Pecheyran, C. | bioRxiv Science

bioRxiv2025

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.

ecology↗

bioRxiv2023

A step towards measuring connectivity in the deep-sea: elemental fingerprints of mollusk larval shells discriminate hydrothermal sites

Deep-sea hydrothermal-vent systems are under investigation for base and precious metal exploitations. The impact of mining will depend critically on the ability of larval dispersal to connect and replenish endemic populations. However, assessing connectivity is extremely challenging, especially in the deep sea. Here, we investigate the potential of elemental fingerprinting of mollusc larval shells to discriminate larval origins between multiple hydrothermal sites in the Southwest Pacific Ocean. The gastropod Shinkailepas tollmanni represents a suitable candidate as it uses capsules to hold larvae before dispersal, which facilitates sampling and ensures mineralization occurs on the site of origin. Multielemental microchemistry was performed using cutting-edge femtosecond laser ablation Inductively Coupled Plasma Mass Spectrometry analysis to obtain individual measurements on 600 encapsulated larval shells. We used classification methods to discriminate the origin of individuals from 14 hydrothermal sites spanning over 3,500 km, with an overall success rate of 70%. When considering less sites within more restricted areas, reflecting dispersal distances reported by genetic and modelling approaches, the success rate increased up to 86%. We conclude that individual larval shells register site-specific elemental signatures that can be used to assess their origin. These results open new perspectives to get direct estimates on population connectivity from the geochemistry of pre-dispersal shell of recently settled juveniles.

ecology↗