ALBERT

Description: 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, Gpi= -12.2% day−1; fastgrowing larvae, Gpi=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〈0.05). However, the instantaneous protein growth rates were significantly higher for all larval 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 (positive Gpi) 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.

Description: Herring larvae were sampled during the lllLS in the ICES area IVa from 1990 to 1997. Larval growth was determined based on otolith microstructure analysis from larvae of 7-25mm standard length. The data were correlated to the North Atlantic Oscillation (NAO) and the intlow of Atlantic water (AW) at the same period. The study showed a strong correlation between positive NAO years with a higher inflow of AW and subsequently higher larval growth rates. which could be an indication of favorable growth conditions

Description: Cod larvae from laboratory rearing experiments aged from 1 to 12 d after hatching, both fed and deprived of food, were analysed. The number of increments on the otolith and the width of these increments were determined together with the RNA/DNA ratios on the same individual larva. Alizarin marking of the otoliths was performed to confirm the formation of daily increments. Cod larvae reared at 6*C formed the first ring right after hatching and deposited increments on a daily basis. A comparison of the measurements between the right and the left lapillus showed that these can differ, if the radius is taken. The lapillae core showed especially high individual variability, whereas the sum of the increments did not differ between both lapilli. Until Day 10 after hatching, while the larvae were still feeding on their yolk, the external food situation did not affect the increment width of the lapilli or the RNA/DNA ratios. In larvae older than 10 d the width of the daily increments was dependent on the nutritional situation and RNA/DNA ratios decreased in starving larvae in comparison to feeding larvae. RNA/DNA ratios and increment widths were correlated.

Description: Anchovy larvae (Engraulis anchoita) were sampled from an area of upwelling in the subtropical waters over the Brazilian shelf and from an area over the Argentinian shelf characterized by a tidal mixing front. They were analysed for their nutritional condition by measuring RNA/DNA ratios. The degree of starvation occurring in the two geographic areas was correlated to length distribution, developmental stage and position in the water column. It was shown that starving larvae (5 to 80 %) were mainly found in the smaller length classes (4 to 10 mm SL), but some larvae between 16 to 20 mm SL were also determined as starving (4 to 6 %). Larvae found in the stratified water column had higher RNA/DNA ratios reflecting a better nutritional condition than in the mixed water body. The degree of starvation at the same developmental stage was higher in Brazil than in the Argentinian area. RNA/DNA ratios of larvae from the Argentinian area showed a significant shift to higher values (indicating a better condition) than in Brazil. Results on the nutritional condition of the larvae based on RNA/DNA ratios were compared with zooplankton data and larval gut analyses determined on the same cruise. Speculations on the survival strategies of these two anchovy populations are given