Encouraging results from freshwater eel culture
Science Centres: Fisheries
The international market for cultured eels exceeds 100 000 tonnes. Both Southeast Asia and Europe, which are the primary markets, have experienced significant reductions in supplies of glass eels over recent years, limiting the possibilities for further growth of their eel farming industries. There is increasing interest in eel farming in New Zealand to supply these overseas markets.
A huge size range of eels was evident after only 9 months of culture.
Growth of cultured shortfinned eels.
Eel farming was trialled in New Zealand in the 1970s, but proved uneconomic. Since then there have been marked changes in technology. The development of recirculating systems for growing eels at high density in heated water now provides much greater production efficiency.
NIWA is currently determining the best culture and husbandry techniques for aquaculture of eels in fresh water (as first reported in Aquaculture Update No. 29 (2001)), and will shortly be starting trials in salt water. Over the past year NIWA has been involved in a cooperative culture venture with Gould Aquafarms of Leeston, Christchurch. Glass eels have been grown in aquaria with a recirculation system that uses glass-plate lamellar separators to trap large particulate matter and a biofilter to maintain water quality. Although the water was unheated, the well insulated surroundings meant that temperature remained between 18 and 28 °C, averaging 18–19 °C in winter and 24–25 °C in summer. These comparatively warm temperatures enabled year-round growth of the eels. Ammonia levels were checked daily because free ammonia is extremely toxic to eels and the toxicity increases markedly with increased pH.
The 700 glass eels introduced into four aquarium tanks in early November 2001 were initially fed on natural foods like minced tubifex worms or fish (fish roe had been used in the Christchurch laboratory trials). Within 2 weeks, the glass eels were feeding on imported artificial food, which was fed as sticky dough. The feeding rate throughout the 9 months of culture was about 2.5% of total body weight per tank per day, and the pellet size was gradually increased from 0.4 to 2 mm. Growth was assessed by periodic bulk weighing of the eels in each tank, but with the largest 20 eels individually weighed at the last two weighings. Grading into three arbitrary size categories reduced obvious size hierarchy effects. Apart from some initial escapees, mortality was very low.
At 9 months 54% of the eels were classed as “small” and averaged only 3.5 g. The “large” eels (34%) averaged 62 g at this time, with the 20 largest eels, which ranged from 114 to 275 g, averaging 170 g. Obviously, eels have potential for rapid growth, as evidenced by the 12 fish (3%) that exceeded the target weight of 150 g within the 9 months.
The growth achieved in these trials is extremely encouraging, and indicates the potential of New Zealand freshwater eels as a culture species. Our future work will explore ways of reducing the extreme variation in growth (3–267 g in 9 months) seen in the trials. The usual method to reduce size variation is to increase the frequency of grading, but the stress of grading can result in the eels not feeding for several days afterwards. Crowding the eels might possibly result in a reduced size-hierarchy effect and more uniform growth at the increased stocking density.