Dual impact of temperature on growth and mortality of marine fish larvae in a shallow estuarine habitat

High individual growth and mortality rates of herring Clupea harengus membras and goby Pomatoschistus spp. larvae were observed in the estuarine habitat of the Gulf of Riga, Baltic Sea. Both instantaneous mortality (0.76–1.05) as well as growth rate (0.41–0.82 mm day−1) of larval herring were amongst highest observed elsewhere previously. Mortality rates of goby larvae were also high (0.57–1.05), while first ever data on growth rates were provided in this study (0.23–0.35 mm day−1). Our study also evidenced that higher growth rate of marine fish larvae did not result in lower mortalities. We suggest that high growth and mortality rates primarily resulted from a rapidly increasing and high (>18 °C) water temperature that masked potential food-web effects. The explanation for observed patterns lies in the interactive manner temperature contributed: i) facilitating prey production, which supported high growth rate and decreased mortalities; ii) exceeding physiological thermal optimum of larvae, which resulted in decreased growth rate and generally high mortalities. Our investigation suggests that the projected climate warming may have significant effect on early life history stages of the dominating marine fish species inhabiting shallow estuaries.