Salmon and recreational fisheries monitoring: Eels

Although there is currently no exploitation of European eel (Anguilla anguilla) in Scotland, Marine Scotland Science is committed to monitoring stocks as part of the Scotland RBD Eel Management Plan. This was developed to comply with Council Regulation (EC) No 1100/2007, establishing measures for the recovery of the stock of the European eel. These data contribute to an annual assessment of eel stocks communicated to the International Council for the Exploration of the Sea (ICES) via the ICES/EIFAC Working Group on Eel (WGEEL), and to the tri-annual assessment of eel management plans that all EU member states must report to the European Commission.

This requires estimation of the biomass of adult silver eels leaving Scotland to breed. Marine Scotland Science measures silver eel escapement at three trap sites: the Girnock, Baddoch and Shieldaig. These three catchments occupy three different altitude bands, and are assumed to be representative of Scottish eel habitat as a whole. Total silver eel output (Fig. 1) is then calculated in proportion to the total wetted area of eel habitat available in each of these altitude bands in Scotland RBD. Production in the transitional waters of the RBD is assumed to be equivalent to that in the lowest altitude freshwater zone.

The EU regulation sets an escapement biomass target of 40% of the spawning biomass that would have left the coast under pristine conditions. Because there has never been a regulated fishery for eel in Scotland, historical data on eel escapement is sparse, and the pristine escapement for Scotland RBD is therefore estimated partly on the basis of the long-term historical record of eel escapement at the Girnock trap, and partly by reference to data from similar areas outside Scotland.

Fig. 1 (above): Estimated escapement of silver eel from Scotland River Basin District (black line). The blue line shows the target escapement, set by European Regulation 1100/2007 at 40% of the estimated escapement in pristine conditions .

Since there is now no fishery for eel in Scotland there is relatively little scope for further reduction in anthropogenic mortality. The future output of silver eels from Scotland will therefore depend principally on the recruitment of glass eels, which in turn will depend on the fate of eel elsewhere in its range.