Depredation

Depredation, removal of fish from a longline, is a learned behavior performed by several species of toothed whales, mainly by sperm whales (Physeter macrocephalus) and killer whales (Orcinus orca). This behavior, once learned by some individuals, spreads in a population through social learning.

Depredation causes economic loss for fishermen and in some fisheries there is also a potential for the whales to get entangled in the fishing gear which is hazardous both for the whales and for the crew in fishing boats.

The way depredation takes place (detection of fish, operation around the longline, is the whole fish or only part of it removed etc) varies between the fisheries and the whale species involved. In our study area the sperm whales remove entire fish. We do not have data yet on the extent of hooks being removed/damaged, which is not only a problem in terms of gear loss but could also be a problem for the whales.

Our project focuses on a novel behavior performed by sperm whales in arctic Norway, off Vesterålen islands, where sperm whales have been taking Greenland Halibut (Reinhardtius hippoglossoides) from longline fishing vessels since August 2014. The Norwegian longline fleet has experienced this problem with Greenland Halibut fishing since mid 1990`s in Greenlandic waters, but the behavior has not been documented in Norwegian waters before.

The waters off Vesterålen have long been known to be a good sperm whale "ground" due to a deepwater canyon (Bleik Canyon) which has been a favoured feeding ground for sperm whales. Since 2010 this area has been of less importance and the whales have started feeding in shallower waters. The underlying reason for this change is not known, but is probably connected to ongoing changes in the Norwegian sea ecosystem.

Our project is unique since we have been able to document depredation behavior in a fishery from the very beginning  (thanks to valuable data provided by the fishermen and whale watching companies operating in our study area). We are interested in understanding the number of whales involved at present and possible increase in this behavior, how the whales detect the longliners hauling in their catch and how they operate in removing the fish. Our main goal is to work together with the fishery to find solutions that will prevent the increase of this behavior or even remove it all together.