EXECUTIVE SUMMARY: The ICES Workshop on Eel Stocking (WKSTOCKEEL), chaired by Derek Evans, UK, met in Toomebridge, N. Ireland, UK, 20–24 June 2016. This workshop was convened to up date knowledge on the net benefit of stocking (the practice of adding eels to a waterbody (recipient) from another source (donor)), to the recovery of the eel stock, and to make proposals for research to fill any crucial knowledge gaps that prevent a definitive advice on stocking as a stock conservation measure. The definition of net benefit of stocking was taken as “where the stocking results in a higher silver eel escapement biomass than would have occurred if the glass eel seed had not been removed from its natural (donor) habitat in the first place”. Nineteen EU experts participated in the meeting, representing 6 countries. ICES has repeatedly reviewed the issues surrounding capture, transfer and stocking of European eel, almost as a standing item on its annual agenda. The most recent (2015) advice reiterates many previous conclusions. It includes evidence that translocated and stocked eel can contribute to yellow and silver eel production in recipient waters, (but that evidence of contribution to actual spawning is limited by the lack ofknowledge of the spawning of any eel) and that Internationally coordinated research is required to determine the net benefit of stocking on the overall population, (including carrying capacity estimates of glass eel donor estuaries as well as detailed mortality estimates at each step of the stocking process).
The use of stocking is listed in the EU Eel Regulation 1100/2007 as one of a range of management measures that could feature in an Eel Management Plan, and as such be eligible for grant support from the European Fisheries Fund. By 2013 stocking of glass eel was undertaken in 16 Member States. Whilst stocking is a measure featuring in many EMPs, only six achieved their EMP stocking target. The conclusions from WKSTOCKEEL echo many of those from the most recent reviews and the latest advice and recommendations from ICES (2015) given that many of their concerns remain un- addressed. Studies were found to lack controls and/or a simultaneous assessment of the life history of those glass eel left in situ. This in effect means that, whilst a local benefit may be apparent, an assessment of net benefit to the wider eel stock is unquantifiable. For the (lifetime) natural mortality, there is little information available, and no reporting obligations. The contribution of stocking derived silver eel to the spawning stock is still not quantifiable and is limited by the lack of knowledge on the spawning of any eel.
As a consequence of the above conclusions, the knowledge base for the assessment of the net-benefit of stocking is extremely weak. Until such research needs to address the knowledge gaps have been undertaken, there is no basis for the evaluation of individual stocking cases. Such research needs identified included carrying capacity estimates of glass eel donor systems, lifetime mortality estimates, mortality estimates within commercial stocked eel trade channels and the observation and origin assessment of silver eel spawning in their natural environment.