Depredation
Depredation is the problem of whales taking fish from fishing gear and is a problem that is growing around the world. This is a problem not only for the fishers but also for the killer whales and sperm whales who are feeding in this way. The problem is linked to diminishing natural food supplies for these species. 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 are most affected, whereas depredation of net fisheries by these species is very rare.
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. Gear damage, by comparison, has been relatively minor and rare, and the whales do not appear to pose a danger to fishers.
Whale populations are harmed by depredation in at least two ways. First, efforts to deter depredating whales can cause injury. They can become entangled in fishing gear, 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 the animals.
Second, dependence on depredation can cause whales to lose natural behaviours, harming their populations in the long run.
Depredation is a severe problem in Alaska. In the Alaskan panhandle, sperm whales depredating the valuable sablefish (black cod) fishery are the main problem. Along the NW coast of Alaska and the Aleutian Island chain, the main problem is killer whales that raid sablefish and halibut fishers. In the southern ocean, the lucrative toothfish (Chilean sea bass) fishery is seriously affected, whereas in the tropics, various species of tuna are taken.
In B.C. there are increasing reports of fish being taken, especially from sport fishermen.
The fishermen in this YouTube video were surprised by a group of killer whales that took half their 50 lb chinook salmon (warning: language not suitable for children and other non-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 several years. At present, the fishers most affected are commercial salmon trollers and sport fishers targeting chinook and coho salmon.
The Solution:
A symposium in BC in October 2006, organized by the Vancouver Aquarium brought together experts 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. It is very difficult to control the behaviour of whales that have become dependent on depredation.
Various research projects are being initiated to look for ways 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.
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
Visit www.killerwhale.org/depredation
For links to papers on depredation visit: www.killerwhale.org/depredation/abstracts.html
DFO bulletin on predation: http://www.pac.dfo-mpo.gc.ca/species/marinemammals/WhaleDepredation_bull2_e.pdf
