Growth and condition of sprat (Sprattus sprattus) larvae inferred from otolith microstructure analysis and RNA/DNA ratio in the Bornholm Basin (Central Baltic Sea) during spawning season 2001

The aim of the study was to investigate the growth and condition of sprat (Sprattus sprattus) larvae as survival characteristics in relation to environmental changes during spawning season 2001. Spatial and temporal variability in growth and condition were inferred from otolith microstructures, RNA/DNA ratio and an individual-based model. The variation in growth and condition were presented both on population and individual levels. Based on the estimated-age and length relationship, somatic growth in sprat larvae was higher in May than in April. With respect to May, horizontal variability in somatic growth was found on 6 subareas in the Bornholm Basin. There was a trend that the larvae encountered in the northern part of the basin experienced a higher somatic growth in comparison with the central and the southern regions. Within the vertical environment, the growth of sprat larvae tended to be higher at the upper layer than at the lower layer. Sprat larvae were largely in good during the two months. However, the RNA/DNA ratio tended to be higher in May compared to April. Within the vertical environment, it was obtained that larvae at the upper layer were in a better nutritional condition than those in the lower layer. These findings were also confirmed by the instantaneous protein growth rates (Gpi) of the larvae. No diel variation in RNA/DNA ratio was found according to different photoperiods. The application of an individual-based model which combined hydrodynamic and biological modeling using age and growth estimated from otolith structures showed that the model was appropriate to reconstruct the growth trajectory of sprat larvae. The model results showed a high agreement with nutritional condition data for the May sampling. Spatial and temporal variability in growth and condition of sprat larvae was largely due to temperature differences. A higher temperature in May than in April as well as warmer water masses in the upper than in the lower layer resulted in better survival characteristics of the larvae collected from the latter compared to the earlier sampling. Other factors that accounted for this variability were food availability, oxygen concentration, and photoperiod.