Laboratory calibration of optimal growth to deduce in situ feeding conditions of early juvenile sprat Sprattus sprattus from otoliths

We performed an ad libitum experiment over 30 d to generate otolith growth reflecting optimal fish growth for use in estimation of in situ food availability via otoliths in post-larval sprat (Sprattus sprattus L.). Between 16 and 22 degrees C, no difference in length or wet mass growth was detected in contrast to a significant and direct increase from 5.5 to 7.9 mu m d(-1) in otolith and 8.7 to 10.5 mg d(-1) in dry mass growth. The different responses in length/wet mass growth and otolith growth were likely caused by the onset of lipid storage, which we assume to be triggered onto genetically. We estimated a 10-fold increase in food demand from metamorphosed sprat (30 mm length) to juveniles (50 mm length). According to a bioenergetic approach, juveniles needed a concentration of 3 and 5 individuals l(-1) assuming ad libitum feeding on Acartia sp. at 16 and 22 degrees C, respectively. We described increment width as a function of temperature (IW = -0.0326T(2) + 1.6472T - 12.506) and compared this reference with increment widths of Baltic Sea recruits at corresponding temperatures. On average, increments in 2007 were smaller than the laboratory reference, suggesting sub-optimal feeding conditions. In 2003, mean increments were larger, except for early born survivors which exhibited poorer growth. These early born sprat encountered high temperatures after metamorphosis, leading to a higher food demand. Our findings highlight the importance of food availability in near-shore nursery areas and the impact of the right seasonal timing of the juvenile stage on recruitment strength in Baltic sprat.