A glass eel fishery exists downstream of the Arzal estuarine dam in the Vilaine (Brittany, France). Catch statistics were collected between 1996 and 2000, and processed using a subsampling technique which allowed data from a reliable subsample to be extrapolated to the whole fishery. During the same period, glass eel migration into fresh water was monitored using a glass eel trap located on the dam -the upstream limit of the fishery. The analysis of glass eel biology and exploitation shows that the glass eel fishery is very intensive and that there was more or less no escapement during the fishing seasons studied. The proportion of the stock successfully migrating towards fresh water, as compared with the total catch is estimated to range between 0.3 and 3.9%. In such a fishery, the fishing effort affects the abundance. Consequently, the total catch has to be used instead of catchper-unit-effort to estimate abundance.
The construction of the Arzal dam eel ladder in 1996 allowed enhancing fluvial recruitment from a negligible level, limited to yellow eels crossing sluice and dam overflow, to a level ranging from 0.2 to 2.4 million glass eels per year. The effect of such recruitment on the eel population (Anguilla anguilla) of the Vilaine watershed was analysed at 19 electrofishing sampling stations. From 1998 to 2003, average densities of eel varied from 0.34 to 0.72 eels.m -2 , with a maximum reached in 1999 of 0.82 eels.m -2 . Fluvial recruitment failure, together with density dependent mortalities, explains the drop in yellow eel densities observed from 1998 in the downstream area (< 50 rkm) and from 2000 in the middle stream area (50-100 rkm). The largest densities consisted of a large proportion eels classified as age 1 and the increased density in middle stream sectors was interpreted as the consequence of density-dependent migration at the periphery of a saturated area. The construction of 13 eel ladders on the Vilaine waterway in 1999 and 2000 was followed by increased densities in the upstream area (> 100 rkm) in 2001. The comparison to an electrofishing survey performed at 17 stations in 1981, ten years after dam construction, confirmed that ladder installation increased densities by a factor 6 and modified the population structure, with densities of eels age 0 and 1 multiplied 29 fold. This change corresponded to reduced escapement rates to the glass eel fishery (1% to 5%). Considering this result, a preliminary escapement target of 240 glass eel per km 2 of watershed area or 1 500 glass eel per ha of water surface, is proposed and discussed for glass eel fisheries.
To determine whether thyroidal status is related to migratory and settling behavior in Anguilla anguilla glass eels, we sampled glass eels showing different migratory behaviors in the Arzal dam, which constitutes the tidal limit of the Vilaine River (France). We collected 4 groups of glass eels: flood and ebb tide glass eels were netted in the estuary during the flood and ebb tides, respectively, trap glass eels were sampled from the eel ladder trap, and bottom glass eels sheltering on the bottom of the estuary during flood tide. We measured individual whole-body triiodothyronine (T 3 ) and thyroxine (T 4 ) levels for these groups, and calculated total thyroid hormone (TH) levels as T 3 + T 4 contents reflecting the thyroid gland secretory activity, and T 3 :T 4 ratios reflecting T 4 outer-ring desiodase (T 4 ORD) activity. Trap glass eels had the highest TH levels, indicating an activation of the thyroid gland via the thyreotrop axis. This could be responsible for the behavioral transition (loss of circatidal rhythm and switch to counter-current swimming) at the tidal limit and active colonization of river habitats by glass eels. Ebb and flood tide glass eels had similar TH levels that were lower than those of trap glass eels, indicating a lower thyroid gland secretory activity in the former. Ebb tide glass eels had higher T 3 :T 4 ratios than flood tide and trap glass eels, indicating physiological stress related to inefficient use of tidal streams. Bottom glass eels had the lowest TH levels, and high T 3 :T 4 ratios similar to those of ebb tide glass eels; this suggests that physiological stress induced by frequent counter-current swimming leads to precocious settlement of glass eels in estuarine habitats. Our data supports the critical role of the thyroid status in the migratory and settling behavior of glass eels in estuarine and marine habitats.
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