MARINE MAMMAL RESEARCH UNIT - UBC

No apparent shortage of prey for southern resident killer whales in the Salish Sea during summer

No apparent shortage of prey for southern resident killer whales in the Salish Sea during summer
Marine Mammal Research Unit

 

Researchers found four to six times more Chinook salmon in the Salish Sea during the summertime compared with the numbers of fish available to the growing population of northern resident killer whales.

A young southern resident killer whale calf (J56) carrying a dead fish between her teeth while swimming next to her mother (J31) in the Salish Sea. (photo A.W. Trites, University of British Columbia).

Despite the common belief that there are fewer Chinook salmon available during the summer for the declining southern resident killer whales to consume in the Salish Sea compared to the densities of fish available to the growing population of northern resident killer whales, scientists at the University of British Columbia and Fisheries and Oceans Canada have discovered the opposite.

In a newly published paper in the Canadian Journal of Fisheries and Aquatic Sciences, researchers report that the numbers of Chinook salmon in the Salish Sea in summertime are four to six times more abundant for southern resident killer whales than for northern resident killer whales. 

Dr. Sato checking echosounder equipment used to detect fish from onboard the research vessel. (photo A.W Trites, University of British Columbia).

“People have been talking about a prey shortage as if it’s a fact, but this is the first study to quantify and compare the amount of Chinook salmon available to both southern and northern resident killer whales,” said lead author Dr. Mei Sato, a Research Associate at the Institute for the Oceans and Fisheries at UBC at the time of the study and now an Assistant Scientist at Woods Hole Oceanographic Institution.

While the small southern resident population numbers 73 individuals (as of October 2021), the increasing northern resident population numbers around 300. In recent years, southern residents have also been returning later than normal to the inside coastal waters of British Columbia and Washington.

A southern resident killer whale swimming past a school of salmon near the mouth of the Fraser River. (photo. K. Holmes, Hakai Institute and University of British Columbia).

“Measurements from drone footage have shown the southern resident killer whales are thinner on average than the northern residents — which supports the conclusion that the southern residents are experiencing a food shortage,” said co-author Dr. Andrew Trites, a Professor and Director of the Marine Mammal Research Unit (MMRU) at the Institute for the Oceans and Fisheries at UBC. “Our findings suggest that this food shortage is probably not occurring during the summertime when they have traditionally fed in the Salish Sea.”

A southern resident killer whale feeding on Chinook salmon returning to the Fraser River. (photo A.W. Trites, University of British Columbia).

For the study, the team spent the summers of 2018 and 2019 surveying two areas known to be important foraging habitats for resident killer whales — the Johnstone Strait for northern residents and the Juan de Fuca Strait for southern residents — where killer whales intercept migrating Chinook salmon returning to the Salish Sea.

Using state-of-the-art fish finders, the team compared the number and size of Chinook salmon in the two straits.

Dr. Sato watching the prey assessment data streaming onto her laptop from the waters below the research vessel. (photo A.W. Trites, University of British Columbia)

The team collaborated with sport and commercial fisherman, the Sport Fishing Institute of British Columbia, and whale watching companies to design the survey and conduct fish sampling. The collaboration identified the best timing to conduct field work based on peak salmon availability and killer whale foraging behaviour.

“We got help from sport and commercial fisherman, chartered fishing vessels and used echosounders to detect the number and size of fish in the water”, said Dr. Sato. “Killer whales use a similar technique to locate the fish — only it’s much better than the electronic one we use because they can distinguish between different fish species in the water!”

The team also captured sample fish to validate the acoustic signals from their fish finder and distinguish between different fish species, as killer whales prefer larger, older Chinook salmon.

Verifying the type of fish detected hydro-acoustically was done by simultaneously catching fish seen on the sounder, such as this Chinook salmon.

The researchers found that while the distribution and sizes of fish were similar in both straits, there were four to six times as many fish in each patch in the Juan de Fuca Strait.

Estimating prey fields for resident killer whales is a tall order, and this is the first study to use acoustic surveys to estimate the relative abundance of large salmon in the Salish sea. Dr. Stéphane Gauthier, a scientist with Fisheries and Oceans Canada, believes that these measurements should be expanded, and that this study highlights the importance of assessing salmon distribution at sea using hydro-acoustics methods.

While the study looked at the availability of salmon, it did not assess other factors that might prevent the killer whales from catching the fish, such as higher vessel presence and noise in the Juan de Fuca Strait compared to Johnstone Strait.

Now that there is evidence that southern residents are not facing a prey shortage in their traditional summer feeding areas in the Salish Sea, Dr. Sato and Dr. Trites suggest that the focus of research efforts should be directed to other factors, areas and seasons.

“We don’t really know about prey availability outside of the Salish Sea and outside the summer season. To answer the question of whether there’s an overall prey shortage, we still need to figure out what’s happening during winter and spring, whether there are significant fluctuations year-to-year and in the different locations these whales travel to, like California,” said Dr. Sato.


The study was funded by Fisheries and Oceans Canada under research permits issued by DFO, NOAA and UBC.

 


 

PublicationsPublication


2021
 
Southern resident killer whales encounter higher prey densities than northern resident killer whales during summer.
Sato, M., A.W. Trites and S. Gauthier. 2021.
Canadian Journal of Fisheries and Aquatic Sciences, dx.doi.org/10.1139/cjfas-2020-0445
abstract
The decline of southern resident killer whales may be due to a shortage of prey, but there is little data to test this hypothesis. We compared the availability of prey (Chinook salmon) sought by southern residents in Juan de Fuca Strait during summer with the abundance and distribution of Chinook available to the much larger and growing population of northern resident killer whales feeding in Johnstone Strait. We used ship-based multifrequency echosounders to identify differences in prey fields that may explain the dynamics of these two killer whale populations. Contrary to expectations, we found that both killer whale habitats had patchy distributions of prey that did not differ in their frequencies of occurrence, nor in the size compositions of individual fish. However, the density of fish within each patch was 4-6 times higher in the southern resident killer whale habitat. These findings do not support the hypothesis that southern resident killer whales are experiencing a prey shortage in the Salish Sea during summer and suggest a combination of other factors is affecting overall foraging success.
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