engleski

Simultaneous monitoring of the European eel (Anguilla anguilla) and its benthic environment by DGPS/video supported visual census in the Croatian Adriatic

The European eel, Anguilla anguilla, is critically endangered and according to European council regulations member states need to establish stock recovery measures. Such plans must estimate eel escapement to the sea, thus life stage and habitat-based assessment of coastal eels is crucial, and considering their red-listed status, non-invasive assessment methods are desirable. Within the scope of a pilot eel monitoring, the spatial and temporal distribution of eel have been investigated in the shallow benthos of the Central Croatian Adriatic. Using geo-referenced and video supported SCUBA visual census, transects of up to 1000+ m were followed during the day and then with DGPS sub-meter accuracy re-navigated at various night times. Two bays were investigated at various study sites per bay and over complete moon cycles in the spring and fall of 2009/10 and in the spring of 2011. Eel presence and habitat occupation at the instant of observation were recorded along with their exact geographical location. This permitted the quantification of eel densities and habitat preferences at various scales. Significant positive associations with dense seagrass patches were discovered for the eel, and such habitat preference explained most of the eels’ large scale distribution. Furthermore eel abundance and activity showed significant temporal variation, as they peaked several hours after sunset, at quarter moons, and in the fall. Due to the direct encounter by the observer, additional data, including eel size and behavior are gathered. A future addition of stereo imaging of the censused eels is expected to allow for the identification of life stages (yellow vs. silver eel). The applied method is non-invasive, time and cost effective, combines eel census with simultaneous monitoring of the eels’ benthic environment, can collect data from large areas at various scales, and allows for frequent revisits of monitored locations with a high level of spatial precision. As a future EU member Croatia soon needs to report coastal eel stock assessments and a spatial expansion of the ongoing pilot monitoring scheme with added non-sacrificial life stage recognition may allow for the required estimates of eel escapement to the sea.

european eel; Anguilla anguilla; Novigrad Sea; monitoring

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