Unexpected recent records of twaite shad (Alosa fallax) in two north-eastern iberian rivers: Recolonization or recovery of remnant populations?

Abstract

The presence of twaite shad Alosa fallax (Lacépède, 1803), an anadromous fish that migrates to lower and middle river reaches to spawn, has been reported in the Fluvià and Ter rivers (NE Spain) after several decades of absence of confirmed records. From 2015 to 2017 a total of 32 specimens, including one juvenile fish, were captured by angling in the River Fluvià, and a single individual was captured in the River Ter. The origin of these fish in the Fluvià may be the recovery of a scarce local population that remained undetected or a recolonization by individuals straying from neighbouring populations. The presence of a single individual in the Ter and the absence of records for over a century in this river suggest that this specimen may have come from a nearby population. If the establishment of a twaite shad population in the Fluvià is confirmed in the next few years, the construction of fish passages would increase the availability of spawning habitats and improve the species’ conservation prospects.