Publication Abstract

Modelling of Anguilla anguilla production and silver eel escapement for the River Severn, compliance with targets and development of management plans

Modelling of Anguilla anguilla production and silver eel escapement for the River Severn, compliance with targets and development of management plans

A.M. Walker, M.W. Aprahamian, M. Pawson, B. Williams, A. Bark and B. Knights

The River Severn catchment supports the largest glass eel fishery in the UK, with a current annual catch of around 10 t. Since the 1980s, there has been a decline in the catch and CPUE (an index of recruitment) of glass eel of ~70%, with the main decline happening in 1983/1984. Yellow and silver eel fisheries in the Severn catchment are negligible. Comparison of the results of eel-specific surveys undertaken in the late 1990s and between 2002 – 2004 with those during the early 1980s indicates that there have been no significant overall changes in eel distribution, density, biomass or size structure of the eel population in the Severn. This suggests that escapement of silver eels is similar to that during the late 1970s and early 1980s, when the glass eel fishery was much larger (~40 t). The European Commission Eel Recovery Plan requires that Member States develop management plans in order to ensure at least 40% of the potential escapement of silver eels Anguilla anguilla is achieved in a River Basin District (WFD). However, as eel production and silver eel escapement is not measured in most catchments, a modelling approach is required to estimate potential and actual escapement, and to assess the likely effects of management measures. Two modelling approaches have been applied to data for the Sveren catchment.

In this paper we describe the use of Severn is probably not sufficient to meet the 40% management target.   However, with this model, it is not possible to assess whether removal of the glass eel fishery would help to achieve compliance (through increased recruitment), or whether the main constraint of silver eel production is the perceived decline in the amount and suitability of habitat available to eels in the Severn since the 1950s. To address this, we have applied a Scenario-based Model for Eel Populations (SMEP) to model yellow eel populations and silver eel escapement from the Severn catchment. SMEP considers both the biological characteristics of the eel population and a number of potential anthropogenic influences on that population. Biological characteristics modelled include growth, natural mortality, sexual differentiation, maturation and migration, and the model can use site-specific data to calibrate the output. Since the Severn is relatively data-rich, we are able to assess the effects that changes in recruitment, habitat (quality and quantity) and/or accessibility have on mortality, production and eventual silver eel escapement through time.

The presentation will also discuss the need for an enhanced (over current) monitoring programme to complement the development of tools to set reference levels and assess compliance with the EU target, and to quantify the effect of remedial measures.

Reference

A.M. Walker*, M.W. Aprahamian, M. Pawson*, B. Williams, A. Bark and B. Knights (2006) Modelling of Anguilla anguilla production and silver eel escapement for the River Severn, compliance with targets and development of management plans. ICES Annual Science Conference, Theme J: Is there more to eels than SLIME?

A.M. Walker*, M.W. Aprahamian, M. Pawson*, B. Williams, A. Bark and B. Knights

ICES Annual Science Conference, Theme J: Is there more to eels than SLIME?