Talk Defragmentation of the motorway “A63 Landes” for the critically endangered European mink: evaluation of the use and effectiveness of underpasses restored on existing hydraulic bridges

Creation of underpasses on roadways is a main action for reducing wildlife road mortality and habitat fragmentation, particularly for the critically endangered European mink Mustela lutreola.

As part of the carriageway widening of the motorway A63 between Salles and Saint-Geours-de-Maremne, through 104 km of the remaining French European Mink population in south-western France, 33 existing hydraulic bridges were rehabilitated to implement underpasses from 35 to 90 meters long.

Various types of passageways were built using diverse materials and techniques specially adapted to the local characteristics of each bridge, taking into account the hydraulic constraints and the limitation of impacts on watercourses and habitats during the fieldwork.

Evaluation of the use and effectiveness of 28 underpasses were monitored between April 2013 and December 2014, during seven sessions of about 50 days. Eighty-two passageways of six different types (wood or concrete ledges, pontoons …) were monitored using ink footprint traps.

Eighteen species or species-group were identified, showing the crossing of Mammals through all underpasses and 81/82 passageways. Small Carnivores were detected on 24 underpasses, among which 11 including Otter Lutra lutra, and 9 including European or American mink Neovison vison.

At the end of the study, five indicators were used to determine the level of utilization of the passes by the fauna: the frequency of detection of the « small » micromammals, the rate of crossing of « large » micromammals and of small carnivores, the specific diversity of small carnivores (including Otter) as well as the total specific diversity.

The utilisation rate by the mammal fauna differed between underpasses: a “very low” rate was recorded for seven of them, a “low” rate for six, a “middle” rate for seven, and a “good” rate for the eight last. Causes of this effectiveness variability could not be clearly identified, except some problems of connexion to the river bank, or of access to the bridge. However, the utilisation rate by small carnivores tripled from 2013 to 2014, suggesting a habituation (reduction of shyness), thus a probable increase with time of the effectiveness of the underpasses, after 40 years of partitioning by the former road RN10.

These rehabilitations confirm the possibility of adapting the underpasses and their effectiveness to all the types of existing bridges, since we break preconceived ideas and since the design and the fieldwork are accompanied by a mammal specialist of these topics.

bridge underpasses; defragmentation; effectiveness; endangered European mink; monitoring; motorway; passageways; restoration; use.