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

Within the scope of a recently started monitoring, the spatial and temporal distribution and activity patterns of the European eel, Anguilla anguilla have been investigated in the shallow water benthos (<6m) in Novigrad Sea, Central Croatian Adriatic, using a novel visual census method. Transects of various length (up to 1000+ m) with video assisted habitat tracking, DGPS tracking, and depth sounding were followed during the day and then with sub-meter accuracy repeated at various night times. No markers of any kind were needed. The sampling scheme was applied at various study sites within the area (1.5 km2) and over the course of a moon cycle and in two seasons (fall 2009 and spring 2010). Eel presence and habitat occupation at the time of observation were recorded along with their exact geographical location. A joined analysis of eel observation data and results from the video image analyses of the transected benthos permitted the quantification of habitat and depth preferences and avoidances. Significant positive associations with dense seagrass patches of high Zostera marina proportion were discovered for the eel, and habitat preference explained most of the eels’ large scale distribution within the study area. Furthermore eel abundance and activity showed significant temporal variation, as they peaked several hours after sunset and varied within the moon phase cycle and seasonally. Due to the direct encounter of the eel by the observer, additional data, including: eel size, behavior, and color phase (silver vs. yellow) can be gathered. 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.

Anguilla anguilla; seagrass; visual census; DGPS; Croatian Adriatic

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