Toothed whale swallow fishing hooks. Gillnetting and Seine netting are significant causes of mortality in cetaceans and other marine mammals. Porpoises are commonly entangled in fishing nets. Whales are also affected by marine pollution. High levels of organic chemicals accumulate in these animals since they are high in the food chain. They have large reserves of blubber, more so for toothed whales, as they are higher up the food chain than baleen whales. Lactating mothers can pass the toxins on to their young. These pollutants can cause gastrointestinal cancers and greater vulnerability to infectious diseases. They can also be poisoned by swallowing
Michael Bigg key discoveries of Dr. Bigg's team was that there were two types of killer whales living near the British Columbia coastline: "residents" which ate almost exclusively fish, and "transients" which hunted marine mammals and other warm-blooded prey. Resident and transient killer whale societies are separate, and the two types tend to avoid each other. The resident killer whale population of British Columbia and Washington is further divided into two "communities", one northern and one southern, that do not interbreed. Another discovery was that resident killer whales have one of the most stable social structures of any animal species. Pods of