In the wild, these species live on copepod (zooplankton) in the sea during the first phase of life. As it is difficult to cultivate copepods in large quantities, farmed fish larvae feed on rotifiers and Artemia instead, which are grown in separate tanks before being harvested and given to the fish larvae as feed.
Artemia is a crustacean that lives in salt-water lakes. Artemia become encased in hard capsules (cysts) when the conditions are too saline, and these cysts are collected and then sold to the fish farm industry and aquarium industry. The nutrient content of Artemia is poor compared to the nutrient requirements of marine larvae. Therefore, Artemia are first fed with an emulsion that slightly alters the nutrient content before they are fed to the marine fish larvae. This is called enriching Artemia. Work is underway on reducing the time the larvae depend on Artemia by developing a better dry feed and improving feeding regimes. Better feed means that some fish farm facilities are now giving halibut larvae dry feed earlier than 100 days after hatching.
Rotifiers (wheel animals):
Fish larvae usually graze on early stage copepod, which reach a length of 0.5 to 4 mm when they are fully mature. Rotifiers are small enough to replace the early stage copepod.
The rotifiers, or wheel animals, have been given this name because they have a ring of cilia around their mouths, which they use to move (rotate) and catch food with. They often live in brackish water and are common all over the world. Many wheel animals that are used in fish farming are taken from their natural habitats and are then cultivated so that they can be used as feed for fish larvae. As for Artemia, the composition of nutrients in rotifiers is not optimal to meet marine fish species nutrient requirements. The wheel animals are usually enriched with extra nutrients before they are fed to the fish larvae. It is also possible to adjust the conditions in the culture.
Unlike Artemia, which come in their own cysts, fish farmers produce rotifiers by maintaining cultures. This means that these animals reproduce in the tanks at the fish farm facility. Research conducted at NIFES has shown that it is possible to increase the amount of the minerals zinc, selenium, manganese and copper in the wheel animals by adding more of these minerals to the wheel animals’ feed in order to optimise the content in relation to the nutritional needs of the fish larvae.