Towards a standardized characterization of the potentially migrating silver European eel (Anguilla anguilla, L.)

Abstract

We defined a standardized method for discriminating candidate silver eels that may undergo catadromous migration in the following season from the sedentary fraction of a population. A combination of two qualitative criteria (state of differentiation of the lateral line and colour contrast) and one quantitative criterion (Ocular Index OI) was used to determine the development toward silvering. In the non-migratory phase, we found a gradient of the three criteria between yellow (0 criterion), presilver (1 to 2 criteria) and silver (3 criteria) eels. In the migrant phase, silver eels had ended their metamorphosis process and were characterized at the same time by the presence of the 3 silvering criteria. A mark-recapture survey using PIT-tags provided evidence that only identified silver eels (3 silvering criteria present) in the catchment actually emigrated the following season. Moreover, the use of a single criterion of silvering among the three generated large variation in the estimated proportion of candidates for emigration which varied between-22 % and + 63 %. Such a result confirmed that a multicriteria approach is needed to characterize in a standard way the potentially migrating silver eel.

Silver eel downstream migration in fragmented rivers: use of a Bayesian model to track movements triggering and duration★

“…As mentioned in the introduction, the two aspects have generally been analysed independently depending on the type of available data. Analysis of migration from captures in a specific trap is suitable to analyse migration triggering (Acou et al, 2005;Trancart et al, 2013) while radiotracking data are appropriate for analysing movements both in terms of distance travelled (Verbiest et al, 2012) and behaviour at specific dams (Jansen et al, 2007;Bau et al, 2013). The main strength of our study is that it analyses three elements simultaneously: migration triggering, distance travelled and the impact of obstacles.…”

“…All eels fulfilling the 2% ratio rule were tagged except a few individuals that had been injured during the catching process (other individuals were in good health). According to Durif et al (2005) and Acou et al (2005), they were all silver eels (Table S1) and consequently expected to migrate in the short terms.…”

“…Eels were anaesthetized in a solution of acetyleugenol (∼1.1 mL/L), measured, and weighed. Their head lengths and heights, eyes (vertical and horizontal lengths) and pectoral fins were measured and their stages of maturity was checked according to Durif et al (2005) and Acou et al (2005) indices. A coded ATS (Advanced Telemetry System) radio-transmitter with a pulse rate of 45 ppm (F1820 frequency 48-49 MHz, length 43 mm, diameter 12 mm, weight 8 g, minimum battery capacity 95 days or F1815 frequency 48-49 MHz, length 36 mm, diameter 12 mm, weight 7 g, minimum battery capacity 65 days) was implanted in the body cavity by surgical incision as described by Baras and Jeandrain (1998).…”

Silver eel population size and escapement in a Mediterranean lagoon: Bages-Sigean, France

“…Silver stage was determined based on the combination of two macroscopic criteria: a differentiated lateral line (presence of black corpuscles) and a contrasting colour (dark dorsal surface and a white ventral surface). Although the ocular hypertrophy values (OI > 6.5) should be considered as a criterion for silver eel determination (Acou et al, 2005) it was not calculated in the field for logistic reasons. However, Durif et al (2005) showed that the increase in the ocular diameter appears before the pigmentation change, therefore we considered it valid to use only the two criteria.…”

Assessing sexual maturity of feminized Japanese eel Anguilla japonica by measuring eye size

“…Pankhurst (1982) described a correlation between eye index and sexual maturity for this species treated with an exogenous hormone and suggested its availability as a method of assessing sexual maturity during induced maturation. Since then, the eye index has often been applied in field studies of A. anguilla (e.g., Acou et al 2005Acou et al , 2006Durif et al 2005) and A. rostrata (e.g., Cottrill et al 2002). In order to verify the effectiveness of this index for feminized A. japonica in a laboratory, we investigated the relationship between eye index and sexual maturity, and responsiveness to hormone treatment.…”