Date of otolith first increment formation in Baltic sprat Sprattus sprattus and its relation to onset of maturity

This study explores the date of first otolith increment formation in Baltic sprat Sprattus sprattus and relates the onset of maturity at Age 1 to the larval traits that are tied to the formation of the first increment. From the otolith microstructure analyses in larvae we found that regular deposition of increments is closely related to larval length, the first clear increment being identified at 16 mm length. Otolith distance from the centre to the first clear increment was independent of temperature, whereas the width of subsequent regular increments increased with increasing temperatures. These results are interpreted in the context of the interaction of larval length, somatic growth and environmental temperature. The date of first increment formation was further estimated in mature and immature Age 1 sprat caught from 1998 to 2000 using a technique that we developed, based on the effect of temperature on larval-stage otolith microstructure after first increment formation. The overall result was that larvae that reach the length of 16 mm early (late) in the season in warm years are more (less) likely to mature the following year. In cold years, like 1998, the probability of maturating at Age 1 was independent of the date at which larvae became 16 mm long, and other processes such as juvenile growth rates, feeding conditions and year-class abundance are suggested as factors that may influence the onset of maturity