Culture of harpacticoid copepods: potential as live feed for rearing marine fish.

Copepods are useful as food for marine fish cultivation, in terms of both nutrition and ease of culture. Harpacticoid copepods are favoured over calanoids, since harpacticoids, as a result of their benthic habitat, can be reared at much higher densities. However, their benthic nature also makes mass culture difficult, since large surface areas must be provided. Within Harpacticoida, Tisbe spp. seem most useful, having high overall fecundity, and positive phototaxis of the nauplii. Harpacticoids can synthesise de novo several nutritionally important essential fatty acids (EFA), making them desirable as food for rearing marine fish. However, a diet rich in EFAs (e.g. animal derived feed) improves the productivity of copepod cultures, suggesting that the synthesis of EFA is rate-limiting for their reproduction. The nature of the substratum is also important in maintaining a good population, since harpacticoid biomass is more dependent on surface area than volume of a culture. Heterogeneous substrates can support large cultures because of their high surface area, but efficient cleaning methods are necessary. Frequent harvesting of populations will maintain good water quality and an overall low density of sexually mature copepods, raising naupliar productivity overall. Over-harvesting will naturally deplete the population. Harpacticoids are generally tolerant of environmental fluctuations but they do have temperature and salinity optima, and these will be species- and strain-dependent. Harpacticoid copepods are better food for fish larvae than Artemia, because of their ability to synthesise EFAs. The nauplii of harpacticoids are energetically poor but appear to have an appetite-stimulatory effect. Uneaten nauplii grow within the fish rearing tanks and graze on the walls, building up their own nutritional value and maintaining tank hygiene.