The general distribution pattern and mixing probability of Baltic sprat juvenile populations

Transport of Baltic sprat Sprattus sprattus L. larvae spawned on different spawning grounds was investigated by detailed drift model simulations for the years 1979–2002. Modeling approaches with and without diurnal vertical migration were applied. We used recently collected data on spawning location, vertical distribution and the timing of spawning as input to a particle tracking model. Results of this modeling study enabled the identification of potential nursery grounds for sprat originating from different spawning grounds. On average, westerly winds are prevailing over the Baltic Sea. This leads to on average higher abundance of juvenile sprat along the southern and the eastern coast lines of the Baltic. The horizontal distribution of simulated larval or 0-group sprat is consistent with the observed distribution of 0-group sprat obtained from the hydroacoustic field surveys. In addition, this analysis identifies the potential for advective mixing between juveniles originating from different spawning sites or from the same spawning site throughout the spawning season. High spatial overlap was found between Arkona- and Bornholm Basin larvae hatched at the beginning of the spawning season. Mixing probability of sprat juveniles between Arkona- and Gotland Basin as well as between Bornholm- and Gotland Basin were on relatively low levels.