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By Akvaforsk - Solveig van Nes of AKVAFORSK has found one of the key factors that influences the sexual development of halibut larvae. Since females grow significantly larger than males, farmers can now direct their production toward fewer males and higher profits. The method is simple: Use the same temperature as the ocean!

Cold Water Results in Fewer Males - By Akvaforsk - Solveig van Nes of AKVAFORSK has found one of the key factors that influences the sexual development of halibut larvae. Since females grow significantly larger than males, farmers can now direct their production toward fewer males and higher profits. The method is simple: Use the same temperature as the ocean!

van Nes presents her research findings in her doctoral dissertation, which she will defend at the Norwegian University of Life Sciences (UMB) on Friday.

Halibut is a product in high demand, especially female halibut since they become much larger than their male counterparts. Because of the favourable characteristics of the female halibut, van Nes wanted to find a method of increasing the number of farmed females. The sexual development of fish larvae in several different species appears to be sensitive to temperature. It was therefore interesting to investigate whether the sexual development of farmed halibut was also affected by temperature. In the wild, halibut larvae live at a temperature of 5-7° C, whereas in fisheries the temperature is 11-13° C. The advantage of the high temperature is that it accelerates the growth of the larvae, which then develop more quickly through the demanding larva stage and into a more robust stage.

In her research, van Nes found that increasing water temperature in the larva stage resulted in a larger number of males. At a temperature of 13° C during the larva stage, 62 per cent of the larvae developed into males. She also examined several genes that are involved in sexual development, and successfully isolated two aromatase genes responsible for the production of the female hormone oestrogen. One of these genes proved to be particularly sensitive to temperature, thus hampering oestrogen production at high temperatures. This is the reason for the large number of males that develop at 13° C. In contrast, this aromatase gene is more active in cold water, causing more halibut larvae to develop into females so that the proportion of males to females is close to 50-50.

van Nes is not surprised that it is the natural temperature that provides the best conditions for the sexual distribution of fish since this is also true for several different species. "However, it is not always easy to evaluate the effects of temperature on important characteristics during production. If we conduct more research in this field, we will have a basis for re-evaluating the optimal temperature for halibut," explains the doctoral candidate.

Solveig van Nes, 33 years old, is from Halden, Norway. She has a master's degree in marine biology/genetics from the University of Groningen in the Netherlands. She defends her doctoral dissertation on Friday, 20 January 2006, at 12:15 pm in the Ur Building at UMB in s, Norway. Her adviser is Dr. ivind Andersen of AKVAFORSK. The evaluation committee consists of Dr. David Penman (University of Stirling), Dr. Mariann Rand-Weaver (Brunel University) and Dr. Hans Magnus Gjen (UMB). The title of her lecture is "Applications of research of sex determination in aquaculture".

Source: Akvaforsk Institute of Aquaculture Research - February 2006

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