Search

Anthropogenic barriers are widely known to negatively impact the spawning migrations of anadromous fishes, by delaying or preventing passage upstream. Although the impacts of barriers on emigrating post‐spawned adults are less well studied, they could potentially impact the fitness and subsequent return rates of iteroparous species. In this study, passive acoustic telemetry was used to track the emigrations of 53 twaite shad Alosa fallax in the River Severn basin in their first spawning migration a year after being tagged, giving insights into their emigration movements and the impacts of anthropogenic weirs on these movements. A. fallax began their emigrations after spending varying amounts of time and migrating various distances within the river, with late‐emigrating individuals moving fastest and most directly. Emigrations became faster and more direct the further downstream individuals were from their furthest upstream extent. Downstream passage delays at weirs increased emigration times by a median of 61%, with environmental conditions (i.e., temperature, flow, and tidal influence on river level) having little influence on downstream passage at weirs with no modifications to facilitate fish passage. As weir‐induced emigration delays are suggested to deplete energy reserves (when energy levels are already depleted post‐spawning), limit spawning opportunities (by preventing access to downstream spawning habitat), and expose individuals to increased predation risk and suboptimal conditions (e.g., high temperatures), these delays can potentially diminish the benefits of iteroparity. The evidence presented here suggests that more consideration should be given to the potential impacts of anthropogenic barriers on the emigrations of iteroparous species when assessing river connectivity or undertaking barrier mitigation. [ABSTRACT FROM AUTHOR]

Fragmentation of watercourses poses a significant threat to biodiversity, particularly for migratory fish species. Mitigation measures such as fishways, have been increasingly implemented to restore river connectivity and support fish migration. The effects of such restoration efforts are typically tested using telemetry and fisheries methods, which do not fully capture the broader population movements that may have important consequences for population viability. We performed a before‐and‐after control‐impact (BACI) study using genetic tools (SNPs) to investigate the effect of a newly implemented fishway, aiming to enhance upstream spawning migration of brown trout (Salmo trutta Linnaeus) in a reservoir with two headwater tributaries fragmented by man‐made weirs. Another reservoir with two barrier‐free tributaries was also analysed as a control. Our results showed that the isolated brown trout population was spawning in the reservoir before the installation of the fishway, and we found genetic structuring and differentiation between fragmented headwater tributaries before the fishway construction, but not in the control reservoir. Unexpectedly, after the fishway construction we observed signals consistent with increased genetic differentiation between populations of newly recruited juvenile fish in the reservoir tributary and fish in the reservoir. We propose this was caused by newly enabled philopatric behaviour of brown trout to their natal spawning tributary. In contrast, we did not find any genetic changes in the tributary without a fishway or in the barrier‐free reservoir system. Given the scarcity of similar studies, we advocate for an increased use of genetic analyses in BACI studies to monitor and evaluate the effect of efforts to restore habitat connectivity and inform future management strategies. [ABSTRACT FROM AUTHOR]

Reversing the negative impacts that anthropogenic habitat fragmentation has on animal movement is a key goal in the management of landscapes and conservation of species globally. Accurate assessment of measures to remediate habitat fragmentation, such as fish passage solutions in rivers, are imperative but are particularly challenging for territorial species, which are less likely to leave their existing home range, or populations composed of both migratory and resident individuals (i.e., partial migration). This investigation quantified the movements of translocated (captured upstream of the impediment and released downstream) and non‐translocated (captured and released downstream of the impediment) riverine brown trout (Salmo trutta L.), a species known to perform a homing movement, through a fish pass using passive integrated transponder (PIT) telemetry. A significantly higher proportion of translocated fish approached, entered, and passed (on a wider range of flows) compared to non‐translocated fish, consistent with the theory that motivation is a key driver in fish pass use. Translocated fish that entered the pass were significantly larger than those that approached but did not enter, presumably due to physiological capability. Translocated fish were a more reliable indicator of the fish passage solution effectiveness than non‐translocated fish. Our findings hence imply that many fish passage solutions globally, and potentially measures to remediate habitat fragmentation for other taxa, may have been mistakenly assessed for unmotivated animals. Studying both non‐translocated and translocated fish is recommended to provide more accurate and cost‐effective fish passage solution assessments. [ABSTRACT FROM AUTHOR]

Anthropogenic structures extensively fragment riverine systems, reducing longitudinal connectivity, inhibiting migration and leading to severe declines in many fish populations, especially for diadromous species. This study investigated the upstream spawning migration of anadromous river lamprey (Lampetra fluviatilis) in a heavily fragmented tributary of the Humber Estuary, the location of one of the largest UK river lamprey populations. Overall, this study quantified river lamprey migration, spawning habitat distribution and historic river levels to develop a novel empirical index to understand the impact of man‐made barriers and prioritise their remediation. Passage at all weirs only occurred during episodic high river levels, often after prolonged delays with no lamprey passing below average levels for the time of year or utilising the fish pass at the first weir (T1) at the tidal limit. Barrier passage opportunities at the first four weirs were only possible for 30.3%, 38.7%, 52.1% and 6.7% of the migration period, but were lower and severely limited in 15 of the last 21 years. In addition, more lamprey (60%, n = 18) were last detected in reaches with no spawning habitat than in spawning habitat (40%, n = 12). Given the impassibility of, and lack of retreat from, T1 to other Humber tributaries, the River Trent is currently considered an ecological trap for a large proportion of lamprey that enter from the Humber Estuary. This passage should be urgently remediated, per the prioritisation index presented here, to aid river lamprey conservation, especially given their status as a designated feature of the Humber SAC. [ABSTRACT FROM AUTHOR]

