Decadal Trends in the Migration Phenology of Diadromous Fishes Native to the Burrishoole Catchment, Ireland

Abstract

Migration is an important ecological trait that allows animals to exploit resources in different habitats, obtaining extra energy for growth and reproduction. The phenology (or timing) of migration is a highly heritable trait, but is also controlled by environmental factors. Numerous studies have reported the advancement of species life-events with climate change, but the rate and significance of such advancement is likely to be species specific, spatially variable and dependent on interactions with population and ecosystem changes. This is particularly true for diadromous fishes which are sentinels of change in both freshwater and marine domains, and are subject to considerable multiple stressors including overfishing and habitat degradation. Here, we describe trends in the migration phenology of three native Irish migratory fishes over half a century, Atlantic salmon (Salmo salar), brown trout (Salmo trutta) and European eel (Anguilla anguilla). The trends were derived from daily counts of 745,263 fish moving upstream and downstream through the fish traps of the Burrishoole catchment, an internationally important monitoring infrastructure allowing a full census of migrating fish. We found that the start of the seaward migration of eel has advanced by one month since 1970. The commencement of the salmon smolt migration has advanced by one week, although the rest of the migration, and the entirety of the trout smolt run has remained stable. The beginning of the upstream migration of trout to freshwater has advanced by 20 days, while the end of the run is more than one month later than in the 1970’s. The greatest phenological shift has been in the upstream migration of adult salmon, with at least half of migrating fish returning between one and two months earlier from the marine environment compared to the 1970’s. The earlier return of these salmon is coincident with reduced marine survival and decreasing body size, indicating considerable oceanic challenges for this species. Our results demonstrate that the impacts of climate change on the phenology of diadromous fish are context-dependent and may interact with other factors. The mobilization of long-term datasets are crucial to parse the ecological impacts of climate change from other anthropogenic stresses.

Development and field validation of RPA‐CRISPR‐Cas environmental DNA assays for the detection of brown trout (Salmo trutta) and Arctic char (Salvelinus alpinus)

“…The lakes, drained by 45 km of rivers and streams, form a gradient from the oligotrophic, acid‐neutral Lough Bunaveela and Lough Feeagh to the tidal transitional waters found in Lough Furnace. The direct tidal connection of Lough Furnace to the Atlantic Ocean and subsequent channels to Lough Feeagh, the natural Salmon Leap and man‐made Mill Race, make the Burrishoole perfect for monitoring migratory and resident fish populations (de Eyto et al, 2022). In April 2021, thirteen aquatic sites were sampled in duplicate throughout the Burrishoole Catchment (Figure 1).…”

“…Marine Institute, 2020), to indicate the occurrence of fish species in the recent past. These data were collected using three-pass removal sampling with backpack electrofishers (Hedger et al, 2013), seine net sampling on Feeagh and Bunaveela (Matthews et al, 1997) or from manual counting of fish moving through permanent fish traps (de Eyto et al, 2022).…”

“…Positive detection of brown trout at all sites throughout the catchment using RPA-CRISPR-Cas, aligns with previous data collected using traditional survey techniques. It is likely that these signals are from resident brown trout populations present in the catchment, rather than anadromous populations which are known to be in decline (de Eyto et al, 2020). Anadromous forms of brown trout are at a high risk of predation and infestation with pathogens and parasites during the marine stage of their life cycle (Jensen et al, 2019).…”

“…This has been seen in the Burrishoole catchment with high marine mortality of smolts paralleled by poor growth of surviving fish (de Eyto et al, 2022). Differentiating anadromous and resident populations is crucial when considering conservation, however to date there has been a lack of genetic differentiation reported within sympatric populations (Charles et al, 2005;Hindar et al, 1991).…”

Autumn outmigrants in brown trout (Salmo trutta) are not a demographic dead‐end

“…Data on how timing and size at outmigration relate to characteristics of returning fish remain sparse for brown trout, owing to challenges in tracking the life histories of individual fish. The current study builds on previous work on seaward migration timing of brown trout in the Burrishoole catchment, mid‐west Ireland (Byrne et al ., 2004; de Eyto et al ., 2022; Mills et al ., 1990; Poole et al ., 1996, 2007), using new individual‐level information. The aims of this study were to (a) further characterise the bimodal timing patterns in terms of age, size and sex of juvenile outmigrants, and (b) use a novel combination of genetic tagging with physical tagging [passive integrated transponder (PIT) tags] to test for differences between spring outmigrants and autumn outmigrants in their return rates, size and phenotype at return, time spent away and specific growth rates.…”