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With sea lice being a major challenge for the salmon industry, at Aquaculture Europe 2011, Herve Migaud from the University of Stirling looked at what factors are limiting wrasse production. Charlotte Johnston, TheFishSite editor reports.

Lice hamper growth and promote other diseases, said Mr Migaud. It is estimated that sea lice cost the Scottish salmon industry 30 million a year.

Not only is it affecting production and welfare, but there are public perception issues arising as well.

Whilst current medications used are working, increased resistance to these products is a huge issue.

Trials are ongoing to look at vaccines and breeding selection tools, however time is against us, he said.

Wrasse polyculture is seen as a green solution, the only solution, said Mr Migaud.

Wrasse have been used since the 1970s, however uptake has been limited due to the available alternatives.

"Wild harvested wrasse are not sustainable," he said. "Therefore we must culture them ourselves."

The technology to harvest wrasse is still in its early stages. Key challenges include the capture and establishment of wild wrasse broodstocks, control of sexual differentiation and egg production, development of larviculture / juvenile production protocols (feeding, stocking, environmental conditions) and wrasse deployment strategies (feeding in the absence of sea lice, reuse, replacement, transfer).

Current best practice guidelines from Norway, where the methodology is established, suggests that cleaner wrasse need to be included in salmon pens at the level of one to four per cent to ensure effective lice management.

Therefore a large quantity of wrasse are needed to meet requirements for salmon.

There are two main species of wrasse, Ballan wrasse (Labrus bergylta) but also goldsinny (Ctenolabrus rupestris).

Wrasse begin life as females, however some become males later in life - usually after a few years of breeding as females they become males.

Successful captive breeding requires due consideration of both the reproductive physiology and the social biology of wrasse. Establishing appropriate breeding populations is difficult to achieve. The sex ratio in wild ballan populations is approximately 0.1 males per female (Dipper et al. 1977), and fish >18 years of age (>400mm) are predominantly, but not exclusively, male.

To identify sex, Mr Migaud recommends three different approaches.

1) Ultrasound scanning for non-invasive sexing as routinely used for sexing and staging in salmonids, Atlantic cod and flatfish species. This however would only work on mature wrasse.

2) Fish morphometric truss measurements commonly used to differentiate ontogenetic changes within fish species.

3) Development of a vitellogenin agglutination test.

To date spawning of wrasse in captivity has proven be erratic and problematic. There are several possible reasons for this which include stocking density, territoriality and spawning behaviour, said Mr Migaud.

Preliminary work has been carried out looking at three photoperiod regimes (two months advanced, ambient, two months delayed) for out of season production.

Concluding, Mr Migaud said that a reproductive strategy is needed to understand sex differations, confirm fecundity and aid broodstock development in wrasse.

November 2011

Charlotte Johnson

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