Ecomare

Dieren en planten

Water en land

Mens en Milieu

: Spawning grounds and nurseries, Ecomare
©

Reproduction of fish

The releasing of fish eggs and their fertilization is called spawning. The females lay the eggs and the males fertilize them by releasing their seed over the eggs. Fish seed is referred to as milt or soft roe. Most fish species always spawn in the same place: the spawning grounds. These grounds must have specific features for most fish. Herring spawn on shell or gravel banks, such as along the British coast. Lesser sandeel spawn on sandy bottoms. When the eggs hatch, the larvae spend the first part of their life as zooplankton. They are transported by sea currents to places where they grow into adult: the nurseries.

Wadden Sea as nursery
The Wadden Sea used to be the nursery for many flatfish. The flatfish spawned in places in the North Sea where the sea currents carried the fish larvae to the Wadden Sea. The Wadden Sea was an ideal place for young flatfish to grow up. There is plenty to eat and it is warmer than the North Sea in the summer since the Wadden Sea is so shallow.

Nowadays, not many flatfish are found in the Wadden Sea. They haven't disappeared from the system since there is plenty of adult flatfish swimming in the North Sea. That means that the larvae grow up elsewhere. Fishery biologists think that the reason is related to the Wadden Sea being too warm. There is a limit to how warm the water can be for young flatfish, so they have chosen cooler, deeper water for growing up.

Another presumption is related to the sea currents. It's possible that the larvae used to 'automatically' swim into the Wadden Sea. However, there have been many interventions along the coast, such as the new Maas plain and the dam by Eierland on Texel. These promontories can influence the current so that the larvae miss the entrance to the Wadden Sea and are carried to other parts of the North Sea.
Millions of eggs
Egg of a lesser spotted dogfish, with embryo, Ecomare
©
Fish often have many offspring, especially those species whose eggs drift around in the water. The 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 species, such as the lesser spotted dogfish and the thornback ray, lay even fewer eggs, around 140 per year.
Ovivipary
Some fish don't lay eggs. Instead, the females retain the eggs in their body until the embryos are ready to hatch. Fertilization takes place in the body. The males of these fish species have a special fin for bringing the sperm into the female's body. This mode of reproduction is called ovivipary. All the food the embryo needs is in the egg. Therefore, the young fish are not linked to the mother's body with an umbilical cord for receiving food. The eggs are only in the body for safety reasons. The young are born alive, as mini specimen. Examples of viviparous fish are guppies, eelpout and various species of sharks.

Seahorses and pipefish have a different method. The males of these species brood the eggs in a practically enclosed brood pouch. Technically speaking, this is outside of the body however there's not much difference. These males actually 'give birth': they release clouds of mini seahorses and mini pipefish from the pouch with jerky movements.