Pasteurella piscicida in Israeli aquaculture.
The first outbreak of Pasteurella piscicida in Israel occurred in seawater ponds at the Mediterranean coast, in juvenile seabream, in April 1993. In October 1993, P. piscicida was detected in hybrid striped bass, in brackish water fish ponds, 25 km away. In 1994, an outbreak of the disease among juveniles of hybrid striped bass, in that farm, caused direct losses estimated at 1 million US $. In June 1994 it was diagnosed in a hatchery on the Red Sea coast, in 84-day-old gilthead seabream and a week later in juvenile seabass. After 7 months, the food chain, the larval rearing department and the nursery were dried out and sterilized thoroughly, including tanks, equipment and pipes. After the clean up P. piscicida was found again in the first batch, in 75-day-old seabream. About 750 000 fish died. In March 1995 an outbreak, of P. piscicida in a new hatchery at the Mediterranean coast, caused the loss of 500 000 seabream and seabass.
The bacterium was identified at the National Center for Mariculture (NCM), Eilat, Israel. An antibiogram showed that the bacterium was most sensitive to two antibiotics, Flumequine and Oxolinic acid (MIC 0.2 ppm). Although Flumequine was added to the feeds, fish mortalities in some Eilat batches reached 80%. The disease also gradually affected younger (40-50) day-old fish. A new antibiogram with a recent isolate (Eilat, May 1995), 11 months after the first antibiogram, showed an increased resistance of the later isolate to some of the antibiotics previously used.
Different species of fish seem to have different reactions to P. piscicida. Seabream and seabass exhibit mortalities in the nursery from around 70-80 days post hatch (0.3-0.5g). The fish that survived did not display disease symptoms again. Red drum Sciaenops ocellatus larvae and juveniles seem to be resistant to the bacteria, while hybrid striped bass seems to be highly susceptible showing repeated infections up to market size.
Early detection of the bacterium is essential for proper treatment. Detection by bacteriological means or ELISA kits are not always practical or even feasible when fish larvae are involved. The NCM is developing a PCR test for the identification of P. piscicida, similarly to the test that was developed for another important fish pathogen, Mycobacterium marinum. Such a test is expected to be highly sensitive, allowing detection of the disease in carriers and perhaps tracing the sources of infections.
(Ardag, P.O. Box 1742, Eilat 88116, Israel)
The effect of genetics and environment on the maturation and larval quality of the giant tiger shrimp Penaeus monodon.
One of the major difficulties in domesticating the giant tiger shrimp, Penaeus monodon, has been the lack of ability to obtain large numbers of good quality spawners from pond-reared animals. Although a proportion of successful spawners have been obtained from ponds the productivity of these animals has not been high, and more often than not productivity is poor. This has reduced the capacity to develop and maintain selected stocks, although there have been examples of the small-scale maintenance of broodstock over a decade. The challenge remains to develop technologies for large-scale production of high quality larvae for industrial application.
Research to date has focused on the role of nutrition and rearing environment in the successful maturation of penaeid prawns, and on the quality of the larvae produced. However, novel approaches to this problem are needed, together with the recognition that fundamental aspects of the biology of penaeid shrimps will require investigations before the problem of reliable production of high quality larvae will be solved.
Recent research at the Australian Institute of Marine Science has been targeted at the genetic and hormonal control of reproduction and egg quality. Information on the hormone systems of Crustacea is extremely limited compared with that of vertebrates, and work is concentrating on fundamental aspects of the genetic and hormonal controls on these systems. The paper will review published information on the maturation and larval quality of penaeids and discuss novel approaches that may provide the solution to a critical problem preventing further development of shrimp farming.
(Australian Institute of Marine Science, PMB No. 3, Townsville, Qld 4810, Australia)
Commercial feasibility of semi-intensive larviculture of Atlantic halibut (Hippoglossus hippoglossus L.).
Production described is in 75 m3 large floating units in a pond in western Norway. Production and supply of larval food is mainly based on the ponds regular zooplankton production, starting with dormant eggs hatching in March. At the time of metamorphosis in June, zooplankton production is declining, possibly due to a regular increase in temperature, and Artemia is supplied until metamorphosed juveniles are transferred to be weaned in land based tanks near the pond, in the last days of June. The production in 1994 was 320 000 juveniles at metamorphosis, with an avarage survival of 35% from startfeeding to metamorphosis, and a production of more than 200 000 juveniles for further ongrowing.
A considerable weight variation at metamorphosis is due to a probable individual variation in larval growth conditions in large outdoor tanks with the light regime causing great patchiness and variability both in larval and zooplankton distribution. Growth can further be restricted by the size of the prey, especially in periods with dominance of small developmental copepodid stages.
In a production like this, where production regularity is maintained by the existence of a regular cycle of zooplankton abundance, low production cost is assumed quite possible by combining weaning and ongrowing facilities in one site.
(Stolt Sea Farm AS Bergen, C. Sundtsgt. 57, P.O. Box 1798, Nordnes, N-5024 Bergen, Norway)
The use of behavioural observations to increase production of cold-water marine fish larvae.
The use of behavioural observations on larval marine fish has a long history and has added considerable depth to our understanding of the ecological adaptations of larval fish, but has rarely been used in conjunction with larviculture. In much of marine fish larviculture we are trying to domesticate wild animals. Quite often we do not have a good understanding of the ecological conditions to which they are adapted in the wild and do not use appropriate hatchery or rearing protocols. Behavioural observation, in conjunction with growth and survival information, is a powerful tool for understanding the conditions to which larval fish are adapted. Using all this information we can then adjust protocols to enhance production. An area which could benefit from this approach is feeding. For example, behavioural observations have recently been used to modify the feeding protocol for Atlantic salmon (Salmo salar) and Atlantic cod (Gadus morhua) in sea cages. In this paper we report on the results of two experiments carried out on feeding frequency and weaning of two species of marine larvae.
In the wild, fish larvae are likely exposed to prey on a periodic basis as zooplankton is known to be patchily distributed in the sea. In marine fish larviculture, larvae are typically exposed to prey on a continuous basis. Results of experiments carried out on lumpfish (Cyclopterus lumpus) fed the same density of live prey in pulses (twice or three times a day) or continuously, demonstrated that growth and survival were not affected. Foraging behaviour was much higher in the pulse treatments. A second set of experiments was performed on weaning of Atlantic wolffish (Anarhichas lupus) larvae. Live prey and pelleted food was offered to larvae from the onset of first-feeding. Observations on larvae indicated that for the first couple of weeks, they fed mostly on live prey with some pellet feeding. The pattern changed and by 8 weeks larvae consumed all the pellets, then took live prey. In this example, the behaviour of the larvae suggested a schedule which could be used to wean wolffish from live prey to pellet food.
(Ocean Sciences Center, Memorial University of Newfoundland, St. John's, Newfoundland, Canada A1C 5S7)
