Morphological Discrimination of the Silvering Stages of the European Eel

International audienceA lack of knowledge on the transition from the resident to the migratory phase has led to a series of studies on the silvering process. Silvering marks the end of the sedentary growth phase and the beginning of the migratory phase. A six-stage classification was developed to describe the physiological and morphological events that occur during this metamorphosis and the subsequent migration. Stages corresponded to a growth phase (I to FII), a premigrant stage for females (FIII), and migrating stages for both sexes (FIV, FV, and MII). Here, the objective was to develop a silver index using only external measurements to assess the degree of metamorphosis of eels, based on the same data set that was used in the former study. It consisted of a large number of both resident and migratory eels that were sampled at different times of the year with different types of fishing gear and at several locations representing various types of habitats. Discriminant Analysis was applied on external measurements only: (body length, body weight, pectoral fin length, and eye diameters). Total percentage of correct reclassification into the six silvering stages was 82%. The silver index (classification functions) was able to identify 91% of the migratory eels. This method, associated with proper sampling, could be utilized for the quantification of potential spawners given that they all reach their spawning grounds in the Sargasso Sea

Evaluating stocking of YOY pike Esox lucius as a tool in the restoration of shallow lakes

1. Stocking of piscivores in shallow, eutrophicated lakes to reduce cyprinid densities is a common approach in lake restorations. Young-of-the-year (YOY) pike Esox lucius are frequently used to reduce cyprinid densities, but their effectiveness is equivocal. This study uses a simple model to assess the efficacy of pike stocking as a lake restoration tool. 2. The model evaluates YOY pike consumption in relation to different survival and growth patterns, timing of stocking, degree of piscivory of pike as well as YOY cyprinid growth patterns and productivity. The parameters used in the model are obtained from field surveys in eight study lakes stocked with YOY pike as well as from the literature. 3. Our model showed that all parameters studied were important for predicting the effects of pike stocking on cyprinids. In particular, body size at stocking, cyprinid production and pike survival were good predictors of biomanipulation success. 4. However, our model showed that only under very specific circumstances will stocking of YOY pike result in a reduction of YOY cyprinid densities large enough to affect water quality. 5. The findings of this study question the efficacy of pike stocking as an appropriate and reliable tool for restoration programmes in shallow lakes, and indicate that when used efforts should be made to optimize the timing of stocking in relation to YOY cyprinid production and to increase the stocking body size of the pike