Comments on ‘Weak population differentiation in northern European populations of the endangered anadromous clupeid Alosa fallax’

Abstract

In a recent paper, Volk et al. (2007) reported statistically significant, but weak genetic differentiation among ‘spawning populations’ of the endangered twaite shad Alosa fallax (Lacepede). The authors state that this suggests ‘substantial gene flow among the majority of individual estuary–river spawning locations’, an assertion that could have implications for the management of this endangered species. The sampling regime upon which this study is based is flawed, however, and may underestimate the degree of population differentiation for A. fallax from northern Europe. A fundamental precept of genetic studies of fish population structure is that samples need to come from spawning aggregations on the spawning grounds in order for clear interpretations of population structuring to be reached. Volk et al. (2007) state that A. fallax ‘spawning biology and habitat requirements are not well described’. Aprahamian et al. (2003), however, provide a comprehensive overview of A. fallax biology, including spawning biology and spawning habitat requirements within specific rivers. Alosa fallax is an anadromous clupeid, which are believed to home to their natal river, and are known to spawn in tidal fresh water and non-tidal portions of rivers as far as 400 km upstream (Aprahamian et al., 2003). Similarly, the congeneric allis shad Alosa alosa (L.) (Bagliniere et al., 2003) and American shad Alosa sapidissima (Wilson) (Limburg et al., 2003) are known to make extensive upstream spawning migrations. Despite a reportedly high degree of spawning-site fidelity (Melvin et al., 1986), A. sapidissima are known to exhibit forays into multiple estuaries along the Atlantic coast during their annual spawning migration. Adults tagged in the Connecticut River have been captured in the subsequent annual spawning migration in estuaries ranging from Pamlico and Albemarle Sounds (North