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Glass eels, the young life stage of the endangered European eel entering the European continental waters, are exploited for human consumption (direct, or on‐grown) and restocking. Restocking is considered as a potential contribution to the conservation of the stock. For this, the mortality of glass eels during capture should be low. We studied the handling mortality of glass eels obtained by commercial push net fisheries in different rivers in France in 2019 and 2020. The direct mortality of glass eels in 41 fishing trips of 29 boats varied from zero to 3.1% (mean 0.3%). Skin lesions identified by staining occurred in 31% (range: 4%–98%) of the eels. Post fishing mortality of glass eels varied from zero to 67.2%. The mean total fishing mortality was with 7.4% (range 0%–56.2%), considerably lower than in previous studies. The Sustainable Eel Group (SEG) issues certificates to fishers applying best practices for a responsible fishery. Comparing certified to uncertified fishers, the certified fishers have on average significant lower lesions and post‐fishing mortality rates, However, some uncertified fishers have an equal or better glass eel quality than certified fishers. Noting that certified fishers score better on average, and avoid the worst scores, we conclude that the SEG‐certification scheme for responsible fishing is effective. [ABSTRACT FROM AUTHOR]

This study describes a mass marking method suitable for European glass eel Anguilla anguilla, avoiding osmotic shock and reducing fish stress. During a single day, 360,000 glass eels intended for a stocking operation were bathed into an alizarin red S ( ARS) solution, at a density of 346 ind.·l−1 before being released into a freshwater marsh along with unmarked glass eels. After 15 days, examination of marked and unmarked eels kept in enclosures in the marsh showed 100% of otoliths had been successfully marked and that no excess deaths related to marking process had occurred. Seven months after stocking, results from biometric measurements and otolith analysis, carried out on a sample of 53 elvers caught by electrofishing and belonging to the size class of the 0+ cohort, suggest that: (i) the proportions of marked and unmarked remained unchanged, hence suggesting that the marking process had not caused excess mortality, (ii) there was no difference in length, body mass or condition factor between marked and unmarked individuals, demonstrating that marking had caused no harmful effect after 7 months, and (iii) ARS marks in otoliths remained clearly visible. The benefits of this ARS mass marking technique are discussed through a stocking operation. [ABSTRACT FROM AUTHOR]

Charrier, F., Mazel, V., Caraguel, J-M., Abdallah, Y., Le Gurun, L. L., Legault, A., and Laffaille, P. 2012. Escapement of silver-phase European eels, Anguilla anguilla, determined from fishing activities in a Mediterranean lagoon (Or, France). – ICES Journal of Marine Science, 69: 30–33.Escapement of silver eels from a Mediterranean lagoon was estimated by a capture–tagging–recapture and automated tag-reading study. The population of silver-phase eels in the lagoon was estimated to be 13.2 kg ha−1, with an escapement rate from the commercial fishery of 76.8%. [ABSTRACT FROM PUBLISHER]

As a key parameter in the management of fish populations, individual growth rate ( GR) variations were examined in the European eel ( Anguilla anguilla; > 150 mm) using extensive mark-recapture surveys in the lotic habitats of two small rivers of western France: the Frémur, supposed to be saturated, and at the same latitude, the Oir with densities fivefold lower than those of the Frémur. In both systems, generalised linear models were used to test whether spatiotemporal factors such as dominant habitat type or local density affect GR variability. In the presumed unsaturated system, the Oir, GR variability is mainly explained by a set of habitat suitability drivers (density, dominant habitat type). In the Frémur, GRs appear independent of differences in habitat density or productivity. Below saturation, an increase in density will decrease the GR through intraspecific competition. At saturation, intraspecific competition reaches such high levels that regardless of eel density and productivity, the resources available by individual are similar throughout the system. In these circumstances, the effect of density on growth was presumed undetectable. Despite these contrasted results, mean GRs observed in both catchments were closed (~20 mm·year−1). This is an unexpected result as GR is expected to be higher in unsaturated systems. This similarity could be explained by the difference between the two systems in terms of: (i) sex ratio (the Frémur is dominated by male, whereas the female is dominant in the Oir), (ii) habitat type distribution or (iii) possible interspecific competition (important salmonid populations in the Oir). [ABSTRACT FROM AUTHOR]

Abstract: 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. [ABSTRACT FROM AUTHOR]

Estimating accurate age-specific survival probabilities and understanding the processes (density dependent or independent) that regulate this demographic parameter are fundamental to propose sustainable management options for the endangered European eel ( Anguilla anguilla)., In the present study, we used extensive mark-recapture data sets (13 and 17 years) on eel >150 mm to estimate accurate natural survival probabilities of two eel sub-populations, from the Oir and Frémur rivers, western France, and then we analysed survival probabilities in relation to density-independent (temperature) and density-dependent factors to identify those causing survival variability., The Frémur and Oir rivers are two small (<100 km2) river systems in close proximity (65 km apart). The Frémur River is a small river obstructed by dams with medium water quality, high eel recruitment and density (0.37 eel m−2), and a male dominant population; whereas, in the Oir River, low densities of eels were observed (0.04 eel m−2) and the sex ratio was skewed towards females. Furthermore, previous research suggested that the lotic habitats of the Frémur River have reached carrying capacity, whereas habitats in the Oir River are below habitat saturation., In the Frémur River, there were significant spatial and temporal variations in the survival probabilities. However, survival probabilities observed in the Oir River were stable over time and space. The results highlight that the differences in the characteristics of the two systems and the two sub-populations prompt different responses to regulatory processes., The contrasting pressures applied on these two sub-populations impact survival, which possibly lead to different life history strategies such as sex differentiation. [ABSTRACT FROM AUTHOR]

1. The drastic decline in European eel Anguilla anguilla stock is now widely recognized. However, while various causes for this decline have been identified, the relative importance of each cause remains unclear. 2. During the catadromous migration of silver eels, the negative impact of dams is frequently highlighted, but mainly for powered dams (with turbines) or where connectivity is completely ruptured. Mechanical impact due to turbine blades is often considered the most important cause of mortality of silver eels during downstream migration. Consequently, non-powered dams equipped with spillways are often considered safe for the passage of migrating silver eels. 3. We hypothesized that, to understand the negative impacts of dams, a much wider context must be considered than turbine mortality alone. Using an acoustic telemetry survey of silver eels, we demonstrated the negative effects of non-powered dams on downstream migration. 4. Five main impacts on eel populations were highlighted: (i) the attenuation or loss of triggering factors, leading to an absence of or delay in migration; (ii) extra delays and extra distances travelled when crossing the dam; (iii) extra energetic costs of the additional distance travelled as result of exploring the dam and the reservoir to find other escape passages; (iv) the selection of a more risky behavioral phenotype, i.e., bold eels; and (v) direct blocking once migration has started. Mortality was evaluated as a supplementary impact. Some of these effects (attenuation of triggers, extra delays to cross the dam) might be more important than the same effect from powered dams, probably due to the constant high water discharge required for turbines that facilitate the passage of eels. 5. As these "safe" dams are very widespread, they must be considered a potential threat to effective eel conservation. [ABSTRACT FROM AUTHOR]