 Whales can also be threatened by humans more indirectly . They are unintentionally caught in fishing nets by commercial fisheries as bycatch and accidentally swallow fishing hooks . <unk> and Seine netting is a significant cause of mortality in whales and other marine mammals . Species commonly entangled include beaked whales . 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 litter , such as plastic bags . Environmentalists speculate that advanced naval sonar endangers some whales . Some scientists suggest that sonar may trigger whale beachings , and they point to signs that such whales have experienced decompression sickness . 
