In recent days we have been seeing calls in the media to engage in an all-out cull of pinnipeds (seals and sea lions), arguing that we need to act urgently to stop pinnipeds from eating salmon smolts that take food away from southern resident killer whales. We do know from recent research conducted by the Salish Sea Marine Survival Project in the Strait of Georgia that pinnipeds, in particular harbour seals, are responsible for consuming up to 40% of out-migrant Chinook and 47% of out-migrant coho. Is an all-out cull the answer, or is selective predator control of specific individual seals a better approach?
What seal data do we have?
The best available research tells us that there are several marine mammal predators responsible for consuming Chinook salmon, and that their populations are increasing—some dramatically in the period from 1975 to 2015. The killer whale population, despite what has been reported in the media, has increased from 292 to 644 individuals. Harbour seals increased from 210,000 to 355,000; California sea lions increased from 5,900 to 47,000; and Steller sea lions increased marginally from 74,400 to 78,500. Consumption of Chinook shows that killer whales are the largest source of predation, responsible for 10,900 metric tonnes, followed by harbour seals at 2,500 metric tonnes, then Stellar sea lions at 1,200 metric tonnes, and California sea lions at 600 metric tonnes.
That sounds fairly small, but when you dig deeper to look at the actual number of individual Chinook salmon consumed it tells a shocking story. Overall, predation has increased from 5 million individual Chinook in 1975 to 31.5 million in 2015. What is not represented well in the analysis is that reporting consumption in tonnes mis-represents the real problem—that most consumption takes place by harbour seals eating out-migrating Chinook smolts. Harbour seals account for an increase in individual consumption from 3.5 million in 1975 to 27.4 million in 2015!
While these are shocking numbers, and perhaps they also help explain the mystery of why some salmon stocks have been declining due to something bad happening in our ocean, we still don’t know all the answers of precisely which harbour seals are the bad guys.
Seal research methodology
Researchers started looking closely at specific estuaries and tried to determine if out-migrant smolt predation was attributed to certain individual harbour seals or if all harbour seals were to blame. They came to Vancouver Island and studied animals at a number of locations near the Qualicum River. By placing devices to measure prey events, researchers determined that only a small portion of harbour seals actually had become out-migrant smolt experts. Another interesting finding was that harbour seals had a preference for larger smolts. Perhaps this could explain why steelhead, for example, who migrate as the largest of all out-migrant smolts, have been in significant decline since the early 1990s.
Size may matter, as there are some incidents of differential ocean survival between different Chinook runs that have distinctly different out-migration strategies. Chinook have 2 types of races—ocean-type Chinook migrate to salt water in their first year, whereas stream-type Chinook spend one full year in fresh water before migrating to the ocean as larger smolts than their ocean-type cousins. Ocean-type Chinook tend to utilize estuaries and near-coastal areas. Much more research needs to be done to investigate if there appear to be clear trends between larger smolt size and predation preference.
Harrison River Chinook are ocean-type, migrating to sea at small size and within a few months of emerging from the gravel. In recent years, Harrison Chinook have outperformed other upper Fraser Chinook, which are largely stream-type and spend up to a full year in fresh water growing larger before out-migrating. One hypothesis is these stream-type larger smolts may represent more appealing prey for awaiting harbour seals, and possibly there is a connection or pattern in that regard. Again, much more research is necessary to determine if there is a scientifically valid correlation.
What should we do about the seals?
While the jury is no longer out on the impact of harbour seal predation, what we can do about it is very much in debate. The current population of harbour seals had been sharply increasing until approximately 20 years ago, and it has since stabilized. Has the population reached natural carrying capacity, or are they being controlled by transient killer whales that are their natural predator? It is likely both. Many anglers regularly report harbour seals travelling several miles upstream in coastal rivers in search of smolts and any adult salmon and steelhead they might encounter. This could be a sign that some seals are attempting to expand their range as other food sources become less available to them.
Regardless, many people believe the current population of harbour seals has increased as a result of human caused conditions. As such, they argue we must take action to reduce the population down to levels seen in the 1970s. That represents a decrease from the population today of around 355,000 back down to 210,000, or a broad based cull of 145,000 animals. They argue a responsible harvest and sale of these animals would provide economic value to BC while reducing predation of Chinook, in turn creating more fish available to assist SRKW recovery.
Is a wide spread seal cull the answer?
In a word— no. Many in the scientific and recreational fishing communities are looking to new research, which indicates that not all harbour seals are indeed smolt predators. They suggest that the responsible science-based approach is to design any new pinniped control programs around identifying the specific problem animals, at specific high-predation sites. Robust and highly selective methodology is preferred as opposed to a general cull. Another unanticipated problem with a widespread seal cull would be removing a critical food source for transient killer whales. A widespread cull could lead to impacts on these precious whales. The juice is not worth the squeeze.
- Pat Ahern
- November 30, 2018
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