On the application of models of European eel (Anguilla anguilla) production and escapement to the development of Eel Management Plans: the River Severn

Abstract

Aprahamian, M. W., Walker, A. M., Williams, B., Bark, A., and Knights, B. 2007. On the application of models of European eel (Anguilla anguilla) production and escapement to the development of Eel Management Plans: the River Severn. – ICES Journal of Marine Science, 64: 1472–1482. The European eel stock has declined significantly since the 1980s, and the Eel Recovery Plan of the European Commission requires Member States to develop river basin Eel Management Plans (EMPs) that will achieve an escapement of silver eels that equals or exceeds 40% of the escapement biomass that would be produced in the absence of human activities. However, because silver eel escapement is not quantified within the UK, a modelling approach is required to estimate potential and actual escapement, and to assess the likely effects of management measures. We focus on two approaches developed in the UK, the Reference Condition Model (RCM) and the Scenario-based Model for Eel Populations (SMEP), and illustrate how such approaches can be used in EMPs using selected data from the River Severn. The RCM results indicate that the yellow eel population in the River Severn basin may be just 30–40% of the potential density indicated by reference conditions derived from a selection of rivers between the late 1970s and the early 1980s. The challenges of applying a model designed to be as realistic of eel production as possible, and the limited data on natural eel habitat and eel production in the Severn, preclude a SMEP analysis similar to that of the RCM, but simulations based on a simplified basin description and eel survey data from the early 1980s illustrate the potential of this model to assess compliance and test management scenarios.

Escapement, route choice, barrier passage and entrainment of seaward migrating European eel, Anguilla anguilla, within a highly regulated lowland river

“…Due to a lack of quantitative data describing 11 current escapement for many European catchments, several countries have adopted 12 modelling approaches to estimate eel densities and escapement under scenarios with 13 and without human induced stress (e.g. the Probability Model; Scenario-based 14 Management of Eel Populations, SMEP) (Aprahamian et al, 2007). Impacts of 15 fishing and the operation of hydropower plants and pumping stations are however 16 currently underrepresented in many models due to insufficient empirical data 17 (Aprahamian et al, 2007;ICES, 2011a).…”

Status of eel fisheries, stocks and their management in England and Wales

“…Though it is accepted that the assessment does not take into account the impact of anthropogenic influences, it does compare current eel densities with those derived from glass eel recruitment prior to the recruitment collapse in 1983-1984. More generally, the Reference Condition Model (RCM: Aprahamian et al, 2007) is being used to assess compliance for the RBDs in England and Wales and for the cross-border RBD with Scotland, and an example assessment of the River Ellen (north west) is presented here. In many rivers of England and Wales, the density of eel naturally declines with distance upstream from the estuary Ibbotson et al, 2002).…”

“…Data for 12 rivers surveyed in the 1970s and early 1980s have been used to create a model that predicts the yellow eel population (in terms of densities along the river) that would have been expected before the major decline in glass eel recruitment across Europe in 1983-1984. The most significant factor explaining variation in density profile between rivers (assuming constant recruitment) was found to be river gradient (Aprahamian et al, 2007). By plotting the rate of decline in eel densities against the gradient of each river catchment, it is possible to predict the natural rate of decline for any river.…”

“…Several spatial, life history models are under development, and some were reviewed in the EU-SLIME project (Dekker et al, 2006). The Scenario-based Model for Eel Populations (SMEP) (Aprahamian et al, 2007) and GlobAng (Lambert and Rochard, 2007) share common approaches to modelling eel production within river basins. While there is a lot of growth data eels from UK rivers (Aprahamian, 1988(Aprahamian, , 2000Bark et al, 2007), there is a limited understanding of other processes, or how they might be influenced by density and habitat.…”

A spatial assessment model for European eel (Anguilla anguilla) in a delta, The Netherlands

“…Unlike marine fish stocks where member states collate fisheries data to be used in a single stock assessment, during the last 5 years almost all countries have developed their own national assessment model for eel (e.g. Aprahamian et al, 2007;Bevacqua et al, 2007;Ciccotti et al, 2012;Oeberst and Fladung, 2012). The models developed for eel stock assessments are mostly dynamic demographic models of an eel population in a single catchment using age, stage or cohorts (Dekker, 2000;Oeberst and Fladung, 2012).…”