy failure rates to their decline.
The research team collected fecal samples from both southern and Alaska resident killer whales from 2011 to 2021, analyzing the DNA in these samples to identify their diets. They found that the summer months reveal a higher consumption of chum and coho salmon among Alaska residents, while southern residents primarily eat Chinook salmon during this time.
“Chinook salmon are vital for all resident killer whale populations due to their size and nutritional value,” stated Van Cise. “However, our findings indicate that consistent access to other fish like chum and coho salmon, or even flatfishes like arrowtooth flounder, is crucial for the health and survival of this population.”
In recent years, the team has gathered more fecal samples outside of summer, revealing a surprisingly varied diet among resident killer whales. They found that these whales also consume sablefish, arrowtooth flounder, lingcod, Pacific halibut, and big skate, which contradicts the assumption that they only eat salmon. The two populations have different non-salmon preferences and switch between species at varying times, reflecting a balance between the local availability of fish and the matriarchs’ knowledge of reliable hunting grounds.
“The survival of her family depends on the reliability of the foraging sites she knows each year,” Van Cise noted.
Both in the U.S. and Canada, resident killer whales have gained public attention, especially concerning the plight of southern residents. The research team believes their findings and future dietary investigations are critical to supporting these whales’ recovery efforts.
“While it’s essential to protect key Chinook salmon populations to aid the recovery of the endangered southern resident killer whales, we have learned from this study that we also need to consider the entire ecosystem of fish that make up their diet,” concluded Van Cise.
The co-authors of the study include M. Bradley Hanson, Candice Emmons, and Kim Parsons from NOAA’s Northwest Fishery Science Center, Dan Olsen and Craig Matkin from the North Gulf Oceanic Society in Alaska, and Abigail Wells from Lynker Technologies. Funding for the research was provided by the North Gulf Oceanic Society, the National Fish and Wildlife Federation, Shell, SeaWorld, NOAA, the Exxon Valdez Trustee Council, and the U.S. Marine Mammal Commission.
