A sealion with a fish in its mouth.

A scientific report published in the journal Nature in 2017, “Competing trade-offs between increasing marine mammal predation and fisheries harvest of chinook salmon” is one of a number of ongoing debates about whether it’s feasible to support the recovery of salmon-eating marine mammals, and have a viable salmon fishery; others go as far as to advocate for “removal” of some of the predators (sea lions and seals) to increase fisheries catch.

As Southern Resident Killer Whales are dependent on salmon, we thought it was worth a “deeper dive.” There is a problem, no one can dispute that. But is the problem over-consumption of salmon by marine mammals, or is it decades of habitat destruction, overfishing, and mismanagement of Pacific salmon, particularly Chinook? Let’s first go back in time to see how we’ve gotten to where we are today.

In the 1800s, naturalist and whaling captain Charles Scammon reported, such an abundance of whales in California that some said you could walk across Monterey Bay without getting your feet wet! Clearly, at this time, the ocean could support many more marine mammals than today, unfortunately also leading to the poor assumption that the bounty of the ocean was limitless, and could, therefore, withstand “extraction.”

What those early whalers could not have conceived was the technological advances that would follow, enabling commercial whaling on an industrial scale; factory fishing fleets at sea for weeks, scouring the depths of our oceans with sonar, no place a refuge. As we started to find the bottom of this bottomless supply, we needed to remove competitors.

In the 1970s, campaigns such as the Save the Whale movement lead to governments enacting legislation to ban the hunting of marine mammals. This protection coincided with new legislation to improve air and water quality. Our aquatic ecosystems were severely damaged, with decades of industrial chemical and toxic waste dumping in our rivers and oceans— based on the notion that the solution to pollution is dilution.

It’s a testament to the resilience of nature, that populations of seals and sea lions were able to bounce back at all considering the harm. But bounce back they did, and within 30 years, some considered their populations in the Pacific almost back to pre-hunting levels.

Unlike seals, Pacific salmon have not recovered, either from overfishing or from decades of habitat destruction, damming of rivers, logging, and pollution. As previously discussed in our “deeper dive” into hatcheries, bridging the gap by raising and releasing billions of hatchery-raised fish, is not increasing the fishing catch. So, what is the catch? It turns out, adding fish to a system that cannot support its wild counterparts is not the solution!

So here we find ourselves today, with government scientists making models to estimate the number of salmon being removed by salmons’ natural predators.

An early drawing of killer whales.

As you can appreciate, advocates for removing salmon predators are not considering a wild, fully-stocked natural ecosystem, capable of feeding the marine life abundance “enjoyed” by Scammon and his crew. But instead, one that has been unnaturally transformed such that it can no longer support current populations—Exhibit A—Southern Resident Killer Whales. And yet we do not look to our own past actions, we seek to lay the blame on nature itself, sometimes even endangered whales!

It should be noted that a study of harbor seals in the Salish Sea showed that they’re not totally dependent on salmon—hake, pollock, and herring are about 80% of their diet, with salmon perhaps less than 10%. Though, as we previously noted, hatchery salmon at release are smaller than wild salmon of the same age, thus increasing the likelihood of seals eating them, as the hatchery size is an ideal seal ready-meal. By deviating from nature’s selection, we are exacerbating the problem we are now seeking to fix by “removing” seals!

This is not to discount the collective consumption of Pacific salmon by seals. Even if salmon is a small percentage of their diet when eaten by a lot of seals it totals a large number of salmon. But in a healthy ecosystem, a predator cannot out-consume its prey if it’s to avoid extinction. The population size of predators is controlled by the amount of food in their habitat, and the population size of the prey is controlled by predators—something biologists refer to as carrying capacity—the maximum an environment can sustain indefinitely. Seals in the Salish Sea have reached this limit, and their population is controlled by Bigg’s killer whales (the mammal eaters of the Salish Sea, also known as Transients).

Predators play a huge role in managing the health of the ecosystem. Sick or weak prey are the easiest to catch, thus ensuring that the fittest are the ones to survive to reproduce. Seals and killer whales provide a service to the ecosystem not easily mimicked by humans.

Continued below...

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Harbor seals’ preference for pollock actually benefits salmon populations, as pollock are predators of young salmon. A reduction in pollock stock is also increasing herring in the Salish Sea, and this benefits the entire ecosystem. With the restoration of seals, we can start to see a natural re-wilding of the Salish Sea with the welcome return of herring specialists such as humpback and minke whales. Though some may even consider the return of whales as a threat to fisheries, in fact, whales provide benefits, acting as an ocean fertilizer, and transporting important nutrients into the Salish Sea, vital to species at the bottom of the food web, supporting predator populations, including prized fisheries species, such as salmon.

What is certain is that removing marine mammals will not increase the fisheries catch. The root cause of population declines in wild salmon must be resolved with the restoration of rivers and other salmon important habitat. Continuing to release hatchery fish cannot and will not restore salmon populations to previous historic levels, and without top predators, it’s likely the catch will continue to fall, as ultimately the release of thousands of additional hungry hatchery mouths could destabilize the entire ecosystem until there is no viable fish catch even for humans. Who wins then?