Scientists have known for years that Chinook salmon are important to southern resident orcas, but Chinook are not the only fish the whales eat. At the moment, chum salmon are returning to Puget Sound, and recent orca sightings suggest that the whales may now be feeding on chum.
Harbor seals also eat Chinook salmon, but also chum, coho and other fish. They seem fond of smaller fish like herring and juvenile salmon. Oh, what a tangled food web we weave… Can we really say that seals are stealing the lunch from killer whales?
Southern resident orcas are considered endangered. Puget Sound Chinook and steelhead are threatened. Harbor seals seem to be everywhere, hardly struggling to find food, at least as far as anyone can tell. So is it time to bring the powerful influence of humans into the equation by forcefully reducing the harbor seal population in Puget Sound?
It’s a question that people have been pondering for years, but I’m not sure we’re much closer to an answer. A new report, which I will discuss, offers some options for the Salish Sea.
Meanwhile, a recent permit will allow more than 700 salmon-eating sea lions to be killed on the Columbia River, but that has nothing to do with Puget Sound. Before addressing the problem of seals in the inland waterway, some key questions need to be answered, as discussed in a story I wrote last month for the Encyclopedia of Puget Sound.
Some of the most important questions surround how much salmon the seals are actually eating and how they fit into the complex food web that involves all kinds of fish and marine mammals. We can’t forget, for example, that transient killer whales eat a fair number of harbor seals, so it’s not a one-way street.
A recent study examined the bones from harbor seals that died years ago to determine if today’s seals are eating higher or lower on the food web. It’s a fascinating study involving stable isotopes from amino acids found in the bones. I believe I was able to explain simply enough the basic techniques. See Encyclopedia of Puget Sound, Sept. 8, 2020.
On the experimental front, a new acoustic device is being tested as a deterrence for harbor seals and California sea lions that have been feasting on threatened salmon and steelhead coming through the Ballard Locks on their way into Lake Washington.
The device mimics the sound of a killer whale slapping the water with its tail. The idea is to startle the fish-eating pinnipeds and move them away from the fish ladder, where they often pick off fish trying to make it over the dam. For details, listen to the story by KUOW reporter Eilis O’Neill, or check out the news release from Long Live the Kings, one of several organizations partnering in the project.
A new report released in September offers a list of actions that could be taken to reduce seal and sea lion predation in the Salish Sea. The technical report (PDF 4.4 mb) summarizes the discussions from a November workshop attended by 75 U.S. and Canadian experts.
Author M. Kurtis Trzcinski of the University of British Columbia divides the suggestions into four categories:
Vary hatchery production:
Salmon and steelhead hatcheries should experiment with releasing young fish all at once or over longer periods of time to see what is most effective at reducing seal predation. Larger releases might “flood the predator field” so that more of the fish get away. Fewer fish coming out of a hatchery at any one time might attract less attention and increase survival.
One could also change the release location to see if there are places where the hatchery fish have a better chance of surviving. One could also hold the fish for longer or shorter times in the hatchery to see whether larger fish survive better or worse than smaller ones.
Another idea related to hatcheries is to produce forage fish, such as herring, with the idea that an abundance of forage fish might provide an alternate prey for seals and sea lions, thus reducing predation on salmon.
Enhance fish survival
Leaving aside seals and sea lions, these ideas relate to habitat efforts to increase survival of salmon and steelhead in the streams and estuaries. Improving stream flow and assuring proper temperatures could be critical factors, along with enhancing habitat for better food and protection for the growing fish.
Enhancing habitat to increase survival of other species, such as forage fish, could help with salmon and steelhead survival.
Discouraging seals and sea lions from eating salmon and steelhead could take the form of harassment, removing or relocating haul-out areas, or requiring marinas to build structures to keep pinnipeds off docks and floats.
Harassment with noise or physical disruption could be scheduled at key times, such as during salmon out-migration or return to the streams. But workshop participants gave the idea a low chance of success.
Preventing seals from hauling out, especially near salmon migration routes, might work in one area, but it probably would move the animals to another location with uncertain effects.
Another idea was to inject the animals with a contraception to control the population, although a project involving the handling of thousands of seals and sea lions would be immense.
Killing seals and sea lions could be accomplished through hunting, which would require the hunters to use the animal for food or other purposes, or culling, which means killing the animal for the sole purpose of reducing the population.
Some experts proposed running an experiment by reducing the population through culling and then measure the effects on fish populations. Others suggested removing all the seals in one area and comparing the effects to a similar area where seals were not removed.
Preliminary estimates say it would take the lethal removal of 50 percent of the harbor seals — or about 20,000 animals in the Salish Sea — to push Chinook and coho salmon toward recovery. In addition, about 3,000 animals would need to be killed every year to maintain a stable population.
Uncertainty of such actions is high. Some say that other predators might need to be removed as well to keep them from simply eating the fish saved by eliminating seals. Birds, otters, raccoons and large fish are among the predators that could become a concern.
Officials in both the U.S. and Canada are considering their next steps, including an action plan that would probably include research to improve our understanding of the food web.
Related articles from the Encyclopedia of Puget Sound: