Unlike large whales, most dolphins do not migrate with the seasons. Yet they often travel great distances in a day, mainly in search of food. By staying on the move, dolphins never eliminate a food source by eating all the fish or squid in one area. Many prey species keep moving as well, and dolphins follow them. Pilot whales seem to track the movements of squid, while common dolphins trail various fish that sometimes swim hundreds of kilometres.
Food may not be the only reason dolphins and porpoises move, though. In some seas, spinner dolphins and certain porpoises seem to enter shallow waters to avoid attack by sharks or orcas. And dolphins that form allmale or all-female groups must set out eventually in search of mates.
The distance travelled varies by species, time of day and season. Spinner dolphins around Hawaii range up to 50 kilometres a day. They feed close to the coast during daylight hours but move offshore at night to feed in deep waters. Resident orcas off Vancouver Island may venture 120 to 160 kilometres a day in summer when salmon are swimming in great schools from the open sea to the rivers. In winter, food supplies are even more dispersed, and orcas spread out over larger areas.
Coastal bottlenose dolphins have home ranges that vary according to gender and age. In Sarasota Bay, Florida, females with calves have the largest and most productive home ranges - about 40 square kilometres.'
Nursing females require the most food. The males, meanwhile, have to roam over less productive areas.
River dolphins and some porpoises have much smaller home ranges, Generally restricted to rivers, a few river dolphins move out into the open sea. Harbour porpoises in the Bay of Fundy have been followed for a fourday period, during which they travelled 40 kilometres. The longest distance covered in one day was 20 kilometres, while the average was 10 kilometres -less than 1 0 percent of the distance some orcas traverse. In winter, many harbour porpoises in this area move offshore, presumably to enjoy the warmer waters touched by the Gulf Stream.
Like land mammals and birds, many dolphins and porpoises have home ranges to which they return year after year. These ranges are not considered territories. To qualify as a territory, an area must be defended -either from other species or from members of the same species. There is as yet no evidence that dolphins and porpoises use aggression to maintain exclusive use of a habitat.
Rauno Lauhakangas