Population-specific bycatch risks in two vulnerable anadromous clupeids: insights from otolith microchemistry
Otolith microchemistry analysis revealed that bycatch of European shads--allis shad Alosa alosa (L. 1758) and twaite shad Alosa fallax (Lacepede 1803)-- in western Iberian commercial fisheries removes individuals from a wide array of natal origins, with the most abundant source rivers suffering the heaviest losses. Spatial variation in bycatch risk was evident: specific marine areas exhibited high natal-origin diversity, reflecting complex dispersal. A. alosa showed extensive medium- and long-distance movements-- including rare longitudinal displacements along the Cantabrian slope--and greater natal origin diversity than A. fallax, whose dispersal was largely restricted to middle-distance, latitudinal migrations. In both species, bycatches were dominated by the most abundant continental populations--Mondego and Minho for A. alosa, and Ulla and Minho for A. fallax--suggesting that these rivers function as source populations exporting individuals to sink populations through marine dispersal. Despite their differing dispersal ranges, both species displayed dual resident-dispersive contingents coexisting within the same populations, reflecting an interplay of river proximity, philopatry and resource availability. The stronger philopatry and constrained range of A. fallax imply heightened vulnerability to localized bycatch pressure near natal rivers, whereas A. alosas broader dispersal and higher origin diversity expose multiple populations to risk at a regional scale. These species-specific dispersal capacities and metapopulation structures critically shape bycatch vulnerability. Incorporating natal-origin and dispersal data into transnational, ecosystem-based management--such as targeted temporal or spatial fishing restrictions at mixing hotspots--will be essential to safeguard metapopulation dynamics, mitigate bycatch mortality, and maintain ecological connectivity among European shad populations.