Depredation

Depredation is the growing problem of whales taking fish from fishing gear. In British Columbia, killer whales and sperm whales are the cetacean species most commonly implicated in depredation events. This is an issue not only for the fishers but also for the whales who are feeding in this way. Whales may depredate fishing gear because the catch type is part of their natural diet, they have learned to target this easily obtainable food source, or their natural prey species have been reduced. Once one whale in a population learns to feed this way, depredation behaviours are quickly transmitted through the population via social learning. Hook and line fisheries, like trolling and long-lining, are most affected in B.C., recreational anglers are also targeted by the whales.

THE PROBLEM:
The principle problem posed by depredating whales to fishers is the loss of catch, costing potentially tens of thousands of dollars a day. One fisherman in Tasmania estimates that he loses about $7,700 US per day when killer whales target his boat. Near the Crozet Islands, the Patagonian toothfish fishery losses nearly $5 million a year to killer whale and sperm whale depredation (Roche et al 2007). Gear damage, by comparison, has been relatively minor and rare, and the whales do not appear to pose a danger to fishers.

A fisher holds up the remains of a black cod, which was depredated by a sperm whale while the fish was hooked on a fishing line. photo Dan Falvey

Depredation is harmful to whale populations for at least two reasons. First, efforts to deter depredating whales are dangerous for the animals themselves. They can become entangled in fishing gear, swallow hooks, or sustain injuries from encounters with frustrated fishermen. In Chile, for example, rifles, harpoons, dynamite, and bottles filled with fuel have all been aimed at depredating whales. Second, dependence on depredation can cause whales to lose their natural foraging behaviours, harming their populations in the long run.

Killer whales around the stern of a trawler in Alaska. photo Lance Barrett-Lennard

Currently, depredation is a most severe in Alaska and the oceans of the southern hemisphere. In the Alaskan panhandle, sperm whales depredating the valuable sablefish (black cod) fishery are a serious issue, and along the NW coast of Alaska and the Aleutian Island chain, killer whales often raid sablefish and halibut fisheries. In the southern ocean, the lucrative toothfish (Chilean sea bass) fishery is seriously affected by sperm whales and killer whales, and in the tropics, various species of tuna are taken. Worldwide, 11 species of odontocetes have been implicated in the depredation of longline fisheries. Killer whales most commonly participate in depredation in temperate and coastal waters, while false killer whales are most commonly observed depredating in tropical and offshore areas (Hamer et al, 2010).

In B.C. there are increasing reports of fish snatched from lines, especially from sport fishermen. Depredation by killer and sperm whales is not widespread in BC yet, but appears to be increasing and could become a serious problem in the coming years. At present, the fisheries most affected are commercial salmon trollers and sport fishers targeting chinook and coho salmon. Several incidences of depredation are reported each year, but likely many events go unreported. Anyone who experiences a depredation event by whales or dolphins should report it so that scientists and managers may learn more about when and where this problem is occurring.

THE SOLUTION:
symposium in BC in October 2006, organized by the Vancouver Aquarium, brought together professionals from around the world to provide insights into the problem and share potential solutions. Experts at the meeting agreed that depredation is much easier to prevent or control before it becomes an entrenched behaviour. Once a whale has become dependent on depredation, it is very difficult to reverse the behavior. Various research projects are underway to look for solutions to reduce or prevent depredation. Most promising among these are acoustic devices and modifications to fishing gear, particularly the conversion of hook and line gear to pots and traps. Research will be conducted collaboratively with both fishers and researchers.

A sperm whale shadows a fishing boat in Alaska. photo Heather Vukelic

WHAT YOU CAN DO:
Dr. Lance Barrett-Lennard, Senior Marine Mammal Scientist at the Vancouver Aquarium summarized the “take home message” from the 2006 symposium as: “don’t feed the whales.  In practice, this means that fishermen should keep alert for whales, and stop fishing when they approach. If fishermen stop disposing of fish heads and guts while their lines are still in the water, whales won’t learn to raid their lines in the first place.”

If you do witness depredation or if fish has been taken from your lines – please report the incident to the sightings network: 1-866-I-SAW-ONE or sightings@vanaqua.org or our online form.

More Information
For links to papers on depredation click here
DFO bulletin on predation

References

Hamer, D.J., Childerhouse, S.J., Gales, N.J., 2010. Mitigating operational interactions between odontocetes and the longline fishing industry: a preliminary review of the problem and of potential solutions. Report to the International Whaling Commission (IWC) Scientific Committee, Report no. SC/62/BC3. 30pp

Roche, C., N. Gasco, G. Duhamel, &, Guinet, C. 2007. Marine mammals and demersal longlines fishery interactions in Crozet and Kerguelen Exclusive Economic Zones: an assessment of the depredation level. CCAMLR Science 14:67–82.