Using ‘trap and transport’ to facilitate seaward migration of landlocked European eel (Anguilla anguilla) from lakes and reservoirs

Abstract

‘Trap and transport’ (T&T) is employed to facilitate the seaward migration of European eel (Anguilla anguilla) past obstacles such as hydropower facilities. Hence, previous studies assessing its efficacy have focussed on disrupted fluvial systems. The transferability of findings from lotic to lentic systems is uncertain because many of the environmental cues that trigger eel migration in rivers are lacking from reservoirs and lakes, particularly those with limited flow connection to the surrounding catchment. We used acoustic telemetry to compare the migration behaviour of T&T adult A. anguilla which were fyke netted and transported from two disconnected reservoirs (n = 80) to a control group of resident river eel (n = 30) during their migration through the lower River Stour, UK, to the North Sea. Migration patterns and behaviour were broadly similar between the reservoir T&T eel and river eel with 86 and 90 % of each group successfully reaching the sea, respectively. Reservoir eel were larger and at a more advanced stage of migratory readiness (silvering) and commenced migration sooner after release than the river eel, but they descended the catchment at a slower rate. Behaviour in the estuary was highly variable between individuals with residence times ranging from 5 h to 83 days (median = 1.4 days) across all groups. Only one individual failed to migrate through the estuary and most (75 %) reached the sea within five days of entering the estuary. Findings indicate that T&T of adult eel from reservoirs represents a feasible method to allow landlocked individuals to migrate and potentially contribute to the spawning stock, either now or in the future. Results also highlighted the high capture effort that may be required to implement an effective T&T programme. Gaining a thorough understanding of eel abundance and population structure in the source waterbody is desirable before implementation.