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.
A whole life under water
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.
Migration of fish
salmon migration routes
Just like birds, a distinction can be made between resident fish, migratory fish and wanderers. Resident fish spend their whole life in the same area. Dab, whiting and dragonet are found in the North Sea region all year round. However, most fish migrate for various reasons. It could be due to a food shortage. Or a habitat may be less pleasant at certain times in the year due to temperatures that are too low or too high. For example, the tub gurnard is only a summer resident in the North Sea. A third reason is reproduction - the spawning grounds do not always lie in the area where the adults normally live.
An exceptional example is the spawning migration from sea to fresh water, performed by sticklebacks and salmon. These fish species are referred to as anadromous fish. Salmon spawn in the upper reaches of the large rivers, mostly far inland. After two or three years in these rivers, the still young animals go to sea, which often means migrating thousands of kilometres away. When returning to spawn, the animals can recognize the river water in which they were born from its scent.
Four out of 11 anadromous fish species have disappeared from Dutch waters (such as sturgeon and allis shad), and five others are listed by the Trilateral Wadden Sea (Work Group) as vulnerable or susceptible (such as sea trout and twaite shad). The greatest bottlenecks for these species are access to spawning grounds and loss of habitat. Pollution is very likely another limiting factor.
If proper measures are taken, it is believed that the declining populations (not those that have disappeared, since they have often disappeared completely from northern European waters) can be preserved. This means regulations improving passage between fresh and saltwater areas and insuring adequate water quality particularly in the fresh water habitats. An example is the fish passage in the river Lek, which opened in September 2004. The passage consists of a 400-meter long ladder with 24 steps, each step 15 centimeters higher than the previous one. Fish such as salmon, sea trout, barbel and eel can rest in each basin. The fish ladder seems to work well. All of the dams in the Lek and Nederrijn have also been provided with fish ladders since then. Two ladders have been kept under control. There was a notably large number of sea and European river lamprey. Large numbers of the exotic Chinese mitten crab also makes good use of the fish passages.
A special program has been set up for reintroducing houting, family of the salmon. This fish disappeared from Dutch waters around 1930, particularly due to the construction of dams in rivers. Now that fish ladders have been made, new opportunities for this migrating fish have been created. Young houting from Denmark were released in the German catchment basin of the Rijn. Descendents of these pioneers have managed to provide for a growing population of houting in Dutch waters.
Eel live in fresh water. When they are mature, they migrate to sea. One thinks they migrate to the Sargasso Sea to spawn and consequently die. The eel larvae are transported with the Gulf Stream to Europe where they arrive after 2 years, and then swim upriver. The trip up and back is 5000 km! This process has never actually been proven: one has never found a spawning eel in the Sargasso Sea. Eel larvae are found in the regions between the North Sea and the Sargasso Sea. In addition, laboratorium experiments have shown that in theory, eel have sufficient fat stores to make the long trip to the Sargasso Sea.
Wanderers are fish species that 'accidentally' end up in a region where they do not actually belong. In the North Sea region, one occasionally finds a sunfish or a basking shark.
Eat and be eaten
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
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.
Source: de Vleet, Ecomare
