Large-scale rearing of cod fry on the natural food production in an enclosed pond. In: The propagation of cod Gadus morhua L.: an international symposium, Arendal, 14 - 17 June 1983

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