| dc.description.abstract | For four years (1980-1983) populations of 5 day old cod
larvae have been transferred to a dammed seawater pond
(60 000 m^3 ). The water volume had been treated with rotenone
in advance, and the larvae were released during the March -
April zooplankton bloom. The hydrography, standing crops of
zooplankton, fish larvae, and phytoplankton were monitored
frequently. The cod larvae had a very high feeding incidence
and the whole population started to grow quickly in all
years, with few or no starving larvae being observed.
In 1980 and 1981, however, large populations of hydromedusae
were present. These probably preyed upon the cod larvae
until metamorphosis (35-40 days post-hatching). Survival to
that stage was about 2%.
In 1982 a mass mortality of larvae was observed immediately
after transfer to the pond. This mortality was probably
due to decomposing fish and zooplankton killed by the rotenone
treatment, which was carried out at 1 ppm that year
compared to 0.5 ppm in other years. In 1981 and 1982 the
numbers of cod at metamorphosis (12 mm length) were estimated
at 15 000, about the same as estimated numbers of 0-group cod
in June - July.
In 1983, two populations of cod larvae (1.2 million and
0.7 million) were transferred to the pond 10 days apart.
Thirtyfive days post-hatching, about half a million larvae
metamorphosed from the first group (50% survival) and another
200 000 from the second group (30% survival). This improved
survival compared with earlier years was probably due to a
smaller population of predatory hydromedusae. Although cod
larvae populations were considerably larger, their growth
rate was comparable to that in earlier years, due to the
larger food supply in the pond. | en |
