Reproduction range of garfish, Belone belone (L.), in the northern Baltic Sea

The garfish (Belone belone (L.)) annually migrates from the North Sea to the Baltic Proper. Before this study, the known reproduction areas were located in the southern Baltic Sea up to the Islands of Gotland, Saaremaa and Hiiumaa. However, the larvae collected with a beach seine in 2005 and further observations of 0+ larvae and juveniles after that indicated successful reproduction in the northern Gulf of Finland and the southern Gulf of Bothnia. These catches of 0+ garfish and several other observations of garfish larvae and juveniles during recent decades demonstrate that today the species is annually also reproducing in the northern Baltic Sea. The salinity limit for garfish reproduction is not precisely known; however, the lowest reported value 5-6 ‰ fits well with our observations. We recorded garfish larvae along shallow sandy shores where the temperature is higher than in adjacent open water areas. We suggest that the longterm increase of surface water temperatures in the Baltic Sea have enabled the reported expansion of garfish reproduction range

Detailed temperature mapping-Warming characterizes archipelago zones

Rapidly warming shallow archipelago areas have the best energetic options for high ecological production. We analyzed and visualized the spring and summer temperature development in the Finnish coastal areas of the Northern Baltic Sea. Typical for the Baltic is a high annual periodicity and variability in water temperatures. The maximum difference between a single day average temperatures across the study area was 28.3 °C. During wintertime the littoral water temperature can decrease below zero in outer archipelago or open water areas when the protective ice cover is not present and the lowest observed value was −0.5 °C. The depth and exposition are the most important variables explaining the coastal temperature gradients from the innermost to the outermost areas in springtime when water is heated by increasing solar radiation. Temperature differs more within coastal area than between the basins. Water temperature sum was highest in innermost areas, lowest in open water areas and the variation in daily averages was highest in the middle region. At the end of the warming period, the difference in surface water temperatures between the innermost and outermost areas had diminished at the time when the cooling began in August–September. These clear temperature gradients enabled us use the cumulative water temperature to classify the coastal zones in a biologically sensible manner into five regions. Our study shows a novel approach to study detailed spatial variations in water temperatures. The results can further be used, for example, to model and predict the spatial distribution of aquatic biota and to determine appropriate spatio-temporal designs for aquatic biota surveys. The new spatial knowledge of temperature regions will also help the evaluation of possible causes of larger scale climatological changes in a biological context including productivity.Peer reviewe