Modeling marine shad distribution using data from French bycatch fishery surveys

Abstract

In the last few decades, there has been a marked decline in the number of shad (Alosa alosa and A. fallax) landed in France, which prompted the French committee of the Inter- national Union for Conservation of Nature to list shad as a 'Vulnerable' species in 2010. The fresh- water phases of shad life cycles have been extensively studied, but the marine phases remain poorly understood. The present study aimed to provide new insights into shad ecology by describ- ing the marine distributions of twaite and allis shad using a presence/absence model based on bycatch data from commercial fishery surveys. Depth and salinity were identified as the main fac- tors influencing shad distribution. Both species were primarily located in shallow areas, at depths of between 0 and 100 m. As expected for anadromous species, low-salinity areas were preferred. Substrate and latitude played minor roles in the observed distribution of shad. Our results suggest that latitudinal migration between winter and summer habitats does not occur in twaite and allis shad populations. Furthermore, substrate does not appear to be a key factor contributing to shad distribution. A better understanding of the distribution of shad species throughout their life cycles, particularly in the open sea where they are vulnerable to bycatch, would help in the selection of key protected areas for the sustainability of shad populations.