Growth, mortality and feeding of cod (Gadus morhua L.) larvae in enclosed water columns and in laboratory tanks. In: The propagation of cod Gadus morhua L.: an international symposium, Arendal, 14 - 17 June 1983
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In April 1982 cod larvae were reared in four 30 l laboratory tanks and in two 300,000 l columnar plastic enclosures for 3-4 weeks from hatching. North Sea oil "production water" was added to one of the enclosures immediately after introduction of the yolk sac larvae. Natural zooplankton at an initial copepod nauplii concentration of 5-7 l^-1 provided the food source for enclosure larvae. Food in the tanks, mainly copepod nauplii and small copepodites, was maintained at about 300 items 1^-1. Post yolk sac specific growth rates of larvae in the enclosures were 10.0% and 10.4% d^-1. In the laboratory, cod larvae of Norwegian origin grew at 7.8% and 7.7% d^-1, while those of Clyde Sea origin grew at 1.7% and 6.0% d^-1. Natural daily mortality rates were 8.4% and 9.7% in the enclosures and 10.6%, 14.5%, 15.6% and 10.0% in the tanks. 'Production water ' had no detectable effect on the treated larvae. Larvae in both systems started feeding at 4-5 days after hatching and feeding incidence was soon over 60%. The diet of enclosure larve consisted almost entirely of nauplii of the copepod Pseudocalanus elongatus. The relationship between numbers and sizes of food items relative to standard length and the relarionship of maximum gut content weight to larval size were determined. Pump samples revealed that cod larvae in the enclosures preferred the top 7.5 m of the water column.