Shrinkage effect of freezing on herring and sprat in the Baltic Sea

Length and weight are fundamental parameters in life-history modelling and stock assessment. However, freezing conservation, commonly used to preserve biological samples, is known to alter both. Thus, its effect was quantified for two clupeid species, herring and sprat, in the Baltic Sea. A total of 954 clupeids (520 herring and 434 sprat) were collected across three hydroacoustic surveys conducted by Germany and Sweden in 2023 and 2025. A correction equation accounting for freezing-induced bias was derived for both morphometric measurements. The estimated correction factor is higher for herring than for sprat in length (2.7% vs. 1.7%), and for sprat than for herring in weight (3.5% vs. 1.3%), suggesting a species-specific impact of freezing on morphometric measurements. Freezing duration was modelled as a random effect on the correction factor, with an influence detected in all cases. For length, maximum shrinkage was reached after five days for herring (3.6%) and ten days for sprat (2.2%), before decreasing and stabilising at lower values. For weight, maximum reduction was reached after five days for both species (herring: 2.05%; sprat: 6.7%). Although the shrinkage is sensitive to various methodological and biological factors, this study provides a robust basis for understanding and correcting freezing-induced bias in clupeids across the Baltic Sea.