Search

The European eel, Anguilla anguilla, is an emblematic facultative catadromous species that spawns in the Sargasso Sea and grow in continental waters of Europe and North Africa. In most of its growing habitats its population has dropped since 1980. Although Mediterranean lagoons represent particularly important habitats for eel, knowledge of eel ecology in lagoons is not as developed as it is in rivers. Particularly, data on the phenology, drivers and biometrics characteristics of glass eel entering lagoons are scarce. To address this lack of data, the abundance, pigmentation stage, size, and weight of glass eel entering the Bages-Sigean lagoon (western French Mediterranean) were monitored during 647 d from December 2018 to April 2021 using passive floating traps. Simultaneously, different environmental drivers were measured (flow velocity, temperature, rainfalls…). The highest abundances of glass eels were observed between mid-November and mid-March especially when the discharge of the main tributary of the lagoon was higher than its base flow. The glass eels captured during the peak of entrance were less pigmented, longer, and bigger than during other months. This work enabled us to identify periods when anthropogenic activities should be limited to decrease human-based impact on glass eel in similar habitats.

Despite the increasing use of acoustic cameras in fish ecology and fisheries studies, the quantification of biases associated with this method have received little attention. In this note, we used data collected from an ARIS acoustic camera, positioned in a channel linking a lagoon to the sea, to quantify differences in European eel (Anguilla anguilla) counts and size estimates made by two experienced operators. Count estimates made from 58 videos were highly consistent between the two operators (R2 = 0.99), although statistically different. Compared to the known sizes for the 82 eels, sizes estimated manually from video were underestimated. The resulting mean error percentages were significantly different between the two operators (−3.9% ± 8.5 (SD) and −6.6% ± 8.9). This error percentage was significantly influenced by the known size of the eels but not by the detection range. Our results highlighted the importance of taking into account the biases in counts and size estimates in fish ecology and fisheries studies based on acoustic cameras. These biases have to be quantified and, if possible, corrected using similar protocols as described in this study, when multiple operators analyse acoustic videos or when comparing the results from different studies.