Larger salmon post-smolts do not grow as well in
marine cages as traditional 100g smolts, according to new research. A trial undertaken as part of the CtrlAQUA project showed that fish that
were transferred from land-based facilities at 100g grew better than those
transferred at 600g, surprising the researchers involved. Methodology The trial started with 30 gram salmon juveniles
in tanks with recycled water at Nofima's research facility in Sunndalsøra. The control group were smoltified by
light control (winter signal) and were transferred to the sea as 100 gram smolt
in August. The other groups were either exposed to 24-hour light for the entire
land-based phase, or given a winter signal with a short day for six weeks as
the control fish. Both light regimes were tested in both freshwater and
brackish water, and the fish were transferred to the sea at around 200 grams in
October or 600 grams in December. The fish were slaughtered at the end of
November the following year, when they had reached around four and a half
kilos. Results The most efficient production regime in the RAS
involved the use of brackish water combined with 24-hour lighting for the
entire phase. The positive effect on growth by not giving the fish a winter
signal was greatest when the fish were 200 grams, but still noticeable when
transferring the 600 gram fish. The fish with the best overall growth were,
however, those that were transferred to sea at 100 grams after undergoing
traditional smoltification. The fish that were 600 grams at the time of
transfer had the poorest growth in the marine cages. The latter ate little
during the first three weeks in the sea, and had poorer growth in the summer.
Although there were growth differences between the groups, all groups had good
growth compared to the performance in commercial farming, with growth factors
(TGC) between 2.9-3.3 in the sea phase. The larger post-smolts ate little during the
first three weeks in the sea, and had poorer growth in the summer. What was perhaps most surprising, say the researchers,
was that the fish in all of the production regimes endured the transfer to
seawater, even though not all were smoltified in the traditional way by light
control. Survival in the sea phase was 93 percent, and there were no
differences between the groups. "It appears that salmon are quite flexible with
regard to time of transfer to seawater, and this provides opportunities for
several different production strategies," researcher leader Trine Ytrestøyl
says. Sexual maturation Early sexual maturation among male fish is a
problem in land-based salmon production. Sexual maturation is affected by light
regime and temperature that control the growth rate. "We saw a clear tendency towards higher rates of
sexual maturation among male fish in the RAS facility when they had reached 600
grams, compared to when the fish were 100 and 200 grams. But we found no clear
correlation between light regime or salinity in the recycling facility and
sexual maturation," Ytrestøyl said. "In the sea phase we found very few
sexually mature male fish. Perhaps a greater proportion of the male fish would
have become sexually mature if they had remained in a recirculating facility until
slaughter, but this was not tested in this trial." Conclusion The researchers' conclusion is that several
strategies may be used for the production of young salmon in an RAS and that
the most effective production regimes in an RAS don't necessarily result in the
best growth in the sea phase. However, for a fish farmer, it may be of greatest
importance to minimise the time in the sea. Because the fish had different production times
in the land phase, they were transferred at different times in the autumn, at
different temperatures and day lengths. "These are factors that affect the growth
patterns of salmon, and we should therefore be cautious before drawing
conclusions on which production regimes in recirculating facility that are best
with regard to growth in the sea. However, based on the results of this trial,
a traditional smolt of 100 grams was the most effective strategy," Ytrestøyl
says. It remains to be seen whether one would have
achieved the same results if the fish had been transferred to sea in the
spring, at a time of rising sea temperatures and longer days. Sponsored content - article continues below Ytrestøyl nevertheless emphasises that in order
to reduce the risk of escape and sea lice, it is sensible to reduce the time
the salmon spend in the sea. This illustrates that there are many unanswered
questions with regard to how large post-smolt should be produced, and how the
season at the time of transfer affects performance in the sea phase. Research will continue
into these issues at CtrlAQUA.
New trials by Nofima and
CtrlAQUA show salmon transferred to marine sites as large post-smolt don't
always reach slaughter size before those transferred as 100 g smolt © Terje
Aamodt, Nofima
Reproduced with permission. Copyright The Fish Site
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