Abstract
Newly hatched Baltic cod Gadus morhua larvae are typically found at depths >60 m. This is a region of low light and prey availability, hence generating the hypothesis that larvae have to migrate from hatching depth to the surface layer to avoid starvation and improve their nutritional condition. To test this hypothesis, Baltic cod larvae were sampled during the spawning seasons of 1994 and 1995 with depth‐resolving multiple opening/closing nets. Each larva was aged by otolith readings and its RNA/DNA ratio was determined as a measure of nutritional condition. The RNA/DNA ratios of these larvae aged 2‐25 days (median 10 days) ranged from 0.4 to 6.2, corresponding to levels exhibited by starving and fast‐growing larvae in laboratory calibration studies (starvation, protein growth rate, G pi = ‐12.2% day −1 ; fastgrowing 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 and fed laboratory larvae. Only larvae aged 8‐11 days had higher mean RNA/DNA ratios above 45 m than below ( t ‐test, P t ‐test, P Starving larvae were found in all depths sampled (10‐85 m), whereas growing larvae (positive G pi ) were restricted to samples taken shallower than 45 m. These superior growth rates above 45 m corroborate the hypothesis and imply that migration to the shallow water layers is a prerequisite for good nutritional condition, growth and survival of Baltic cod larvae. The frequent occurrence of cod larvae older than 8 days in the deep water in poor condition suggests that a proportion of the larvae will die from Starvation in the deep layers of the Baltic Sea.