An example of meso-scale hydrographic features in the central Baltic Sea and their influence on the distribution and vertical migration of sprat, Sprattus sprattus balticus (Schn.) DANIEL STEPPUTTIS 1, *, HANS-HARALD HINRICHSEN 2 , UWE BO ¨ TTCHER 1 , EBERHARD GO ¨ TZE 3,  AND VOLKER MOHRHOLZ 4 1 Johann Heinrich von Thu ¨nen Institute ⁄ Institute of Baltic Sea Fisheries Rostock, Alter Hafen Su ¨d 2, D-18069 Rostock, Germany 2 Leibniz-Institute of Marine Sciences, University of Kiel, Du ¨sternbrooker Weg 20, D-24105 Kiel, Germany 3 Johann Heinrich von Thu ¨nen Institute ⁄ Institute of Sea Fisheries Hamburg, Palmaille 9, D-22767 Hamburg, Germany 4 Leibniz-Institute for Baltic Sea Research Warnemu ¨nde, Seestrasse 15, D-18119 Rostock, Germany ABSTRACT A wind-driven meso-scale pattern of temperature, salinity and oxygen was found along a transect in the northern Bornholm Basin (southern Baltic Sea). Strong winds caused currents along this transect, which shifted cold intermediate water (minimum: 3.6°C) towards the south. The transect was surveyed with a towed CTD-system and hydroacoustics in parallel to investigate the distribution of sprat, Sprattus sprattus balticus (Schn.) in relation to the observed meso-scale pattern. In those parts of the transect where the cold intermediate water was ob- served, sprat were restricted to water layers below the halocline. In other parts of the transect, sprat moved into higher water layers and occupied a wider depth range. The important factor was temperature, which set an upper limit to the vertical sprat distribution. The development of hydrography, as measured in the field, was evaluated with a hydro- dynamic model. Key words: Baltic Sea, habitat, meso-scale, oxygen, salinity, sprat, Sprattus sprattus, temperature INTRODUCTION The meso-scale includes distances from tenths to tens of kilometres. Good examples of relevant structures in the pelagic zone at this scale are shelf fronts (Le Fevre, 1986). Such hydrographic features are known to influence the distribution of organisms, including adult fish (Le Fevre, 1986; Fiedler and Bernard, 1987; Munk et al., 1995; Casotti et al., 2000; Garc ¸on et al., 2001). The congregation of fish is induced by the aggregation of lower trophic levels in high productivity regions of the front. In contrast to large frontal systems, which often persist over a long time period and are partially predictable, other meso-scale patterns, such as wind- induced hydrographic structures, are expected to be short-term events and hence are difficult to detect or predict. Therefore, the measurement and quantifica- tion of their effect on the pelagic ecosystem presents a great challenge (Garc ¸on et al., 2001). The biological implications of wind-driven currents were shown for egg and larval transport of Baltic cod (e.g., Voss et al., 1999; Hinrichsen et al., 2003a; Voss and Hinrichsen, 2003), Baltic sprat (Hinrichsen et al., 2005; Baumann et al., 2006) and zooplankton species. All these examples were described as passive processes, having an effect on planktonic species or life stages. An active response to wind-induced meso-scale changes of hydrographic properties in terms of vertical migration of individuals has rarely been documented. Raid (1989) described the distribution patterns of phyto- plankton, zooplankton, fish larvae and larger pelagic fish (herring) in relation to meso-scale hydrographic features in the Baltic Sea; however, there was no obvious effect on larger fish. The identification of relevant physical parameters is essential to understand the response of pelagic fish species to hydrographic structures. Potential factors influencing the distribution of Baltic sprat, Sprattus sprattus balticus (Schn.), are temperature, temperature gradient, salinity and oxygen. One limiting factor, low oxygen content of the water, is obvious, but the influences of other factors are under discussion. Some authors have suggested that cold temperature (below 4°C) is another ultimate limiting factor (Rechlin, *Correspondence. e-mail: science@stepputtis.net   Deceased. Received 27 October 2010 Revised version accepted 27 October 2010 FISHERIES OCEANOGRAPHY Fish. Oceanogr. 20:1, 82–88, 2011 82 doi:10.1111/j.1365-2419.2010.00567.x Ó 2010 Blackwell Publishing Ltd.