Minimally Invasive Collection of Biometric Data Including Maturation Stage on European Eel Using Photography

In response to the severe decline of the European Eel Anguilla anguilla stock in recent decades, various data frameworks and research efforts toward improved management rely on the availability of site-specific biometric data. At the same time, scientists are obligated to minimize the negative effects (stress, harm, and sacrifice) of their samplings on individuals and the population without compromising data quality. In-field methods for biometric measurements must be quick, precise, and practical for the user. Essential information that is typically required in (large-scale) eel monitoring programs includes body length, mass, sex, and maturation stage. As live eels are difficult to handle, individuals are typically anesthetized or killed (and sometimes stored frozen to postpone measurements) to obtain the necessary biometrics. The primary purpose of this paper was to explore the suitability of a nonlethal method based on photography for obtaining essential biometrics and maturation stage from live European Eels A. anguilla in a timely manner. In addition, we evaluated the relative accuracy of measuring the parameters that are necessary for assessing maturation stages in eels after defrosting and examined the necessity of correcting for potential shrinkage of eyes and pectoral fin. Both procedures were compared against a standard reference of measurements from freshly killed eels. We found that the minimally invasive method using alive measurements of eels' body length and mass together with digital measurements of eyes and pectoral fin from photographs had the highest agreement for maturation stage outcome with the fresh reference. Our results further reveal the necessity of correcting for shrinkage of eyes and pectoral fins (in addition to length and mass) after freezing to maximize reliability in stage classification. Consequently, we provide specialized formulae to apply shrinkage corrections for eye diameter and pectoral fin length.