NUTRITIONAL CONDITION AND VERTICAL-DISTRIBUTION OF BALTIC COD LARVAE

Newly hatched Baltic cod Gadus morhua larvae are typically found at de pths >60 m. This is a region of low light and prey availability, hence generating the hypothesis that larvae have to migrate from hatching d epth to the surface layer to avoid starvation and improve their nutrit ional condition. To lest this hypothesis, Baltic cod larvae were sampl ed during the spawning seasons of 1994 and 1995 with depth-resolving m ultiple opening/closing nets. Each larva was aged by otolith readings and its RNA/DNA ratio was determined as a measure of nutritional condi tion. The RNA/DNA ratios of these larvae aged 2-25 days (median 10 day s) ranged from 0.4 to 6.2, corresponding to levels exhibited by starvi ng and fast growing larvae in laboratory calibration studies (starvati on, protein growth rate, G(pi)=-12.2% day(-1); fast-growing larvae, G( pi)=14.1% day(-1)) respectively. Seventy per cent of the field caught larvae had RNA/DNA ratios between the mean values found for starving a nd fed laboratory larvae. Only larvae aged 8-11 days had higher mean R NA/DNA ratios above 45 m than below (t-test, P<0.05). However, the ins tantaneous protein growth rates were significantly higher for all larv al age groups in the surface layers (t-test, P<0.05). Starving larvae were found in all depths sampled (10-85 m), whereas growing larvae (po sitive G(pi)) were restricted to samples taken shallower than 45 m. Th ese superior growth rates above 45 m corroborate the hypothesis and im ply that migration to the shallow water layers is a prerequisite for g ood nutritional condition, growth and survival of Baltic cod larvae. T he frequent occurrence of cod larvae older than 8 days in the deep wat er in poor condition suggests that a proportion of the larvae will die from starvation in the deep layers of the Baltic Sea. (C) 1997 The Fi sheries Society of the British Isles.