A Correcting Factor for the Reduction of Body Length and Mass of European Eel After Ethanol Preservation and After Freezing

Measurements of length and mass are used in many research fields, and such data are often collected from samples that have been preserved in ethanol or frozen prior to data collection. Since many preservation methods affect the size and shape of soft-bodied animals, species-specific correction factors are used. Here, we calculated ethanol and freezing shrinkage correction factors for the European eel, Anguilla anguilla Linnaeus, 1758, and investigated how preservation duration and individual size affected shrinkage. We also investigated if freezing had an impact on the size of the eyes and pectoral fins, which could affect maturation stage classification. We found that preservation in 95% ethanol and freezing decreased body length and mass, as expected. Time kept in ethanol did not affect shrinkage. Time kept in freezer had some effect on shrinkage, and the model fit suggested inclusion of days frozen for body mass shrinkage. That, however, only had negligible improvement on the model. For preservation in freezer, shrinkage was greater for lengths below 330 mm and mass below 100 g, compared to eels above these sizes. However, applying a size threshold to the analysis only generated negligible improvement of the model fit, meaning that specific shrinkage factors for different sizes are not needed. We also found that freezing induced shrinking in eye size, while the pectoral fin increased in size. The increase in pectoral fin length is however believed to be due to a measurement deviation. User-friendly formulas for all correction factors are provided. The application of these factors should be restricted to the European eel within the size range used in this paper.