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Escapement of silver eels from the Grand-Lieu Lake to the sea

Acronym: ESGL
Period: 2015 till 2016
Status: Completed

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"Current knowledge about the downstream migration of sexually mature European eels
(Anguilla anguilla) remains incomplete, particularly in still water habitats such as lakes
and wetlands subject to water level management. However, for the management of
this endangered species, it is important to understand migration dynamics, and contribution to the breeding stock. This study aimed to assess the parameters that trigger
and guide the migration of silver eels in the largest floodplain lake and associated
wetlands in France (the sluice regulated Grand-Lieu Lake). A telemetry survey of 50
acoustic and PIT-tagged female silver eels was performed during the 2015–2016 migration period. We deployed a novel telemetric approach, using receivers to delimit
several restricted virtual boxes to determine the instantaneous location of individuals
and to transform simple discrete telemetric data into presence/absence data. The low
numbers leaving the lake centre are probably explained by the lack of orienting water
flows or other environmental clues, but whilst the fate of 34% (17/50) of the tagged
eels is unknown, 18% (9/50) were caught by commercial fishermen. Modelling showed
that detections were not clearly associated with environmental factors typically involved in riverine migrations (e.g. current velocity, atmospheric pressure and temperature) but they were particularly associated with higher and increasing water levels and,
for eels exiting the lake, a sharp increase when sluice gates were opened to an effective gap of >75 cm. It is concluded that management of water levels and sluice gate
opening during the migration period might aid escapement of silver eels."

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