Detection of a New Reproductive Location of Twaite Shad (Alosa fallax) with eDNA in the Ter River, NE Catalonia

Résumé fourni par la source

The Twaite shad (Alosa fallax Lacépède, 1800) is an endangered anadromous fish listed as Vulnerable on the IUCN Red List. In northern Catalonia (NE Iberian Peninsula), the species maintains a small reproducing population in the Fluvià River, while in the Ter River only sparse, irregular records exist and the status of the species has remained essentially unknown. This study aimed to assess the status of Twaite shad in the lower Ter River through a combined approach of scientific rod-and-reel fishing and eDNA analysis, with the specific objectives of confirming adult presence during the spawning season and detecting juveniles as evidence of effective reproduction. Water samples for eDNA analysis were collected at multiple sites along the lower Ter, across five sampling campaigns conducted between May and September 2025. Duplicate samples of mean 32 L each were filtered in situ and amplified using the MiFish 12S marker with twelve PCR replicates per site and sequenced using Illumina high-throughput technology at a 40 M read sequencing depth per sample. Complementary scientific fishing was carried out using a rod-and-reel technique with demonstrated effectiveness for Twaite shad, comprising five spring sessions (40 h of effort) and four autumn sessions (80 h of effort) at key habitat sites. The eDNA approach proved markedly more sensitive than conventional fishing. Notably, high-resolution eDNA confirmed the presence of the species below the Ullà weir, demonstrating its capacity to move upstream past the Torroella de Montgrí gauging station, most likely facilitated by elevated discharge events during spring. A confirmed eDNA detection in late August provides strong evidence of juvenile presence and, consequently, of effective reproduction in the Ter River. Conclusions: This study provides the first evidence that Twaite shad reproduce in the Ter River, confirming the existence of a previously undescribed breeding population. The eDNA approach demonstrated substantially greater sensitivity than conventional fishing methods for this cryptic migratory species. Priority actions include the identification of spawning grounds, the extension of eDNA surveys to reaches upstream of Ullà—now feasible given the current breach in the weir—and the genetic characterisation of this population to determine its origin, diversity, and potential introgression.

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