Copepods as fish food organisms

 Copepods

Copepods are common zooplankton both in freshwater and in brackish water. Their advantages being wide range of body size within and between species. They are natural feeds for larvae and juveniles of many finfish and crustaceans. The early stage nauplii and copepodites can be extremely useful as initial prey for species that have very small mouth gape at first feeding.

In the wild, most marine larvae feed on copepod eggs and nauplii during the first few weeks of life. Since a number of larval fish cannot be reared using rotifers as the first feed but have to be reared on either laboratory reared or wild caught copepod nauplii. Adult copepods range from 0.5 to 5.0 mm. The larval stages consist of six naupliar and six copepodite stages.

4.2. Copepod classification

    Kingdom :Animalia
      Phylulm :Arthropoda
        Sub-phylum : Crustacea
          Class : Maxillopoda
            Sub-class : Copepoda

4.3. Biology and life cycle of copepods

Most adult copepods have a length between 1 and 5 mm. The body of most copepods is cylindriconical in shape, with a wider anterior part. The trunk consists of two distinct parts, the cephalothorax (the head being fused with the first of the six thoracic segments) and the abdomen, which is narrower than the cephalothorax. The head has a central naupliar eye and unirameous first antennae that are generally very long. Planktonic copepods are mainly suspension feeders on phytoplankton or bacteria; the food items being collected by the second maxillae. The male copepods are commonly smaller than the females and appear in lower abundance than the latter. The eggs hatch as nauplii and after five to six naupliar stages (moltings), the larvae become copepodites. After five copepodite moltings the adult stage is reached and molting is ceased. A diapause stage is present in the development of the copepods so as to survive adverse environmental conditions, such as freezing. The major diapause habitat is the sediment, although a minor part of the diapausing individuals may stay in the planktonic fraction, the so-called “active diapause”. Harpacticoid copepods are less sensitive and more tolerant to extreme changes in environmental conditions (i.e. salinity: 15-70 g/l; temperature: 17-30°C) than calanoids and thus are easier to rear under intensive conditions. Moreover, harpacticoids have a higher productivity than calanoids and can be fed on a wide variety of food items, such as microalgae, bacteria, detritus and even artificial diets. However, as mentioned previously, care should be taken in this respect as the lipid and (n-3) HUFA composition of the copepods is largely dependent on that of the diet fed.

 

4.4. Candidate species and culture techniques

The main suborders of copepods found in brackishwater are calanoids (Acartia, Calanus and Pseudocalanus spp.), harpacticoids and cyclopoids. Herbivorous copepods are primarily filter feeders and typically feed on very small particles. But they can feed on larger particles, which give them an advantage over the rotifers. Copepods can also eat detritus. Calanoida is an important order of copepods, a kind of zooplankton. They include 43 families with about 2000 species of both marine and freshwater copepods. Calanoid copepods are important in many food webs, taking in energy from phytoplankton and algae and 'repackaging' it for consumption by higher trophic level predators like birds, fishes and mammals. Many commercial fishes are dependent on calanoid copepods for diet in either their larval or adult forms. Baleen whales such as the bowhead whale eat copepods of the genera Calanus and Neocalanus.

 

4.5. Culture techniques

A continuous production system for the calanoid copepod Acartia tonsa consists of three culture units: basis tanks, growth tanks and harvest tanks. The Acartia tonsa are isolated from natural plankton samples or reared from resting eggs onwards. The basis tanks (200 l grey PVC tanks: 1500 x 50 cm) are run continuously and the eggs produced are used to adjust population stocks. These tanks are very well controlled and kept under optimal hygienic conditions: using filtered (1 μm) sea water (salinity 35 g/l) and fed with Rhodomonas algae. Temperatures are kept at 16-18°C and a gentle aeration from the bottom is provided. Adult concentrations with a ratio of 1:1 males to females are maintained at less than 100 per l by adjusting once a week with stage IV - V copepodites. Approximately 10 l of the culture water is siphoned daily from the bottom of the tanks (containing the eggs), and replaced by new, clean seawater. Eggs are collected from the effluent waters by the use of a 40 μm sieve; production averaging 95,000 eggs/day, and corresponding to a fecundity rate of 25 eggs/ female per day. The basis cultures are emptied and cleaned two to three times per year, by collecting the adults on a 180 μm sieve and transferring them to cleaned and disinfected tanks. Collected eggs are transferred to the growth tanks where maximal densities reach 6000/l. The nauplii start to hatch after 24 h with hatching percentages averaging 50% after 48 h incubation. Initially Isochrysis is given at a concentration of 1000 cells ml/1 and after 10 days a mixture of Isochrysis and Rhodomonas administered at a concentration of 570 and 900 cells ml/1, respectively. The generation time (period needed to reach 50% fertilized females) is about 20 days with a constant mortality rate of about 5%/day. After 21 days, the adults are collected using a 180 μm sieve and added either to the basis or harvest tanks. Harvesting tanks are only in use once the fish hatchery starts to operate. Cultures are maintained in 450 l black tanks under the same conditions as described above. Each tank receives a daily amount of 16 X 108 Rhodomonas cells, harvested from bloom cultures. These tanks are emptied and cleaned more regulary than stock tanks. To facilitate the harvesting of solely nauplii or copepodites of a specific stage (depending on the requirements), eggs are harvested daily and transferred to the hatching tanks; the aeration levels within these tanks being increased to maintain 80% oxygen saturation. Nauplii of appropriate size (and fed on Isochrysis) are harvested on a 45 μm screen and by so doing cannibalism by the copepod adults is also minimized.

 

4.5.1. Use of resting eggs

Many temperate copepods produce resting eggs as a common life-cycle strategy to survive adverse environmental conditions. Resting eggs can tolerate drying at 25°C or freezing down to - 25°C and that they are able to resist low temperatures (3-5°C) for as long as 9 to 15 months. These characteristics make the eggs very attractive as inoculum for copepod cultures. Samples of sediments rich in resting eggs can be stored in a refrigerator at 2-4°C for several months. When needed, the sediment containing the resting eggs is brought in suspension and sieved through 150 μm and 60 μm sieves. The resting eggs is then immersed in the disinfectant, (i.e. FAM-30 or Buffodine); surface-disinfection being needed to eliminate contaminating epibiotic micro-organisms. After disinfection, the eggs are then washed with 0.2 μm filtered sterile seawater and transferred to disinfected culture tanks or stored under dark, dry and cool conditions. Before, starting the surface-disinfection procedure attention must be paid to the physiological type of resting eggs. Some marine calanoids are able to produce two kinds of resting eggs, i.e. subitanous and diapause eggs. Since subitanous eggs only have a thin vitelline coat covering the plasma membrane, they are more susceptive to disinfectants than the diapause eggs which are enveloped by a complex four-layer structure.