Development of Production Technology of Juvenile Artemia Optimal for Feeding and Production of Atlantic Halibut Fry

Abstract

The goal of the studies that are reported in this thesis was to develop adequate methods for the production of good quality fry of Atlantic halibut (Hippoglossus hippoglossus L.). In this connection, prey preferences of the Atlantic halibut larvae is important. When giving en the choice between rotifers and short term (ST) enriched nauplii, the Atlantic halibut larvae showed no preferences for any of these the first two days of the live feed period. From day 3 and onwards the larvae preferred ST nauplii. When the larvae had the opportunity to select between different size classes of A. franciscana, the preferred size was the newly hatched nauplii during the first 3 weeks. Around day 20 at 3-4 mg dry weight (DW), the larvae showed a pronounced shift in their preferred size of A. franciscana, and they started to select for individuals of 1.2-1.4 mm length or longer. The size corresponds to A. franciscana cultivated for 3 days.

The larvae apparently digested the newly hatched nauplii better than the ST nauplii during the first weeks, this presumably because of the high lipid content of the latter. Also, the bigger size classes of A. franciscana seemed to be more effectively digested that the ST nauplii by larvae of less then 2.4mg DW. Bigger larvae showed almost equal and very efficient digestion of all size classes of A. franciscana. The larvae never selected positively for ST nauplii, and use of newly hatched nauplii for some time may be beneficial based on the selection and digestion patterns of the larvae. However, the newly hatched nauplii do not contain DHA (docosapentaenoic acid) and are therefore nutritionally inadequate for the larvae. This size class of Artemia may be beneficially used for a few days after onset of exogenous feeding provided that the nutritional status of the larvae at the onset of exogenous feeding is adequate and that the nutritional quality of the feed used later is satisfactory.

The larvae selected positively for larger A. franciscana in a period before the reached the size when they normally start to accept formulated food. Prey of the size 1.2-1.4 mm (i.e. 3-day-old A. franciscana) seemed to be the optimal size for the larvae after about day 20 in the lived feed period. It was possible to produce 3-day-old A. franciscana with 18 mg DHA g-1 DW and a DHA/EPA ratio > 1.0 by using a diet containing more than 39% lipids and 10% DHA, respectively. A high DHA/EPA ratio (>1.0) is needed after the production because A. franciscana appears to transform DHA to EPA (eicosapentaenoic acid) relatively efficiently.

Attempts were made to stabilise the content of DHA in A .franciscana in the larval tanks by incubating them with the DHA rich microalgea I. galbana. However, 6 mg C 1-1 or higher levels of this alga were negative for the DHA contents of ST A. franciscana which then decreased more rapidly during the first 24 h than at lower algal concentrations. This was most likely due to a replacement of lipids by algal cells in the gut of the nauplii. In the case of 3- and 4-day old A. franciscana, the effect of addition of I. galbana on the DHA level in the animals was small for the algal levels normally used in the larval tanks (1-2 mg C1-1).

The microalgea Tetraselmis sp. was used to reduce bacterial level and change bacterial flora associated with the live feed. During a 4 h incubation of 2-day-old A. franciscana with this alga, the number of associated bacteria and Vibrio spp. were reduced significantly. This effect was even more pronounced after transfer to first feeding conditions. The composition of the microbial flora associated with the animals became more diverse, and less dominated by Vibrio alginolyticus, which was the dominating species associated with 2-day-old A. franciscana. The results also showed that the algae indirectly affected the larvae through changing the associated bacterial flora of the live feed. Larvae which received algal-treated 2-day-old A. franciscana had significantly lower numbers of Vibrios and bacteria with haemolytic activity compared to larvae fed non-treated- feed. In another first feeding experiment, 4-day-old A-franciscana had high numbers of associated bacteria, and especially of bacteria exhibiting haemolytic activity, even if the procedure for algal incubation was used. Differences in bacterial flora associated with the live feed may lead to differences in growth of larvae.

Atlantic halibut juveniles with better quality was obtained with 20% perfect larvae when juvenile A. franciscana was used as feed compared to about 4% with the use of St nauplii. The 3-day-old A. franciscana seems to be the optimal size of the larvae from about day 20 after first feeding because this size has many useful properties. It is the most preferred size by larvae from this time in the live feed period, it is effectively digested by the larvae and it can be produced with a relatively high DHA content (<18 mg g-1 DW). The main challenge is to improve the microbial quality of this feed.