Behaviour and Adaptation
Size of groupings:
Although the popular impression of dolphins is that they are highly sociable and gregarious, their social structures and group sizes vary considerably between species in different areas and circumstances. Bottlenose and spotted dolphins for example, occur in small groups everywhere but only form large groups offshore. In general, the riverine and coastal species live alone, in pairs or in small groups of perhaps half a dozen. Some continental-shelf, shelf-edge and pelagic dolphins, by contrast, live in herds of hundreds or thousands. The structuring within these herds are complex with some containing all males, some mothers and young only and others with a complete mixture. These alliances also vary in length of time ranging from just a few hours, through several years to permanent associations. To complicate the picture further, mixed-species herds are common: herds of pilot whales may be attended by small groups of bottlenose dolphins; northern right whale dolphins and Pacific white-sided dolphins are often found together as are spotted and spinner dolphins. At times, up to six species may travel and feed in the same small area.
Among the many advantages of living in a group is the increase in the efficiency of searching for and capturing food, along with greater opportunity for reproduction, learning, defence and sensory integration.
Sensory integration is the term for the way in which each animal within a group contributes to the sensory information gained by the group as a whole. This attribute plays a significant part in defence and the search for food. For example, a group of hundreds or thousands of dolphins may be spread over many hectares of sea. If one animal discovers a shoal of fish or a predatory shark, it can immediately pass this information to others in the group, so that all may benefit. A single animal or a small group may remain unaware of the food or a predator and miss a meal or, perhaps, suffer a surprise attack. The variety of sizes of groups may be attributed to the availability of food within a particular region. Small groups of baleen whales for example, have as their main prey, swarms of krill. These swarms are not able to support more than a few animals feeding requirements.
Migration and distribution patterns:
The migratory habits of dolphins are conditioned by their apparent inability to fast for long which means that they need to live in a region where the availability of adequate food is fairly constant throughout the year, or to move seasonally in response to the changing availability of prey. Bottlenose dolphins migrate each year along the coast of North Carolina, often within 100m of the shore, moving south in autumn and north in spring. The seasonal migration of harbor porpoises in and out of the Baltic Sea, through the Lille Belt in Denmark, used to be so large and predictable that a major fishery took advantage of it. However, this migration no longer occurs because this population has been severely depleted by over-fishing, pollution and the effects of coastal development. Dolphin migration along the Pacific coast of Japan, on the other hand, are still large and regular enough to support a fairly extensive drive fishery. Herds of striped dolphins migrate south into Sagami Bay, along the east coast of Izu Peninsula, during autumn and winter. Northbound herds move along the west coast of Izu Peninsula in spring.
Changing conditions:
Profound changes are taking place in the global ocean-atmosphere system. The impact of ultra-violet radiation, as a result of the depletion of the ozone layer, is affecting marine food production in a major way. More UV radiation means less plankton, at least in the short term until resistant strains evolve. Hence the whole base of the marine food chain is reducing. Global warming is also having a serious effect on the ecological balance in the oceans. Warmer oceans are radically affecting the distribution of many cetacean species as they live, or seasonally migrate, to colder waters where food supply is better. While this warming of the oceans may benefit some ocean faring species, others may not be able to adapt and will simply perish.
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