For diadromous fish species, that migrate between marine and freshwater habitats, there is commonly very considerable disparity in knowledge about their ecology in different habitats (Lucas and Baras, 2001; Klemetsen et al., 2003; Armstrong et al., 2018). For the anadromous Atlantic salmon (Salmo salar) there is a relatively good understanding of the ecology of the freshwater phase of the life cycle in published literature, this is in marked contrast to that of the marine phase (post-smolt) (Thorstad et al., 2012; Barry et al., 2020; Flávio, Kennedy, Ensing, Jepsen, & Aarestrup, 2020). In particular, we have only a limited comprehension of their marine habitat use and the migration pathways they use to migrate between their natal tributaries and their marine foraging areas in both directions (Riley et al., 2014). For Atlantic salmon making their outward marine migration, what we do know comes principally from two approaches: the capture of salmon at sea from either targeted (SALSEA-Merge, 2007; CEFAS, Freshwater Fisheries, Environment Agency, and Natural Resources Wales, 2018; ICES, 2020) or opportunistic (Thorstad et al., 2012) capture by fisheries vessels and from telemetry studies tracking fish tagged in freshwater to marine habitats (Went, 1973; Mork et al., 2012; Klimley et al., 2013; Lothian et al., 2018; Newton et al., 2018; Chaput et al., 2019; Barry et al., 2020; Ounsley, Gallego, Morris, & Armstrong, 2020).
The former study approach can provide important, spatially explicit, but point source data on the presence of migrating salmon (Heupel et al., 2010; Hitt et al., 2011; Rowell et al., 2015). This approach has shown us that salmon emanating from rivers in southern Europe migrate north to marine feeding grounds in the Norwegian sea and/or to the north-west Atlantic, to the seas off west Greenland (Dunbar & Thomson, 1979; Klemetsen et al., 2003; Thorstad et al., 2012). Using this approach however, it is difficult to reconstruct migration pathways and passage speeds, in part because it requires fish collection over an enormous spatial area from which pathways need to be inferred.
The telemetry approach applied to seaward migrating smolts has until now, mostly centred around tracking using acoustic technology. Acoustic telemetry studies (both active and passive tracking), typically using fixed position, continuous monitoring acoustic receivers and fish tagged with acoustic transmitters, has rapidly become a common method to investigate fish migration patterns (large and small), site fidelity and diel and seasonal movements (Heupel et al., 2010; Hitt et al., 2011; Rowell et al., 2015) throughout both freshwater and marine environments. This technique has the advantage that it can provide highly precise spatial information and thus more precise migration pathways. The disadvantage, when this approach is used to track anadromous species migrating into marine systems, is that the financial and logistical costs of maintaining suitably positioned receivers increases exponentially with distance from the river from which individuals are being tracked. Collaboration amongst global and regional projects, often with differing primary aims, allow for migration information of fish movements in the spatially extensive marine environments to be obtained in a more cost-effective way.
In one of the few studies to have looked at migration pathways of anadromous Atlantic salmon in the Irish Sea using an acoustic telemetry approach, Barry and colleagues (Barry et al., 2020) analysed fish migrating from Castletown River and the River Boyne, both of which discharge into the west of the Irish Sea (on the east of Ireland). They found three Atlantic salmon smolts tagged in these tributaries were detected in the north suggesting a northern trajectory, after leaving the river mouth, taking them towards the North Channel, giving them access to the North Atlantic. Of these, one fish was detected on one of only two marine receivers, 250km to the north approximately 30 days after leaving its natal river. This finding provided the first evidence of a northerly migration trajectory for Atlantic salmon migrating into the Irish sea. However, due to there being no southern marine receiver array, we cannot completely exclude the possibility of southern migration occurring (Barry et al., 2020).
Here we provide a Telemetry Case Report on sea migrating Atlantic salmon from a river in north-west England discharging into the eastern Irish Sea. We show that Atlantic salmon smolts once entering the Irish Sea took a northern pathway suggesting a northern trajectory towards the North Channel, giving them access to the more open North Atlantic. We extend the maximum distance that a post-smolt Atlantic salmon has been tracked in acoustic studies in Europe to date. This study also demonstrates the need for both collaboration and data sharing between telemetry projects with alternative funding sources and project aims and for cross project detection compatibility to obtain maximum benefit from highly expensive, logistically demanding marine telemetry studies.