SEAL CULL COULD THROW FOOD WEB OFF BALANCE
Harbour seals serve as easy scapegoats for Chinook salmon decline, write Peter Ross and Lance Barrett-Lennard.
Explaining the declining numbers of iconic Chinook salmon is more complicated than one might think, and harbour seals have been increasingly put forth by some as the primary culprit.
Sure, seals eat salmon. But food webs are complicated, and it is easy to gloss over the positive roles that predators play in contributing to healthy and productive coastal ecosystems. Declining salmon abundance is the result of a complex variety of factors, and cannot be solely attributed to harbour seals. In the case of declining numbers of vulnerable Chinook salmon, threats include warming ocean and freshwater temperatures, destruction and alteration of stream and estuary habitats, fishing pressures, pollution, and the salmon’s starring role as prey for other fish, seals, sea lions, birds, bears, dolphins, porpoises and whales — including critically endangered Southern Resident killer whales.
It’s partly out of concern for the latter sparking recent calls for a cull of harbour seals, with those in favour citing a recent explosion in the seal population as principal cause of the decline of Chinook salmon. “Explosion?” Yikes. This is serious, and we had better respond. But, hold on a minute — there has been virtually no change in seal numbers in B.C. in more than 20 years. But for the whales — which face additional threats that include vessel strikes, pollution, underwater noise, and a shrinking gene pool — the problem is, as usual, us humans.
It was also people who almost eliminated harbour seals from B.C. in the 20th century. From 1879-1968, an estimated half a million seals were killed in B.C. for the commercial fur trade and for predator control. Their numbers plummeted to fewer than 10,000 in the 1960s; less than one-tenth of their pre-exploitation numbers. They did have an impressive recovery in the two decades following, increasing exponentially during the ’70s and ’80s, beginning to slow in the 1990s, and have been relatively stable — at about 110,000 animals — in the two decades since. The so-called “explosion” was a conservation success story that took place largely in the 1970s and 1980s, pretty old numbers to cite in today’s news.
With a successful recovery of harbour seals, more interactions with fisheries come naturally compared to several decades ago. Harbour seals are opportunistic predators that feed on bite-sized prey that are abundant or accessible. They take advantage of an easy meal, sometimes targeting those same fish that we value: juvenile salmon heading to out to sea, or adult salmon incapacitated on fishing lines. A healthy population of seals and fewer salmon inevitably leads to more noticeable interactions, including quicklearning “nuisance animals” that quite understandably draw the ire of some anglers and fisheries managers. Calls for action come quickly, and seals are an easy target.
Some research indicates that wild Chinook salmon productivity is negatively related to seal density, yet there have also been many good Chinook years during the past two decades when seal numbers were high. Other studies show only four per cent of the harbour seal diet is salmon; that herring and hake are their primary prey. In fact, 40 per cent of the harbour seal diet is hake, which is a major salmon smolt predator. While appealing to some, a seal cull could actually destabilize the coastal food web, result in an increased abundance of hake, and increase the predation of juvenile salmon by hake.
The harbour seals’ healthy population numbers are also echoed by another wellknown species, the Bigg ’s (or transient) killer whale. The population of these apex predators — which feed on marine mammals and favour seals and sea lions — is on the rise, and the increase in Bigg ’s killer whales is serving to effectively keep the seal population in check, without any human intervention at all. A human-sanctioned cull of the seal population in the Salish Sea would mean a reduction in food for these mammal-eating killer whales.
Unfortunately, history is rife with failed attempts to manage populations by removing predators from the ecosystem. In this case, we need to focus on mitigating our own impacts so as to better protect salmon and their habitat. As tempting as it is to lay the blame on seals, it is we humans that have work to do. Let’s celebrate the vibrant population of these predators — seals were equally abundant during pre-contact times when salmon was plentiful.
Declining salmon abundance is the result of a complex variety of factors, and cannot be solely attributed to harbour seals.
Dr. Peter Ross is the vice-president of research, and Dr. Lance Barrett-Lennard is director of the Cetacean Research Program at Ocean Wise. Ocean Wise is a not-for-profit organization whose vision is a world in which oceans are healthy and flourishing. www.ocean.org