(excerpts from summary of the second year report on the EU Concerted Action project AIR3 CT94 2094 entitled "Utilisation of Copepod Diets for Larviculture of Halibut, Cod and Turbot, and a Review of Published Halibut Research and Cultivation Data", co-ordinated by the Danish Institute for Fisheries Research (DIFRES) in Hirtshals, in collaboration with colleagues from the UK; Sea Fish Industry Authority (SFIA) in Ardtoe, NERC Unit of Aquatic Biochemistry (NERC) in Stirling, Scottish Agricultural College (SAC) in Aberdeen, Norway; Institute of Marine Research (IMR) in Bergen, Spain; Institute of Oceanography (IO) in Vigo, Belgium; Laboratory of Aquaculture and Artemia Reference Centre (ARC), in Gent and with colleagues from Ireland: Aquaculture Development Centre (ADC), in Cork)
Reporting period: 01.12.1995-30.11.1996 (second and final year of project)
For information, contact Josianne Stottrup <SMTP:jgs@dfu.min.dk>
The final meeting was held at Hirtshals, Denmark, during 17-21 November in the form of a Workshop.
Summary:
(...)Batch and continuous production methods for the culture of Tisbe holothuriae have been described and may easily be applied by the industry to replace the current day rotifer production systems. The first 2-3 naupliar stages are available to the fish larvae whereas the more benthic late naupliar and copepodite stages are presumed less available to the pelagic larval stage of marine fish. Thus, few potential benefits of cultures of this copepod species were foreseen partly because these organisms could only replace rotifers and cannot by utilised by larger fish larvae such as halibut and cod. Further, the larval nutritional problems documented so far seem to be associated with the Artemia feeding stage, for which no suitable alternative (in terms of size) is at hand.
In Norway, stocking densities of 500-1500 nauplii and 50-300 copepodites/adults per litre were achieved in outdoor tanks during the period March-October. The results indicate a production potential of 62-162 halibut larvae per m3 copepod culture medium.
In Spain, the influence of food concentration on the reproductive strategy of Euterpina acutifrons was demonstrated. This work was considered instrumental for the further development of mass culture methods for this species: another live prey candidate in larviculture.
In the UK a 150m^3 outdoor mesocosm tank failed to produce sufficient copepods to perform replicated larval feeding trials. As a result the few fish reared exclusively on copepods were used for histological studies.
During the 1996 halibut rearing season the Sea Fish Industry Authority has carried out several halibut larval rearing experiments as part of its research programme. Industry funded research on Artemia enrichment diets has led to advances in the understanding of the processes behind correct larval development. Evidence to support a link between weight-specific-growth-rate through the larval phase and the degree of eye migration has been collected. Failure of pigmentation of halibut fry is still the main problem encountered with the Ardtoe Artemia enrichments. The proposed beneficial effect of lipid manipulation has not yet been demonstrated for this problem and is the subject of continuing investigation.
A review was completed on UK halibut cultivation research. The development of the UK research programme and industry uptake have been described. Production bottlenecks have been identified and discussed in relation to the current research programme. The application of the enclosure technique used primarily in Norway for the rearing of cod, halibut and turbot larvae was reviewed and commercialisation of the methods developed discussed in relation to their constraints.
Samples of intensively cultured Acartia tonsa. and extensively cultured Eurytemora velox were received from DIFRES and SFIA respectively. Carcass and tissue samples from halibut larvae (Hippoglossus hippoglossus) fed Eurytemora or enriched Artemia were also received along with eye and carcass samples from halibut larvae fed a mixture of copepods (mainly Centropages hamatus and Eurytemora affinis) or enriched Artemia from IMR.
Cold storage of Acartia eggs, for periods up to 12 weeks. increased the n-3 polyunsaturated fatty acid (PUFA) content in the newly-hatched nauplii, although this was largely due to increases in 18:3n-3 and 18:4n-3. The fatty acid compositions of Acartia nauplii and adults were readily manipulated using a variety of algal species. In general, Rhodomonas baltica and Thalassiosira weissflogii tended to increase eicosapentaenoic acid (20:5n-3; EPA) more than docosahexaenoic acid (22:6n-3; DHA), whereas Heterocapsa triquetra and Isochrysis galbana tended to increase DHA more than EPA.
