Overexploitation of wild fish populations has greatly increased the importance of aquaculture as a source of fish protein. In both intensive and extensive fish culture systems, relationships between fishes and crustaceans representing the subphylum Copepoda must be controlled to maximize fish production. In aquaculture, copepods serve as:
- Food for small fish,
- micropredators of fish and of other organisms,
- fish parasites,
- intermediate hosts of fish parasites, and
- hosts and vectors of human diseases. In this review, we present an overview of these complex relationships, give examples of interesting cases, and where possible, suggest solutions to copepod challenges.
Food for small fish:
Freshwater finfish seed production often faces a problem of an adequate food supply. Artificial feeds are widely used, but planktonic animals are very important, especially rotifers, cladocerans, and copepods. Virtually all fish feed on plankton, especially in their early life phases.
Planktivorous fish depend on small invertebrates throughout their entire lives. Copepods of the order Cyclopoida are the most important food items in freshwater aquaculture, and their nauplii are especially valuable for feeding fry (Szlauer and Szlauer 1980). Copepods as natural food are either cultured or collected from the wild.
Free-living copepods, and zooplankton in general, may be harvested from the wild. Zooplankton may be collected from specific depths in lakes using a custom-made mechanical device (Szlauer et al. 1978) that operates by propelling water through a huge plankton net using an outboard boat engine.
Send ak (1980) surveyed the possibilities of acquiring different planktonic animals, mainly cladocerans and copepods, from I´ nsko Lake, Poland. According to his studies, annual lake productivity in Poland ranged from 6.25 to 62 g m-3 of water (possibly even 90 g m-3 in eutrophic lakes).
The second major method of acquiring copepods relies on harvesting plankton from small rivers flowing from lakes. Such outlet rivers contain large amounts of zooplankton. The diel and annual plankton flow from P o´ n Lake, Poland by the River P onia has been estimated to be 372 and 135 000 kg, respectively, while the same river removed 65.0 and 25 251 kg, respectively, from Miedwie Lake, Poland (Szlauer 1976, 1977, 1983/84).
In Central Europe, several attempts have been made to actually use copepods and other components of zooplankton for feeding fish in aquaculture (Anwand 1978, Szlauer and Szlauer 1980 1982). The results were promising, but better infrastructure and funding are needed.
Copepods can also be cultured to supply food for fish. Culture methods for marine copepods are well advanced (Ogle 1979, Ohno and Okamura 1988, Payne and Rippingale 2001), but relatively few attempts have been made to culture freshwater species.
One example may be a method of mass culture of Paracyclops fimbriatus developed recently by Szlauer (1995) using observations made from a mass occurrence (13 000 individuals L-1) during experiments on municipal sewage sludge.
Taiwanese scientists have been successful at rearing brackish-water copepods. Prof. Shin-Hon Cheng has developed a method of culturing Apocyclops royi on a semi-industrial scale (Cheng et al. 1999 2001).
Some 20 metric tons of different brackish-water and marine copepods are exported each year from Taiwan to Japan (Dr. Masato Kubota, pers. comm.). There are very few cases of using copepods as fish feed on an industrial scale. One of the most recent examples is an American company (Argent) that is marketing a product that they claim is made of cyclopoid copepods.
According to the supplier, these copepods are a“selectively bred, biologically engineered microorganism cultured in a pristine Arctic lake.” These copepods are reported to contain the highest known concentration of the fundamental and critical biological pigment, astaxanthene, accounting for their striking orange coloration.
Complimenting this pigmentation are previously unknown levels of highly unsaturated fatty acids (HUFAs), and especially those of the famed “omega-3”family. The high levels of omega-3 HUFAs are essential for aquaculture applications involving fish larvae and typically exceed 40 times those obtained from hatching the highest quality Artemia eggs.
A number of companies offer dried copepods as food for aquarium fishes; for example, Sera Products from the UK sells FD- Cyclops.
Authors:
Wojciech Piasecki, Andrew E. Goodwin, Jorge C. Eiras, Barbara F. Nowak
