Spatial ecology of white-clawed crayfish Austropotamobius pallipes and signal crayfish Pacifastacus leniusculus in upland rivers, Northern England

The American signal crayfish Pacifastacus lernusculus, an invasive species widely introduced throughout Europe, is a major threat to native European crayfish species and is causing increasing concern because of its wide impact on aquatic ecosystems. This thesis investigates the within catchment expansion of signal crayfish populations in two upland rivers and the spatial ecology and movement of the introduced signal crayfish and the indigenous white-clawed crayfish Austropotamobius pallipes. Populations of signal crayfish are established and expanding on the upland rivers Wharfe and Ure. On the Wharfe the signal crayfish population is well established and now occupies about 30 km of river and is currently expanding at a rate in excess of 2 km year(^-1). On the Ure the signal crayfish population is younger and currently occupies 1.6 km and is currently expanding at about 0.5 km year(^-1). The range expansion is biased towards downstream in both rivers, by a ratio of about 3:1 (downstream:upstream).The movements and dispersal of white-clawed and signal crayfish was studied utilising a combination of radiotelemetry and internal and external Passive Integrated Transponder (PIT) tags. Radiotagged adult signal crayfish were capable of substantial active movements (maximum movement 790m in 79 days). The level of movement of adults suggests they may have the potential to be responsible for the observed rates of population expansion. Although the movements of radiotagged adult signal crayfish within main river channel were equally distributed upstream and downstream, in-stream barriers both natural and artificial were found to limit the upstream movements of PIT tagged crayfish and this may contribute to the observed downstream bias of signal crayfish population expansion. The movements and dispersal of PIT tagged white-clawed crayfish within a small upland high gradient stream were strongly biased towards downstream. Maximum movement of radiotagged adult signal crayfish occurred during midsummer. Temperature appeared to be a major factor influencing the timing and extent of movements between tracking periods although there was a large variation between individuals. All significant downstream movements made by crayfish were active movements and not the result of passive movement during periods of high discharge. There were no sex or size differences in the dispersal and movement of radiotagged and PIT tagged signal crayfish whilst in PIT tagged white-clawed crayfish size, sex, injuries and duration of tracking influenced extent of movement. The expansion of the signal crayfish population in the River Wharfe appears to lead to the progressive loss of white-clawed crayfish populations where they come into direct contact. Limited differences in the microhabitat utilised by the two species were found where the species were syntopic, suggesting the potential exists for direct competition between the two species. In addition signal crayfish showed greater movement and dispersal than white-clawed crayfish. This may contribute to the ability of signal crayfish to colonise rivers rapidly and may also offer a competitive advantage over white-clawed crayfish thus contributing to the observed replacement. The results are discussed in the context of the conservation and management of crayfish and the ecology of invasive species

Catchment-wide interactive effects of anthropogenic structures and river levels on fish spawning migrations

Worldwide, rivers are extensively fragmented by anthropogenic structures, reducing longitudinal connectivity, inhibiting migration and leading to severe declines in many fish populations, especially for diadromous species. However, few studies have determined the effects of annual differences in hydrology on catchment penetration past barriers to spawning habitats. We investigated the upstream spawning migration of 120 (n = 61 & 59) acoustic tagged river lamprey (Lampetra fluviatilis) across two contrasting (dry and wet) years in the River Yorkshire Ouse, England. Overall, significantly more lamprey reached spawning habitat (76% vs 39%) and penetrated significantly further upstream (median [km] from release, 53.9 vs 16.8) in the wet year than the dry year. Passage at weirs was almost exclusively during elevated river levels, which directly and collectively influenced catchment-wide distribution, especially in the dry year. Indeed, higher proportions entered two upper tributaries in the wet year (9.8% vs 27.1% and 9.8% vs 30.5%), due to increased passage efficiencies at the two main river weirs (60.5–87.5% and 54.5–83.8%), and reached assumed spawning locations 66.5% and 10.9% quicker. By contrast, there was no difference in numbers of lamprey entering, or time taken to arrive at assumed spawning location, in the two lower river tributaries between years. Our study supports the landscape-scale paradigm for ecosystem restoration because of the observed catchment-level effects of hydrology and barrier distribution on fish migration. Connectivity restoration for migratory fish should be implemented at a catchment scale, with planning incorporating spatial information regarding accessibility to key habitats to reap the largest gains

Short-term effects of low-head barrier removals on fish communities and habitats

Barrier removal is increasingly being seen as the optimal solution to restore lotic habitat and fish communities, however, evidence of its efficacy is often limited to single sites or catchments. This study used a before–after methodology to examine the short-term (average, 541 days) effects of low-head (0.1–2.9 m) barrier removal at 22 sites distributed across Denmark and northern England on fish density, community, and river habitat responses. Following barrier removal, changes in the aquatic habitat were observed, such that the area immediately upstream of the former barrier location became shallower, with larger substrate and faster flow conditions. The reinstatement of this habitat was especially valuable in Danish streams, where these habitat features are rare, due to the naturally low gradients. Across all 22 sites fish species richness and diversity was similar before and after removal of barriers, likely because of the short study timescale (1–2 years). Across all sites combined, there was an increase in total fish density following barrier removal. A large increase in salmonid (Salmo trutta and Salmo salar) densities following barrier removal occurred at 7 out of 12 Danish sites. No similar response in salmonid density was observed at any of the UK sites which were mostly characterized by high channel gradients and short ponded zones. Two UK barrier removal sites showed marked increases in density of non-salmonid fish species. This study suggests that the removal of low-head barriers can be an effective method of restoring lotic habitats, and can lead to positive changes in fish density in the former ponded zone. The short-term effect of small barrier removal on the fish community is more variable and its effectiveness is likely to be determined by wider riverine processes