The inadequacies of estimating silver eel escapement by studying yellow eels caught in the coastal sea far away from the river mouth. A comment on Rohtla et al. (2021)

Abstract

Abstract For the conservation and management of the critically endangered European eel, it is crucial to maintain an overview on the biomass of the eels moving from freshwater feeding areas to the sea for spawning. Rohtla et al. suggested an assessment of the escapement of eels from the Narva River to the Baltic Sea based on their study of origin of yellow eels from Estonian coastal areas and eels from Finnish coastal areas. Silver eels from the Estonian coast have not been included in the study. The number of descending eels in the Narva River has not been investigated. The sampling areas in the sea were located far from the river mouth, ranging from 100 to 500 km away. The article does not include an analysis of how to calculate the number of eels descending from the river based on the samples used. Thus, estimates can be significantly biased. We demonstrate that the approach used to estimate escapement is methodologically incorrect and needs further development before applying in eel management. This comment recommends methodological improvements for estimating eel escapement using the indirect method.