Ecology of sperm whales (Physeter macrocephalus) in the Eastern North Atlantic, with special reference to sightings and strandings records from the British Isles

The sperm whale Physeter macrocephalus is a common large odontocete inhabiting deep oceans throughout the world where it feeds primarily upon cephalopods, particularly large squid. Human exploitation of the species has occurred on a large scale between the eighteenth and twentieth centuries. Although modern whaling concentrated upon other species like the large rorquals, as new uses were discovered for sperm oil, sperm whale catches increased from the 1950s for a period of about twenty years. After imposing increasingly restrictive catch limits, the IWC banned pelagic catches worldwide in 1979, whilst the remaining coastal fisheries closed during the 1980s. Sperm whales are seasonal breeders but with a prolonged mating season. Females and breeding schools remain in low latitudes throughout the year, whereas adult males join these groups only intermittently and mainly during the breeding season. Outside this period, adult males tend to move into high latitudes in summer remaining there between July and December before returning south again. In Northern Europe, the greatest number of sperm whale records come from the British Isles. In past decades, virtually all sperm whale records around the British Isles were lone, mature males. However, since the mid-1970s, accompanying a marked increase in the number of strandings recorded, there has also been a significant increase in the frequency of animals measuring less than 14 m, with one-third measuring less than 12 m and therefore likely to be sexually immature. Over this same period, groups numbering from three to eleven individuals including subadults have been recorded. Three possible explanations for the recent increase in strandings are: that a greater number of sperm whales are entering the region; there has been a recent increase in mortality (perhaps human induced); or both factors may be operating: a greater number of overwintering whales are becoming exposed to an energy demand from food shortage leading to higher mortality which might be exacerbated by other factors such as contaminants. At present, there is no strong evidence that contaminant burdens are directly responsible for any of the recent strandings. The most parsimonious explanation is that the cessation in the late 1970s-early 1980s of hunting pressure, which traditionally concentrated upon males, has enabled sperm whale populations to recover, at the same time exposing pubertal males to greater competition for females from other males. This may have forced an increasing number of adolescent and young adult males to leave the breeding groups and to move into high latitudes where some have remained for extended periods, running into a seasonal shortage of food by late autumn.