Movement ecology of two threatened anadromous species, twaite shad Alosa fallax and sea lamprey Petromyzon marinus, revealed by acoustic telemetry.

Abstract

Fragmentation of freshwater ecosystems due to man-made barriers (e.g. dams, weirs) poses a major threat to biodiversity, and has driven population declines and extirpations of once-abundant anadromous species. Understanding the movement ecology of anadromous species during all phases of their lifecycle is vital to understand, predict and mitigate the impacts of fragmentation. This is especially crucial for groups such as the Alosinae (shads) and Petromyzontiformes (lampreys), for which movements and space use during both freshwater spawning migrations and the marine life-phase is poorly understood, compared with anadromous species of the Salmonidae family. Here, the movement ecology of threatened twaite shad Alosa fallax (‘shad’) and sea lamprey Petromyzon marinus was investigated using acoustic telemetry during and between their spawning migrations in the heavily fragmented lower River Severn catchment in western Britain. Fish passage studies often consider the impacts of single barriers, yet many rivers actually contain multiple barriers that potentially impose cumulative effects on migrants. Here, sea lamprey were acoustic-tagged at the beginning of their upstream migration in the River Severn catchment, to determine how multiple weirs in sequence impacted their upstream spawning migrations, and to quantify individual variation in movement behaviours associated with weirs. Sea lamprey were impeded and severely delayed by multiple weirs in a cumulative manner, which limited their spawning to atypical habitat. The results also demonstrated the crucial roles of tides and elevated discharge events in enabling weir passage. Exploratory downstream movements made by sea lamprey during delays at weirs represented a large proportion of the total distance moved during their spawning migration. Exploratory downstream movements enabled some individuals to locate alternative spawning tributaries, but these movements were subject to substantial individual variation. Individual variation in passage time at weirs reduced the predictability of arrival timing between downstream and upstream sites, highlighting the stochastic influence of weirs on upstream migration. Iteroparous species that perform multiple spawning migrations throughout their lifecycle provide an opportunity to assess repeatability in barrier passage and space use during their spawning migration, as well as determine movements during their marine phase, using telemetry. However, few studies have applied multi-year telemetry to understand these aspects, particularly amongst alosine fishes. Here, shad were tagged with long-life acoustic transmitters during their spawning migration in the River Severn catchment, in order to 1) assess their movements and dispersal during their marine phase; 2) assess the impacts of man-made barriers on their upstream spawning migration and the environmental and individual factors affecting barrier passage and; 3) characterise and quantify their movement and space use during the spawning migration, test the individual factors affecting freshwater space-use, and assess spatial fidelity to previously occupied areas by individuals tracked over multiple spawning migrations. During their marine phase, shad from the River Severn were detected at spatially distant receiver arrays in coastal waters, providing novel insights into their marine dispersal. Individuals were detected at the Taw-Torridge Estuary, 200 km to the south-west of the Severn’s tidal limit, and one individual was detected in the Munster Blackwater Estuary (Ireland) before returning to the Severn, a minimum movement distance of 950 km. During their upstream spawning migration, weirs consistently acted as impediments to shad, and represented the upstream extent of migration for most fish. At a major navigation weir, higher river levels and temperatures significantly increased passage rates. Previous passage success was a significant predictor of increased passage rates in returning individuals, and passage rates were significantly higher in returning fish than newly tagged fish. Shad were highly vagile during their spawning migrations, and their movement patterns were characterised by multiple upstream and downstream journeys prior to emigration. Their median total movement distance while in the river was 214 km and the median total freshwater residency time was 33 days. Females occupied larger core areas than males, and among returning shad, there was significant fidelity to areas occupied in previous years. This research represents a substantial body of new knowledge relating to the movement ecology of two threatened anadromous fish species. The results highlight the impact of barriers in impeding the upstream distribution of these species within the catchment, as well as the importance of individual variation and spatial fidelity in shaping their movement patterns and distribution.