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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. |
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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.
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A huge size range of eels
was evident after only 9
months of culture.
Growth of
cultured
shortfinned eels. |
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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.
Don
Jellyman,
NIWA [ d.jellyman@niwa.co.nz ]
Mark Lokman, University of Otago |