Predicting and mitigating the impact of anthropogenic barriers on migratory fish requires an understanding of the individual and environmental factors that influence barrier passage. Here, the upstream spawning migrations of iteroparous twaite shad Alosa fallax were investigated over three successive spawning migrations in a highly fragmented river basin using passive acoustic telemetry (n = 184). More fish approached and passed barriers in the lower river reaches than further upstream, with the median cumulative weir passage time of 4.6 (1.8–9.2) days representing 18% of their time in the river. Returning fish in their second year had significantly higher weir passage rates than in their tagging year, with passage rates also positively influenced by previous passage success. Higher water temperatures and river levels also had positive impacts on passage rates. Lower weir passage rates by newly tagged individuals suggest that reliance on within-year passage estimates in telemetry-based barrier impact assessments could result in conservative results, while the higher passage rates of previously successful versus unsuccessful individuals suggest a conserved motivation and/or inherent ability to pass barriers. [ABSTRACT FROM AUTHOR]

Anthropogenic reductions in riverine connectivity can severely impede the migrations of anadromous species. In fragmented rivers, successful migrations depend on the ability of migrants to negotiate barriers or locate alternative passage routes. However, individual variation in the specific aspects of movement that determine migration success in fragmented rivers, is poorly characterised.Here, individual variation was investigated in the spawning migrations of 56 adult sea lamprey Petromyzon marinus, an anadromous, semelparous species that does not show fidelity to natal rivers. The variability and consequences of two key aspects of fish migration within fragmented rivers were tested: passage time (total time taken to pass a barrier) and retreats (exploratory downstream movements after unsuccessful passage attempts). These were tested using acoustic telemetry in the highly fragmented River Severn catchment, western England, U.K.Distinct unimodal, bimodal, and multimodal patterns of variation in passage times were displayed across the different barriers, potentially related to the physical characteristics of the barrier and prevailing river discharge conditions when the barriers were first approached, but were not related to lamprey body sizes. At the first three barriers encountered by upstream‐migrating sea lamprey in the study, between 30% and 46% of individuals made retreat movements, and between 5% and 100% of retreating individuals were able to locate alternative tributaries. Retreating individuals were highly variable in their distance and frequency of retreats; overall, retreat movements comprised 11% (lower‐upper quartiles 0%–52%, range 0%–76%) of the total distance moved before reaching spawning areas. Time‐to‐event analysis indicated that retreat rates reduced as river discharge increased.There was no evidence indicating that individual variation in passage time, or presence of retreat movements at barriers, influenced the subsequent upstream migration speed or final upstream extent of lampreys. While predictability in rank arrival timing was high within three unobstructed reaches, this predictability was disrupted at barriers due to individual variation in passage times.Anthropogenic barriers thus can both disrupt and reveal individual variation in the migration dynamics of anadromous species. Substantial variability in retreat behaviours can be displayed by anadromous species facing delays at barriers, with these behaviours also associated with environmental conditions and the availability of alternative migration routes. Individual variation in exploration and passage time of migrants strongly influence their eventual spawning distribution. [ABSTRACT FROM AUTHOR]

Highlights • Acoustic-tagging protocol for twaite shad was incrementally refined. • All captured shad survived handling, anaesthesia (i.e. MS-222) and tagging. • Surgically implantation was significantly faster than externally mounting the tag. • Movement distance and speed were similar for externally and internally tagged fish. • Internal tagging under anaesthesia is recommended for studying shad migration. Abstract Telemetry investigations to gather essential information about fish migrations are reliant on the behaviour, condition and survival of the animals being unaltered by the tagging procedure. Twaite shad (Alosa fallax Lacépède; 'shad') is a threatened clupeid fish for which there is a considerable knowledge gap on their anadromous movements. They are also reported to be sensitive to handling and anaesthesia, resulting in practical difficulties in tag implantation; previous investigations externally attached tags without sedation. The aim of this study was to incrementally refine the acoustic-tagging protocol for shad via application of a previously un-tried anaesthetic (i.e. tricaine methanesulphonate (MS-222)) and by surgical implantation of the tag in the peritoneal cavity. All captured shad (n = 25) survived handling, anaesthesia and tagging, and were detected moving upstream after release. Surgically implantation (n = 5) was significantly faster than externally mounting the tag (n = 20) and time to recover was similar. Total upstream movement, total movement, residence time in receiver array and speed of upstream movement were statistically similar for externally and internally tagged fish. Post-spawning, a large proportion (68%) of tagged fish returned to the estuary, downstream of the receiver array. Internal tagging under anaesthesia is recommended for studying anadromous movements of shad, given welfare benefits during surgery and once at liberty, thus increasing the likelihood of tagged fish performing natural behaviours. Further, implantation of tags programmed to last many years enables multiple spawning migrations by the same individuals to be studied, which would lead to substantial advances in ecological knowledge and potentially reduce the number of fish tagged. [ABSTRACT FROM AUTHOR]