Fisheries depredation (raiding of fish from fishing gear) by killer whales and sperm whales is a growing problem around the world.
Depredation is likely linked to diminishing natural food sources for whales. The problem is spread through cultural transmission. Killer whales are highly conservative animals and don’t experiment often with new food types, but once one whale takes the plunge, the depredation behaviour can be quickly transmitted through the pod—and eventually through the population. In BC, depredation occurs mostly on commercial salmon trollers and smaller sport fishers targeting Chinook salmon.
Killer whales can be harmed by engaging in depredation behaviour in several different ways. They are at risk of being deliberately injured by frustrated fishermen who lose profits to the depredating whales. They may also become entangled in fishing gear. Finally, a dependence on obtaining food through depredation could cause the whales to lose their natural behavior and foraging skills, much like the problem of ‘garbage bears’, harming the population in the long run.
A symposium organized by the Vancouver Aquarium in 2006 and partially supported by the BC Wild Killer Whale Adoption Program brought together experts from around the world to provide insights into the problem and share potential solutions. Experts at the meeting agreed that the best way to deal with depredation is to prevent it from happening before it becomes an entrenched behavior. 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. Future research will be conducted collaboratively with both fishers and researchers.
Strandings and Entrapments
Stranding (also known as beaching) and entrapment of groups or individuals are potential threats to killer whales. Four mass strandings of killer whales happened in Canada in the 1940s. Further live strandings happened near Tofino in the 1976 and in Dungeness Spit, Washington in 2002. While the cause of these events is unclear, issues such as disease, parasites, and intense human acoustic disturbance may be pre-cursors to these events. Killer whales may also become entrapped in small areas due to geographical anomalies or human-caused disturbance. As reported by Fisheries and Oceans, in 1991, J-pod spent 11 days in the Sechelt Inlet, and appeared reluctant to exit through the small opening at the mouth of the inlet.
The raiding of fishing gear by marine mammals is referred to as depredation.
Photo: Denis Couturier
To avoid the spread of depredation behaviour, fishermen should cease fishing when whales are nearby.