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Thornback ray, Ecomare

Sea fish

There are around 220 species of fish living in the North Sea. A few of them are well known from the dinner table: herring, cod and plaice. The fisheries in the North Sea are intensive, whereby a number of fish species such as cod are being threatened. Other problems in the North Sea are caused by pollution from toxic materials and excessive input of fertilizers.

  • An entire life under water
    Bass, Ecomare

    All fish swim, however there is a large difference in their swimming habits. Stayers such as mackerel, herring and sprat can maintain a fast swimming speed for a fairly long time. However, cod, haddock, sole and plaice are sprinters: they can only swim quickly for a short period of time. This difference can be seen in the muscles. The muscles of stayers are red, a sign of good circulation with lots of oxygen. Sprinters have white muscles and are quickly exhausted.
    Each species of fish prefers to swim in the vicinity where its choice of food can be found. For example, herring, mackerel and sprat are surface feeders and live in the upper water column, usually in large schools. Their food consists of plankton or small fish. The fish which swim through the water column, as opposed to on the bottom, are called pelagic fish. They often have a camouflage color in order not to be too conspicuous for their enemies: a green-blue back and a silver-white belly. This makes it difficult for seabirds looking down into a green-blue water surface to distinguish it, and for other predator fish to see it when looking up against the bright light above.
    In general, one used to believe that fish swam in schools because they would be better protected against predator fish. This may be true, however according to a new theory, swimming in a school also saves lots of energy. Studies have shown that a fish uses the turbulence in the water current induced by the fish swimming in front of it to advance. In this way, a fish only needs to use its front muscles instead of all of its muscles.
    Benthic fish, such as plaice and sole, live on the sea floor and eat benthic animals and small species of fish. For better camouflage, they often develop a color pattern similar to the seabed where they reside. Turbot are even able to change their color to fit in with the surroundings. Benthic fish will dig themselves into the upper layer of sand.
    Predator fish feed mostly on smaller fish. They can live by the bottom (cod and haddock) as well as close to the water surface (garfish, bass and whiting). They are well equipped for their role as predator, being fast swimmers with a large mouth and often sharp teeth slanting backward.

  • Reproduction

    Fish often have many offspring, especially those sorts whose eggs drift around in the water. Cod lays up to a million eggs per year. Many fish eggs and larvae are consumed by all kinds of animals, so that only a small selection actually reaches adulthood. The eggs of species such as herring and lesser sandeel are laid on the sea floor. These eggs are less vulnerable, so that the fish can afford to lay fewer eggs. Herring produces 'only' 30,000 eggs per year. Other types, such as the lesser spotted dogfish and the thornback ray, lay even fewer eggs, around 140.
    Fish have fixed places for reproduction. Herring release their eggs (roe) on gravel or shell banks, such as found along the English and Scottish coasts. After the males fertilize the eggs with seed (milt), the adult fish return to the northern and central North Sea. Lesser sandeel lay their eggs in sand.

  • Eat and be eaten
    Example of a complex foodchain in the Waddensea, Ecomare

    Fish are not the only animals living in the North Sea. An enormous variety of plants and animals are housed under the waves. All life in the sea is dependent upon one another: the one is food for the other. Phytoplankton forms the base for most life in sea. The phytoplankton is consumed by the zooplankton, such as copepods and the larvae of benthic animals and fish. These zooplankton is eaten by all kinds of benthic animals and small fish, such as herring and sprat, and they in turn are on the menu for larger fish, such as cod and haddock. The fish fall prey to seabirds, marine mammals and people. This system of eat and be eaten is called the food chain. In reality, it is much more complicated than described here above. Phytoplankton will eat each other: for example, fish larvae eat copepods. In addition, young fish eat very different prey than adult fish.

  • People and fish
    Euro cutter, fishing, RWS, Directie Noordzee

    Man exercises the greatest impact on sea fish through the sea fisheries. By catching predator fish (cod), the chances for their prey (herring) increases. However, rare species which are commercially uninteresting are also influenced by the fisheries, and often not in a positive manner. Several types of rays and sharks have practically disappeared from the North Sea because they are accidentally caught in the nets of the fishing fleet. Worldwide, the oceans have lost more than 90% of its large predator fish, such as tuna and swordfish since the industrial fisheries developed on a large scale in the 1960s.
    A higher supply of nutrients, especially in the coastal waters and tidal regions, have immense influence on the fish populations. On the one hand, it can be favourable: more nutrients, more plankton, more young fish which grow quickly. However, after a large scale algae bloom, the fish fauna in a particular sea region can run into problems due to an oxygen deficiency. And finally, polluting the seawater with a multitude of toxins also has a negative influence: fish larvae do not survive an oil disaster or an excess of pesticides, and adult fish can become extremely sick from an overdose of toxins.