D. Kahan
Department of Zoology, Hebrew University, Jerusalem, Israel
ABSTRACT
A supply of live food organisms during the early life of fish is a prerequisite for their mass production. In order to diversify the diet, food organisms other than those commonly used for rearing mullet are suggested. Studies on gut contents and feeding behavior of artificially produced Mugil capito fry showed that the following organisms might improve the rearing of mullet fry.
Panagrellus nematodes are recommended for their small size, fast growth rate, nutritional value, tolerance to sea water (after salinity adaptation) and digestibility.
Artemia nauplii, after being cooled at 5°C to slow their movement, can serve as live food for first feeding fry of Mugil capito.
Copepods, the most important natural food of young Mugil, have been grown by an improved method (vegetables instead of micro-algae as food for several marine harpacticoids) to densities higher than previously reported.
RESUME
Pour la production en masses l'utilisation de proies vivantes s'impose pendant les premiers stades de l'élevage de poissons.
Des proies vivantes et des procédés qui diffèrent de ceux normalement utilisés pour l'élevage du mulet sont proposés afin de diversifier l'alimentation. Des études du contenu des intestins et du comportement d'alimentation du larve de Mugil capito (produits par ponte induite) ont montré que les organismes suivants sont susceptibles d'améliorer l'élevage du larve de Mulet:
Nématodes Panagrellus recommandés pour leur petite taille, leur taux élevé de croissance, leur valeur nutritive, leur résistance à l'eau de mer (après adaptation) et leur digestibilité.
Après avoir été refroidis à 5°C pour ralentir leur mouvement, Artémia nauplii peuvent servir de premières proies vivantes pour l'alimentation des larves de Mugil capito.
Les Copépodes, constituant l'alimentation naturelle la plus importante des jeunes Mugil, élevés selon une méthode améliorée (légumes au lieu de micro-algues comme nourriture pour certains harpacticoides marins) à des densités supérieures à celles réalisées jusqu'ici.
Cultivation of mullet was made possible after overcoming problems of induced spawning (Tang et al., 1964; Yashouv, 1969; Shehadeh and Ellis, 1970; Liao et al., 1971; Kuo and Nash, 1975) and rearing of fry (Liao et al., 1971; Kuo et al., 1973; Nash et al., 1974; Sebastian and Nair, 1975). With experience, rearing methods improved and survival increased (Liao, 1975; Houde et al., 1976; Nash et al., 1977). These techniques have been applied with different degrees of success to rear mullet fry in various hatcheries around the world (Cuba, Israel, Italy, Spain, USSR etc.). Nevertheless, mass cultivation of fry is still far from meeting the increasing demand for stocking fish ponds and natural water reservoirs (Oren, 1975; Bar-Ilan, 1975; Pruginin et al., 1975).
Physical, chemical and biological factors impeding the rearing of mullet have been discussed by Nash and Kuo (1975). The main difficulty was attributed to the small size of the newly hatched fry and the limited variety of cultivated organisms mostly Brachionus plicatilis that could be offered as food (Girin and Person-Le Ruyet, 1977; Girin, 1977; 1978; Fujita, 1977).
The presence of various organisms in the gut contents of Mugil capito fry, as well as our observations on their feeding behaviour, indicated that organisms additional to those commonly used, could be offered as food. Furthermore, a diversity of organisms could provide a more nutritious diet.
The studies were carried out on artificially produced fry that were kindly supplied by the late Dr. Yashouv during 1970–71, and by Rothbard in 1978, from the Fish and Agriculture Research Station, Dor, Israel. The fry were kept in asbestos containers in “green water” to which marine plankton was added from time to time. The gut of 1–2 week old Mugil capito contained copepods, lamellibranch and gastropod larvae, algae and detritus. Similar findings were obtained with Mugil cephalus (Liao et al., 1971).
With the aid of a stereoscopic dissecting microscope (15x), Mugil capito fry from the 1970–71 stock were seen feeding eagerly on washed Panagrellus nematodes (Kahan and Appel, 1975). Five hours later, digested nematodes were found in the gut. In nature, too, nematodes have been found in the gut of young mullets (Thomson, 1966; Vallet et al., 1970; Albertini-Berhaut, 1973). Panagrellus nematodes, due to their small size, rapid growth rate and nutritional value, were recommended as food for fish rearing (Larsen, 1941; Bruun, 1949; Sterba, 1967; Ivleva, 1969; Kahan and Appel, 1975). Improvement of growth and decreased mortality was recently obtained with newly hatched fry of silver and common carp when Panagrellus nematodes were added to Aquarian and/or Astra-Ewos dry feed (Kahan, 1978). The drawback, that Panagrellus is not a marine organism, has been overcome by adapting it to tolerate sea water (Kahan and Appel, 1975).
In another series of experiments, M. capito fry were hatched in small aquaria at 20°C. Artemia nauplii that had been cooled at 5°C to slow down their movement could be eaten by the fish fry as their first meal. It had been previously reported that M. cephalus could not handle Artemia prior to the 16th day (Liao et al., 1971; Liao, 1975), while Nash et al., (1974; 1977) reported feeding on Artemia nauplii on the 7th or on the 10th day.
Recently, methods have been developed (Kahan, 1978; 1979) for mass cultivation of various copepods, the most important natural food for young Mugil (Albertini-Berhaut, 1973; Zismann et al., 1975). The density of 160 000 copepods per litre, that was obtained, was higher than the concentration recommended for rearing various marine fish fry (Rosenthal and Hempel, 1970; Liao et al, 1971; Liao, 1975; Nash and Kuo, 1975; Nash et al., 1977; Houde and Taniguchi, 1977, Houde, 1978). High density cultures of copepods were achieved by using vegetables (lettuce, carrots, etc.) as their main diet constituent.
The author's work has been carried out under grants from: Israel Oceanographic and Limnological Research, LTD.; The Robert Szold Institute for Applied Sciences; the National Council for Research and Development and GKSS, Geesthacht-Testerhude, Germany. Thanks are due to Mrs. T. Bar El and Mrs. M. Azury for help in English and French.
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