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Dieren en planten

Reproduction of fish   Nurseries   Marine fauna   Sea fish   

Water en land

Mens en Milieu

Gulls prey upon young fish, Sytske Dijksen, www.fotofitis.nl

Nurseries

If you're lucky, you can see it from the shore: a large cloud of seabirds constantly diving into the sea. That's a sign of a school of fish swimming past. Often times, it is young herring or sprat. In the summer, when the coastal water is warmer, it will be swarms of juvenile flatfish. The North Sea coastal zone is an important habitat for many sea fish, thanks to the large amount of nutrients compared to the deeper seawater further away.

  • A coastal river full with plankton

    Dutch, German and Belgium rivers transport nutrients from farmland to the sea. Upon arrival, this nutrient-rich water is driven north to the Wadden Sea by the sea currents. Such a sea current is called a coastal river. Dissolved fertilizers and nutrient-rich sludge in this water ensure that microscopically small plants are able to grow. In turn, these plants form food for tiny animals, the zooplankton. Fish begin life as plankton consumers. Such plankton-rich seawater makes the coastal river and the Wadden Sea ideal areas where young fish can grow. We call this the 'nurseries' for the North Sea fish.

  • North Sea herring
    Herring, Sytske Dijksen, www.fotofitis.nl

    Adult sea fish must do their best that their offspring can grow up in the coastal river and the Wadden Sea. The larvae live in the flowing seawater, so it is important that they crawl out of their egg at the right place and the right time. Herring are very exact. They migrate every year from the northern North Sea, between Scotland and Norway, to the same areas east of England, where the sea bottom is very gravelly. Here the female herring lay their eggs so that the males can fertilize them. This is called spawning. The eggs hatch after awhile and the herring larvae swim to the surface. They are zooplankton which means they can't swim against the sea current. But the spawning grounds and the timing is chosen in such a way that the larvae arrive just in time in the coastal river to profit from the large amount of food.

  • Herring teens
    School herring, Sytske Dijksen, www.fotofitis.nl

    After awhile, the young herring turn into true fish. The herring teenagers form large schools. Because they are now strong enough to swim against the current if necessary, they stay in the plankton-rich water until they are large and strong enough to travel to the herring grounds in the north of the North Sea. That is where they grow further into adults, and eventually migrate to the same spawning grounds where they once hatched as larva from their egg.

  • Flatfish
    Young plaice, Sytske Dijksen, www.fotofitis.nl

    The coastal river and the Wadden Sea is also a good nursery for flatfish, such as plaice, sole and turbot. It is known that plaice are very loyal to their spawning grounds, while outside of the spawning season they will travel throughout the North Sea. Just like the herring, they choose their spawning grounds in such a way that the larvae reach the nutrient-rich nurseries on time.

    Many sole do the same. There are two groups of North Sea sole which spawn in the spring off the coast of Belgium of France so that the young can reach the coastal river later in the year. The young flatfish live in the coastal zone from small benthic animals. Where there is lots of plankton, lots of benthic animals can also grow, so that there is an abundance of food in the North Sea coastal zone. That is also why biologists and shrimp fishermen find large amounts of flatfish in the coastal waters.

  • Too warm?
    Young flatfish, 'post stamp', Sytske Dijksen, www.fotofitis.nl

    Up till around 1995, the North Sea coastal zone and the Wadden Sea were important nurseries for young plaice. Since then, fewer young plaice have been caught. You still find young sole and turbot. It's not that the plaice have disappeared from the North Sea; the plaice population in recent years is better than ever. Adult plaice still spawn in the old places. It seems that the young plaice have found somewhere else to grow up. Scientists think that the temperature of the seawater, which is gradually warming up, is responsible for the change. The coastal water and tidal channels are probably too warm for the young fish and now choose for cooler, deeper water.