Abstract
Abstract Most research on anadromous fishes has been invested in their freshwater life‐phases, resulting in a relatively sparse understanding of their spatial ecology during marine life‐phases. However, understanding the marine dispersal of anadromous fishes is essential to identify threats and to implement conservation measures that fully encompass their lifecycle. The twaite shad Alosa fallax is an anadromous fish increasingly imperilled across its range due to pollution, harvesting, and impediments to freshwater migration, but little is known about its distribution and movements during its marine life‐phase. Here, the application of acoustic telemetry provided novel insights into the coastal dispersal of twaite shad in the UK and Ireland during 2018–2019, and the freshwater entry of individuals during the 2019 spawning season. Of 73 twaite shad acoustic‐tagged during their upstream migration in the River Severn in May 2018, 58 emigrated from the river. Twelve were subsequently detected 200 km to the south‐west at the Taw–Torridge Estuary between July 2018 and April 2019, where estuarine movements up to 5.8 km inland occurred in summer, winter, and spring. One was subsequently detected in the Munster Blackwater Estuary (Ireland) and then in the River Severn, indicating a minimum movement distance of 950 km. Thirty‐four (59%) of the emigrating individuals from 2018 re‐entered fresh water in the rivers Severn (n = 33) and Wye (n = 2) in April and May 2019. These results suggest year‐round use of estuarine and nearshore habitats by at least a subset of the twaite shad population during their marine phase, providing evidence of potential range overlap between populations that spawn in different areas in the UK and Ireland, which may be facilitated by substantial dispersal. The results also highlight the potential of telemetry for estimating freshwater and marine mortality, and the benefits of sharing detection data across networks.
Highlights
Populations of anadromous fishes are increasingly threatened by anthropogenic disruptions to their lifecycles, occurring in freshwater and marine environments (Limburg & Waldman, 2009)
These results suggest year-round use of estuarine and nearshore habitats by at least a subset of the twaite shad population during their marine phase, providing evidence of potential range overlap between populations that spawn in different areas in the UK and Ireland, which may be facilitated by substantial dispersal
Of the 73 twaite shad acoustically tagged during their upstream spawning migration in the River Severn in May 2018, 58 were detected emigrating from the river; the rate of freshwater
Summary
Populations of anadromous fishes are increasingly threatened by anthropogenic disruptions to their lifecycles, occurring in freshwater and marine environments (Limburg & Waldman, 2009). Key knowledge gaps relate to their spatial ecology while at sea, including habitat use, dispersal, and mortality rates, as well as population-specific distribution and connectivity (McLean, Hay, & Taylor, 1999) Addressing these knowledge gaps may help to mitigate marine-specific threats, including harvesting and accidental bycatch, and to understand the impacts of human-induced climate change (Dunton et al, 2015). An anadromous fish that is becoming increasingly imperilled across its range is the twaite shad (Alosa fallax), which has a distribution across the north-western Atlantic and Mediterranean Their riverine migration period lasts for approximately 3 months, with peak river entry periods varying from February in the south of its range to May–June in the north (Aprahamian, Baglinière, et al, 2003). Four rivers in Ireland support spawning populations: the Munster Blackwater and the Barrow–Nore–Suir river system (King & Roche, 2008)
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