Using these four algal species the DHA/EPA ratio in Acartia could be varied from 0.7 - 9.6. In addition. the level of arachidonic acid (20:4n-6; AA) could be elevated over 10-fold by enriching with either Thalassiosira or Isochrysis. The dietary studies with halibut larvae indicated that feeding copepods was beneficial in allowing normal metamorphosis. More than 60% of copepod-fed larvae (SFIA) had full dorsal pigmentation and eye migration while less than 20% of the Artemia-fed group fell into this category. In eye brain liver and carcass the levels of DHA were higher and EPA lower in the copepod-fed group compared to the Artemia-fed group.
The Eurytemora were split into four size ranges before analysis. The highest concentrations of long-chain PUFA were in the smallest size class (< 64 micro m) with decreasing levels in the largest sizes. As size increased Eurytemora tended to accumulate more saturated fatty acids monounsaturated fatty acids and C 18 PUFA. The eyes from the Norwegian halibut trial were subdivided into copepod-fed, Artemia-fed (pigmented) and Artemia-fed (malpigmented). No differences were observed between eyes from pigmented and malpigmented larvae. The fatty acid compositions of eye total lipid phosphatidylethanolamine (PE), phosphatidylcholine (PC) and triacylglycerol (TAG) showed similar trends. DHA levels were considerably elevated in all lipid classes (2.7 to 41-fold) and EPA levels were lower in eyes from larvae fed copepods compared to those fed enriched Artemia.
The vitamin analysis on copepods revealed equal to rather low vitamin C and vitamin levels compared to what is found in the live prey Artemia and rotifers. The use of copepods for extraction purposes could be excluded since large volumes were either not available or the species that was harvested in mass quantities did not have a particular phospholipid or HUFA composition. Since the incorporation of phospholipids and DHA in Artemia are not very efficient and only limited amounts can be bioencapsulated alternative solutions such as co-feeding diets look more promising.
Regarding halibut market prospects, the continued low and variable quality supply of wild halibut makes the commercial farming of this species an attractive proposition. It is acknowledged however that there is still a need for a substantial R&D programme to overcome the existing technical difficulties and make commercial rearing more efficient and cost effective.
The available literature relating to the economics of halibut production was reviewed. A range from 1.5 to 3.75 ECU in costs of production of halibut fry in intensive hatcheries between the estimates of different workers noted in the literature was found to be essentially explainable by differences in (a) methodology -- ie whether return on capital was included in the costs or not, (b) assumed survival rate -- between 3% and 20-30%, and (c) production system -- ie whether cultured copepods were assumed an essential element in the diet. Depending on the price premium obtainable for large fish for steak production, it was estimated that the price which could be economically justified for the on-grower to pay for all-female juveniles at 40 grams could be as twice that for mixed sex stock.
A comparison was made between the estimated costs involved in the early rearing of larvae in an intensive system, using a diet entirely of cultivated live prey (including copepods), and the estimated costs of a semi-intensive system. in which the larvae are held in plastic mesocosms in a lagoon and fed harvested wild zooplankton. The costs of the semi-intensive system were indicated as somewhat lower than for the intensive system at present, however the risks associated with the dependence on blooms of wild plankton in the semi-intensive system were noted, as was the likelihood of progressive reductions in the costs per juvenile for the intensive system with improvements in survival. and if the necessity for copepods in the diet can be overcome.
Possible advantages of combining salmon and halibut on-growing in sea cages depending on the degree of complementarity existing between the two enterprises were identified as:
1. Reducing risk in terms of income fluctuations, by spreading sales across different markets.
2. Reducing risk of serious disease and pest problems by reducing the concentration of any one species on the sea-cage sites.
3. Increasing efficiency of resource use, by alternate and overlapping stocking of sites, and reducing the need for complete fallowing of sites.
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