| Abstract |
Major migration barriers have been identified as an important
factor in the drastic reduction of freshwater eel stocks at the catchment
scale. In this study, we developed a way to assist decision making in the
management of migration barriers so as to increase eel presence throughout
the Loire catchment (France). Models were developed according to eel
size-class and various different management strategies to improve
accessibility, taking into account: (i) all the migration barriers from the
tidal limit to different upstream limits (downstream–upstream strategy), (ii)
all the migration barriers with a given degree of passability (migration
barrier passability class strategy), (iii) all the migration barriers in a given
sub-catchment area (sub-catchment strategy) and (iv) all the migration
barriers in the entire Loire catchment (global strategy). Eel were actually
found to occur in the Loire at about 50% of the sites sampled. The
improvement of habitat accessibility by removing all obstacles to
colonisation or installing fish passes, for example (global strategy), would
lead to eels being found throughout the whole river. If this management
strategy is impossible because the high cost, two others, also producing
significant eel continental colonisation must be implemented. Either all
barriers from the tidal limit with medium passability and ⁄ or migration
barriers in the sub-catchment area which is closest to the sea and least
accessible must be managed. In both cases, the area significantly exploited
by eels and the probability that this species will be present will be doubled.
However, these results can only be counted on if the fluvial recruitment
increases significantly. |