When does a fish become a fish?

The annual reproduction of fish is quite different from that of mammals. After they have hatched from the egg, fish pass through a larval stage. The larvae of many fish species spend this time in shallow waters away from the parent stock. In a manner of speaking, they live in a different world. At this stage, their numbers can reduce significantly because they are a food source for many other marine fauna. Many can die due to poor environmental conditions. Most fish larvae become juvenile fish in the first year. In fishery biological terms, however, they are only considered offspring or included in stock numbers when they join the parent stock and are large enough to land in fishermen’s nets: in other words when they can be counted. These juvenile fish are known as recruits.

herring larvae

Fig. 5.3a > Twelve hours before hatching: the large, well-pigmented eyes of the transparent herring larvae are particularly striking. © Dr. Bernd Ueberschär, Helmholtz-Zentrum für Ozeanforschung Kiel/GEOMAR

Fig. 5.3b > Eat or be eaten: at eight days old, herring larvae feed mainly on the larvae of smaller crustaceans. They are themselves the prey of larger fish. Only about 1 per cent of herring larvae survive this stage. © Dr. Bernd Ueberschär, Helmholtz-Zentrum für Ozeanforschung Kiel/GEOMAR

herring larvae

Fig. 5.3c > After 30 days the larvae have all the fins of adult fishes. The gills and scales are formed at this stage. The swim bladder is partially formed, so that the larvae can move up and down the water column following the food. © Dr. Bernd Ueberschär, Helmholtz-Zentrum für Ozeanforschung Kiel/GEOMAR

Fig. 5.3d > Still almost scale-free. At 60 days the larvae look like fully-grown herring, but the stomach is not yet fully developed and they have few scales. However, the swim bladder is now fully functional. The larvae can swim strongly and flee from predators. © Dr. Bernd Ueberschär, Helmholtz-Zentrum für Ozeanforschung Kiel/GEOMAR