Martin R. Perrow

Effective conservation management of the Critically Endangered European eel (Anguilla anguilla) is hindered by incomplete understanding of distribution, abundance, and habitat requirements at the catchment scale. All habitats available to eels within a small, highly regulated river catchment, representative of many used across the species' range, were sampled using several methods (including point abundance sample electric fishing and fyke nets) and supplemented by individual telemetry to investigate movements. Eels were found throughout the catchment (59% of n = 131 sites) from the coastal marshes to the headwaters, although the probability of presence declined with distance from the estuary. The lack of a clear relationship with perceived barriers may illustrate a mismatch with the reality experienced by eels, as telemetry identified connectivity across obstacles between paludal habitat and estuary, and detected escapement of mature silver eels from both lotic and lacustrine habitat. Different size/age classes occurred in different parts of the catchment, partly linked to different habitat associations, with coastal paludal habitat supporting >50% of the catchment population and especially smaller (possibly male dominated) yellow eels. Recently recruited elvers were most abundant in the lower reaches of lotic habitat. The largest (probably female) eels were concentrated in lacustrine sites, especially at the ‘end‐of‐the‐line’ in the headwaters. Experiences here suggest that conservation management for eels in small catchments is best focused on: (i) improving connectivity and assisting migration of elvers across ‘problem’ barriers that cannot be removed or modified, (ii) river restoration and rewilding, especially measures that increase instream woody material to provide refuge habitat and, (iii) enhancement or creation (where necessary) of suitable lacustrine habitat to benefit large females in particular. Such action across numerous small river catchments may ultimately help support the recovery of eel stocks.

Contemporary surveys and analysis of previous data identified negative effects of fish stocking for recreational angling in 87% of 23 English lakes (0.2–26 ha and 1–11.9 m mean depth) protected for nature conservation as Sites of Special Scientific Interest (SSSI). All sites except one were in unfavourable condition, with 92% failing to meet targets for characteristic macrophyte species. No lake was judged to have a natural fish community, and 96% had been subjected to fish stocking. The naturalised alien common carp (Cyprinus carpio) was the most widely introduced (87%). The presence of further alien species indicated illegal activity. A combination of estimates from surveys and angler records suggested carp comprised 53% of overall fish biomass density, with values up to 1164 kg ha⁻¹ in individual lakes. Successful recruitment of carp, attributed to a warming climate, was recorded in 44% of the lakes in which it occurred. Consideration of fish impacts on lake ecology from the literature concluded that management of fish stocks, through biomanipulation, was required. The efficacy of biomanipulation was demonstrated by removal of fish from a large exclosure (29% of area) in one study lake and a resultant favourable response of water transparency and macrophyte growth. In the wider SSSI lakes series, we concluded that the retention of carp in any lake was an unacceptable risk to restoration of favourable condition, as were damaging angling practices. A ban on fish stocking in SSSI lakes is recommended, unless fish are introduced to restore natural communities.