Agent-based modelling of juvenile eel migration via selective tidal stream transport

Thomas Benson,Jasper De Bie,Jennifer Gaskell,Paolo Vezza,James R Kerr,Darren Lumbroso,Markus R Owen,Paul S Kemp

doi:10.1016/j.ecolmodel.2021.109448

Thomas Benson, Jasper De Bie + Show 6 more

Open Access

https://doi.org/10.1016/j.ecolmodel.2021.109448

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Abstract

Recruitment of temperate eel species Anguilla anguilla, A. rostrata &A. japonica has declined over the last few decades due to human activities, such as overfishing and construction of migratory barriers (e.g. dams, weirs and sluices) and hazardous energy infrastructure (e.g. turbines, intakes and outfalls). Numerical models, substantiated with data from field and laboratory studies, can potentially predict and quantify the relative impacts of such activities, thereby assisting in the sustainable management of eel populations. Here, we present an agent-based model (ABM) of juvenile eel migration up estuaries. The model includes relevant eel behaviours and environmental conditions that, according to the literature, influence upstream migration. Crucially, by assessing the local salinity gradient and relative flow direction, the modelled eels (agents) self-determine whether the tide is flooding or ebbing and orientate themselves for navigation, with no top-down instructions. This allows the agents to decide which particular behaviour to undertake as part of Selective Tidal Stream Transport (STST). The developed ABM is coupled to a hydrodynamic model of the Thames Estuary and the results substantiated by comparison against eel trap data. Combinations of the various STST behaviours are systematically tested and the influence they have on up-estuary migration is assessed in terms of relative energy expenditure. The parameterised model is then used predictively at Milford Haven Waterway to investigate potential impacts on the juvenile eel population due to entrainment in a power plant cooling water intake and outfall. Results from the Thames model case study indicate that including bed anchoring behaviour is essential for achieving a good comparison with the eel trap data and the choice of salinity detection threshold is also important. If daylight avoidance (diel) behaviour is not included, the most energy efficient migration is achieved using just two STST behaviours (ebb tide bed anchoring and upward migration during flood). With diel behaviour included, energy expenditure is greater, but some efficiency is regained by including all of the STST behaviours. For the Milford Haven case study, the model predicted a juvenile eel intake and outfall entrainment rate of 2.0% and 4.7%, respectively. It is concluded that the ABM is a valuable tool for assessing potential impacts on the recruitment of eels (extendable to other species) and could be used to assist in site-selection and low impact design of energy infrastructure in tidal environments.

Highlights

All anguillid eels have a catadromous life cycle in which the juveniles migrate from the ocean into rivers where they mature for several years and return to the ocean as adult silver eels to spawn (Tesch, 2008)
The agents tended to congregate at the position of the contour of the salinity detection threshold (Sthresh = 0.04 ppt), generally located between about 0 and 40 km from Teddington
The inclusion of downward migration and anchoring at the bed in response to the ebbing tide was found to have the greatest effect on improving the comparison with eel trap data

Summary

Introduction

All anguillid eels have a catadromous life cycle in which the juveniles migrate from the ocean into rivers where they mature for several years and return to the ocean as adult silver eels to spawn (Tesch, 2008). The focus of this study is the swimming behaviour of juvenile European eels (Anguilla anguilla) in the latter stages of their migration as they progress up macro-tidal estuaries to reach freshwater environments. At this stage in their life cycle they are approximately 8–12 cm in length and translucent, so referred to as glass eels, and become elvers when more pigmented. Their decline is thought to be due to various anthropogenic factors (Dekker, 2003; MacGregor et al, 2008; Arai, 2014), such as entrainment in power plant intakes and outfalls or impingement on screens (Piper et al, 2015), blockage due to poorly designed fish passes and weirs (Amaral et al, 2002; Russon et al, 2010; Calles et al, 2013; Vowles et al, 2015), population losses due to commercial fishing (ICES, 2014) and impacts on larval stages due to climate change (Knights, 2003; Bonhommeau et al, 2008)

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