Is Restocking an Efficient Measure to Sustain the Freshwater Eel Populations? Results of a 6 Years Experimental Study in a Natural Freshwater Marsh in Southern Europe

Even though restocking is one of the principal measures proposed to sustain the freshwater eel populations, few studies attempted to demonstrate its efficiency. Here we considered that restocking is useful when stocked eels yield more silver eels than natural migrants and with at least equal reproductive potential. To test this, a restocking experiment was conducted since 2007 in the freshwater marsh “Vigueirat” in south east of France.

Individually marked elvers and yellow eels from different habitats (freshwater, brackhish) were stocked between October 2007 and February 2008. Furthermore since 2008, every January, a significant number of glass eels were added in the marsh. The yield per recruit (number of silver eel per recruits) was estimated for each cohort using a Capture Mark Recapture model (estimates of survival and life stage transitions with E-surge). To evaluate the quality of silver eels produced, we considered pathogen (Anguillicoloides crassus), fat level and the concentration of contaminants (PCBS and heavy metal). Quantity and quality of spawners were then compared between the different stocked cohorts and the natural cohort to assess the efficiency of the stocking experiment and determine the most suitable stocked cohort.