Temperature-induced regional and temporal growth differences in Baltic young-of-the-year sprat Sprattus sprattus

We examined spatial and temporal differences in growth patterns of young-of-the-year (YoY) sprat Sprattus sprattus synchronously sampled in October 2002 from 4 different regions of the Baltic Sea (western, central, eastern, northeastern Baltic), The microstructure of the sagittal otoliths of 427 individuals from 64 sampling sites were analysed to determine the day of first feeding (DFF) and the growth history of YoY survivors. DFF distributions differed markedly between Baltic areas, with a shift to later mean DFFs and narrower distributions from west to northeast. This was consistent with the shift in mean seasonal spawning effort of Baltic sprat, derived from long-term observations (1973 to 2002) of sprat egg abundance in these areas. Otolith growth trajectories (i.e. increment width-at-age) had a characteristic shape related to sampling area and, more importantly, to the time of the year at which the individual started feeding (DFF). During the larval stage, individuals from the northeastern area and those born later in the year had higher growth rates than their earlier born conspecifics, while the pattern was reversed during the juvenile stage. Weekly means of satellite-based sea-surface temperature were used to approximate the potential temperature history of YoY survivors, which significantly influenced the shape of the otolith growth trajectory. We conclude that different DFFs and therefore different temperature histories were primarily responsible for the large-scale spatial growth variability between newly recruited Baltic YoY sprat in 2002.