FBA Issue 32: May / June 2010
An innovative approach to health & nutrition in the rearing of marine fish larvae
Thailand has long been regarded as the pioneer in the rearing of larval marine fish in the South East Asian region, particularly with respect to rearing Sea Bass, Lates calcarifer. Artificial propagation was successfully demonstrated by Wongsomnuk and Maneewongsa (1972).
Since then, the industry has grown in Thailand with many hatcheries scattered along its eastern seaboard, particularly in Chachoengsao, Chonburi and Rayong. Current production estimates are around 800 million 1-inch fry/year, many of which are exported to Malaysia, Taiwan and more recently Vietnam for further grow out in cages and ponds.
In general, hatchery survival rates are fairly low when compared to those of their European counterparts. Hatcheries tend to be smaller and are less capital intensive, with labour being mainly non-technical. General hatchery standards of hygiene tend to be lower and the level of technical expertise with respect to larval nutrition and health is sparse.
However, despite rampant low overall survival rates, the industry is still a highly profitable one. Similar with so many other aquaculture sectors, small changes in routine and product utilization can yield significant differences in survival and growth benefits.
Manipulation of health & nutrition
The simplest area to manipulate and improve upon in terms of fish larval health and nutrition is in the field of bioencapsulation. That is, by nutritionally enhancing the rotifers and Artemia nauplii (brine shrimp), which the larvae of sea bass and other commercial species are fed. Traditional enrichment methods involve concentration of the living food organism in a suspension of nutrients such as essential fatty acids (EFAs) and/or vitamins. As these two food organisms are filter feeders, they concentrate these nutrients within their tissues. After harvesting, they are fed to sea bass fish larvae which benefit from the enhanced nutrient profiles.
There are, however, disadvantages clearly associated with this process. In the case or rotifers exposed to 12 hour enrichment at recommended dosages, high rotifer mortalities are commonplace. Reducing product density reduces product uptake by the target animal and only reduces rotifer mortality rate. This mortality is clearly seen as an accumulation of foam on the surface of the enrichment vessel.
Furthermore, microscopic examination of the rotifers after 12 hour enrichment reveals the rotifers to be very sluggish in their rates of movement compared to non-enriched rotifers and this may well result in further rotifer mortality in the fish larval rearing tanks. In the case of Artemia, the disadvantages may well be considered greater. With 12 hour enrichment times from Instar 2, the nauplius grows rapidly and may well become too large and fast moving for the intended predator fish. Again, there can be naupliar mortality in the enrichment vessel and a concomitant increase in the risk of disease transmission from dead or dying nauplii.
With new enrichment techniques optimising filter feeding particle uptake, the time element can be reduced to a mere 1-2 hours. This leads to reduced labour inputs and easier scheduling of hatching and feeding operations.
The nutritional quality of the prey organism is extremely high. In the case of rotifers there is no significant mortality in the enrichment vessel and the prey organism mobility remains high and in the case of nauplius, growth is limited, therefore the nauplius remains small. Fish larvae therefore find it much easier to capture smaller prey moving within their reactive perceptive fields (RPF) of vision. The risk of contamination is considerably reduced in such a practice.
In summary the short time enrichment process is less work for the hatchery operator and increases nutritional quality for the fish larvae.
What should be used as enrichment materials?
Traditionally this has been heavily skewed in favour of EFAs and vitamins. It is now possible to manipulate individual EFAs depending on the requirement of particular species or particular life stages of a species. As examples, some cultured organisms require EPA whilst others require higher levels of docosahexanenoic acid (DHA) or arachidonic acid (ARA); these can all be controlled and manipulated for the benefit of the target fish/shrimp. Short term enrichment can, however, be applied to a much wider spectrum of nutrients such as proteins and minerals.
In terms of health, it has always been accepted that vaccination is out of the question, although recent work by Shoemaker and Klesius (2005) has shown that immune cells do exist and function in catfish fry as young as 10 days old. There are lessons to be learnt from studying immunity in higher vertebrates and extrapolating these to fish larvae. One such area is passive immunity.
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