Copepods are common zooplankton both in freshwater and in brackishwater. They are natural feeds for larvae and juveniles of many finfish and crustaceans (Figs. 2a and 2b). In the wild, most marine larvae feed on copepod eggs and nauplii during the first few weeks of life.
Because some species of copepods have very small larvae (a necessity for some larval fish species) and can have very high levels of HUFAs and other essential nutrients, they are an excellent food source for first-feeding larvae.
In fact, a number of marine larval fish cannot be reared using rotifers as the first feed but have been reared on either laboratory reared or wild caught copepod nauplii.
Research with several species, such as the turbot and red snapper, has shown that when offered mixed plankton diets, young larvae consume more copepod nauplii than rotifers and prefer copepod nauplii because of the differences in size and swimming patterns of the two prey types.
Consequently, there is considerable interest in the use of copepods as feed sources for small marine larval fish. Copepods are cylindrical with a trunk comprised of 10 segments, consisting of head, thorax and abdomen. Adult copepods range from 0.5 to 5.0 mm.
The larval stages consist of six naupliar and six copepodite stages. The main suborders of copepods found in brackishwater are calanoids (Acartia, Calanus and Pseudocalanus spp.), harpacticoids (Tisbe and Tigriopus spp.), and cyclopoids (see Fig. 2a for shape differences).
Herbivorous copepods are primarily filter feeders and typically feed on very small particles. But they can feed on larger particles, which gives them an advantage over the rotifers. Copepods can also eat detritus.
They differ from Artemia (brine shrimp) and rotifers in that they do not reproduce asexually. Copepods mate after maturing and the female produces 250 to 750 fertilized eggs (rotifers produce 15 to 25 per female). The copepod lifespan is 40 to 50 days (5 to 12 days for rotifers), and it has a longer generation time (1 to 3 days for the rotifer and 7 to 12 days for the copepod).
Unlike the rotifer, copepods are more difficult to culture on a commercial basis. Only a few species of copepods, such as Tigriopus japonicus, have been mass cultured successfully. Even this technique employs the combination of rotifer culture and the use of baker’s yeast or omega-3 yeast as feed.
Unfortunately, the amount of yeast used to produce the copepod and rotifer combination outdoors is fairly high. There are outdoor production systems that can produce large numbers of copepods; however, these systems are very inefficient in terms of number of copepods per liter of culture water.
Considerable work needs to be done on culture and harvest techniques before copepods become as widely used as rotifers. One interesting advantage of copepods is that under appropriate conditions some species will produce a resting egg similar to that of Artemia.
So once commercial techniques are developed, copepod eggs could be collected in large numbers and stored for months, like Artemia (brine shrimp) and rotifer cysts. Photoperiod and temperature largely determine the production of copepod resting eggs.
Laboratory production of these eggs is possible, but has not yet proved to be economically feasible. It is hoped that using copepods as a food source can improve the culture of a variety of species, such as the red drum, by reducing the size variability and mortality.
The use of copepods, especially the harpacticoids (Fig. 2a), is well documented in marine fish culture. Researchers have reared copepods in vessels of 100 liters (26 gallons) and 450 liters (118 gallons) and reported that the system provides 250,000 nauplii per day.
The Japanese have routinely cultured the copepods Tigriopus and Acartia for rearing fish larvae approximately 7mm in length. U.S. researchers compared the growth and biochemical composition of mahi-mahi (Coryphaena hippurus) larvae that were fed brine shrimp, rotifers and the copepod Euterpina acutifrons, cultured in 700-liter tanks.
Larvae fed copepods survived better under stressful conditions. A system for the mass culture of a benthic marine harpacticoid copepod, described by Sun and Fleeger (1995), should be useful for aquaculture.
Other copepods considered to be promising species for mass culture are Acartia clausi, A. longiremis, Eurytemora pacifica, Euterpina acutifrons, Oithona brevicornis, O. similis, Pseudodiaptomus inopinus, P. marinus, Microsetella norvegica and Sinocalanus tenellus.
Cladocerans or water fleas (Fig. 2c), such as Daphnia magna, have been cultured as live food using techniques similar to those described for rotifers. Many laboratories use Daphnia as the invertebrate of choice to conduct toxicity tests because it is easy to culture and maintain in the laboratory.
Cladocerans are mainly freshwater zooplankters; most do not tolerate salinities higher than 3 ppt., and are generally not found in brackishwater. One exception is Diaphanosoma celebensis (=aspinosum). In Asia there is a growing use of this species.
This is a saline-tolerant (1- to 42-ppt) water flea in the 400- to 800- micrometer range that has been successfully cultured in backyard hatcheries. Biomasses of up to 1 kg in 1 cubic meter of water every 3 days have been reached.
To be effective as a replacement, the organism must be enriched before it is fed. This enrichment is accomplished with a source of DHA, but usually not one with an oil emulsion base because of gill and water fouling problems.
Schizochytrium (a spray-dried or drum-dried algae developed by Omega-Tec, Inc.), is the most common enrichment agent used in Thailand for Diaphanosoma. mean densities of 72 to 100 individuals per ml could be maintained on Tetraselmis chui after maximum density was attained (for general culture).
In Thailand, culturists are growing Diaphanosoma on Chlorella sp. In 1998, researchers at SEAFDEC in the Philippines successfully used Diaphanosoma as an Artemia substitute for Barramundi larvae (Lates calcarifer).
Other cladocerans considered promising species are Evandne tergestina, Penilia avirostris and Podon polyphemoides. The cladoceran Moina macrocopa has been used in Southeast Asia as feed for sea bass fry immediately after weaning from Artemia and prior to feeding minced fish flesh.
During this period, sea bass, being a catadromous species (moving into freshwater for a portion of its life cycle), may be reared at lower salinities and fed freshwater zooplankton. This practice is not commonly used or proven to be viable on a commercial scale. A related cladocera, Moina salina, has been used in finfish culture in Spain.
Tintinnid ciliates are consumed by larval fish and crustaceans in the wild and are considered promising candidates for mass production. However, since the technology for mass production of rotifers is well established and microparticulated diets are being co-fed with rotifers or have been developed to partially substitute for live food, the role of copepods, cladocerans and tintinnid ciliates is not as important.
Authors:
Granvil D. Treece and D. Allen Davis
