Dolphins in the North Sea

Dolphins swim in the southern North Sea, porpoises being the most common species. Bottlenose dolphins used to swim here in large numbers, but now when a large group of larger dolphins is seen, it is usually the white-beaked dolphins. Every once in awhile, a common dolphin or a white-sided dolphin is seen or caught. It is very rare to find Risso's dolphins, striped dolphins or pilot whales here. The largest dolphin species, the orca, was an occasional wanderer in the southern North Sea up to 1963. Beaked whales form a separate group and include the northern bottlenose whale and the Sowerby's beaked whale.

The number of dolphins declined sharply between 1940 and 1965. Because they form the top of the food pyramid in the sea, they are very sensitive to the accumulation of toxic materials. And the food for the dolphins, the fish stocks, declined greatly during this period. In addition, the amount of noise under water at sea has strongly increased due to increasing shipping. Dolphins orient themselves with the help of sound and are great bothered by extra noise.

Protection

In 1991, a number of countries signed the Agreement on the Conservation of Small Cetaceans of the Baltic, North East Atlantic, Irish and North Seas (ASCOBANS agreement). By working mutually together, a better protective status for small cetaceans can be reached. The countries oblige themselves to protect the habitats of the small cetaceans, to collect data for scientific studies, to decrease pollution and to spread information. In 2008, this agreement was expanded, so that countries situation around the North Sea and Baltic Sea also oblige themselves to conserve and maintain the habitat of the porpoises and dolphins. Including Ireland, Spain and Portugal will be negotiated in 2008.

The number of dolphins found in the North Sea is used in the research programme as a measurement for the environment. What effects an amount of toxic materials, disturbances and the fisheries in the North Sea have on the animals can be seen by the numbers.

Dolphins do not breath consciously, such as people. They must continually think about their oxygen needs, otherwise they will die. Sleeping in a human sense would lead to their death, therefore they have found a different manner. They can let one half of their brain sleep while the other half takes over a number of functions. In this way, they can sleep safely.

If a dolphin is wounded, other dolphins often help out. They hold it above water until reaching a safe place. That dolphins do the same with humans is probably a fable.

Echolocation system

Dolphins possess an echolocation system to help determine the position of a prey. The animal can send out high tones from its head which reflect against a fish. This reflection is received in its lower jaw. Using echolocation, the dolphin can determing how large the fish is, which direction it is swimming and how fast. The dolphin regulate the strength of the signal. In a noisy sea, the signals must sound harder than in a calm sea.

Dolphins live in a world of sound, which makes them particularly sensitive to excessive noise underwater.

During the cold war, the US and USSR trained dolphins to recognize torpedos and rockets with the help of echolocation.

Dolphins can recognize each other by their own unique whistle. Similar species recognize the noise and use it to call each other. Bottlenose dolphins are constantly calling their 'name' and others answer by repeating the signal and sending back their own whistle.

Weblinks

Table with reports of live dolphins in the North Sea:
http://home.planet.nl/~camphuys/NLflippers.html

An animation of echo-location:
http://rosmarus.com/Info_img/sonar.gif

ASCOBANS:
http://www.ascobans.org/
Participating countries:
http://www.service-board.de/ascobans_neu/bilder/area_map.gif

Source: de Vleet, Ecomare