Anguillid eels.

Anguillid eels have fascinated humans for centuries, but our knowledge of these mysterious fish is still scant. There are 19 species or subspecies in the genus Anguilla, which are found globally, except in the eastern Pacific and southern Atlantic. Their common label 'freshwater eels' is a misnomer - all anguillids are facultatively catadromous, born in marine environments, developing in continental waters, with a proportion never entering freshwater at all. Anguillid eels have several life history traits that have allowed them to exploit a broad range of habitats. As such, anguillid eels play an important ecological role in both marine and freshwater environments as well as being commercially valuable. Because of this, anguillid eels are under threat from multiple stressors, such as barriers to migration, pollution, parasites, disease, climate change and unsustainable exploitation. Six species are listed as Threatened in the Red List of Threatened Species, and four are listed as Data Deficient. Strengthening conservation and management of these species is essential, and further research provides an exciting opportunity to develop a greater understanding of this mysterious clade of fish.

First direct evidence of adult European eels migrating to their breeding place in the Sargasso Sea.

The European eel (Anguilla anguilla) is critically endangered (according to the most recent IUCN assessment) and has suffered a 95% decline in recruitment since the 1980s, attributed in part to factors occurring during the marine phases of its life-cycle. As an adult, the European eel undertakes the longest spawning migration of all anguillid eels, a distance of 5000 to 10,000 km across the Atlantic Ocean to the Sargasso Sea. However, despite the passage of almost 100 years since Johannes Schmidt proposed the Sargasso Sea as the breeding place of European eels on the basis of larval surveys, no eggs or spawning adults have ever been sampled there to confirm this. Fundamental questions therefore remain about the oceanic migration of adult eels, including navigation mechanisms, the routes taken, timings of arrival, swimming speed and spawning locations. We attached satellite tags to 26 eels from rivers in the Azores archipelago and tracked them for periods between 40 and 366 days at speeds between 3 and 12 km day-1, and provide the first direct evidence of adult European eels reaching their presumed breeding place in the Sargasso Sea.

The distribution, ecology and predicted habitat use of the Critically Endangered angelshark (Squatina squatina) in coastal waters of Wales and the central Irish Sea.

The angelshark (Squatina squatina) has the northernmost range of any angel shark species, but there is limited information on its distribution, habitat use and ecology at higher latitudes. To address this, Angel Shark Project: Wales gathered 2231 S. squatina records and 142 anecdotal resources from fishers, coastal communities and archives. These spanned the coastal waters of Wales and the central Irish Sea and were dated from 1812 to 2020, with 97.62% of records within 11.1 km (6 nm) of the coast. Commercial, recreational and charter boat fishers provided the majority of S. squatina records (97.18%), with significantly more sightings from three decades (1970s, 1980s and 1990s) and in the months of September, June, August and July (in descending order). The coastal area between Bardsey Island and Strumble Head had the most S. squatina records (n = 1279), with notable concentrations also found in Carmarthen Bay, Conwy Bay and the Outer Severn Estuary. Species distribution models (SDM) identified four environmental variables that had significant influence on S. squatina distribution, depth, chlorophyll-a concentration, sea surface temperature (SST) and salinity, and these varied between the quarters (Q) of the year. SDM model outputs predicted a larger congruous area of suitable habitat in Q3 (3176 km2 ) compared to Q2 (2051 km2 ), with suitability along the three glacial moraines (Sarn Badrig, Sarn-y-Bwch and Sarn Cynfelyn) strongly presented. Comparison of modelled environmental variables at the location of S. squatina records for each Q identified reductions in depth and salinity, and increases in chlorophyll-a and SST when comparing Q2 or Q3 with Q1 or Q4. This shift may suggest S. squatina are making seasonal movements to shallow coastal waters in Q2 and Q3. This is supported by 23 anecdotal resources and may be driven by reproductive behaviour, as there were 85 records of S. squatina individuals ≤60 cm in the dataset, inferred as recently born or juvenile life-history stages. The results have helped fill significant evidence gaps identified in the Wales Angelshark Action Plan and immediate next research steps are suggested.

A global agenda for advancing freshwater biodiversity research.

Global freshwater biodiversity is declining dramatically, and meeting the challenges of this crisis requires bold goals and the mobilisation of substantial resources. While the reasons are varied, investments in both research and conservation of freshwater biodiversity lag far behind those in the terrestrial and marine realms. Inspired by a global consultation, we identify 15 pressing priority needs, grouped into five research areas, in an effort to support informed stewardship of freshwater biodiversity. The proposed agenda aims to advance freshwater biodiversity research globally as a critical step in improving coordinated actions towards its sustainable management and conservation.

Anguillid eels as a surrogate species for conservation of freshwater biodiversity in Japan.

To monitor and manage biodiversity, surrogate species (i.e., indicator, umbrella and flagship species) have been proposed where conservation resources are focused on a limited number of focal organisms. Using data obtained from 78 sites across six rivers in the mainland Japan and the Amami-Oshima Island, we demonstrate that two anguillids - the Japanese eel (Anguilla japonica) and the giant mottled eel (A. marmorata) - can act as surrogate species for conservation of freshwater biodiversity. Anguillid eels were the widest topographically-distributed species ranging from near the mouth to the upper reaches of rivers. Moreover, stable isotopic analyses indicated that eels are likely one of the highest-order predators in freshwater ecosystems. A significant positive relationship was found between the density of eels and the number of other diadromous species collected. However, the optimal models revealed that both the density of eels and the number of other diadromous species were significantly negatively correlated with distance from the river mouth and cumulative height of trans-river structures from the river mouth to each site. This suggests the positive relationship between eel density and number of other diadromous species was indirect and related to river-ocean connectivity. Given their catadromous life-cycle, and global commercial and cultural importance, as a taxa, anguillid eels can act as indicator, umbrella and flagship species, and a comprehensive surrogate for conservation of freshwater biodiversity.

Evidence for dynamic resource partitioning between two sympatric reef shark species within the British Indian Ocean Territory.

Stable-isotope analyses (δ13 C, δ15 N and δ34 S) of multiple tissues (fin, muscle, red blood cells and plasma), revealed ontogenetic shifts in resource use by grey reef sharks Carcharhinus amblyrhynchos and resource partitioning with silvertip sharks Carcharhinus albimarginatus within the British Indian Ocean Territory marine protected area (MPA). Resource partitioning varied temporally, with C. albimarginatus feeding on more pelagic prey during October to January, potentially attributable to an influx of pelagic prey from outside the MPA at that time. Reef sharks may therefore be affected by processes outside an MPA, even if the sharks do not leave the MPA.

Potential benefits to fisheries and biodiversity of the Chagos Archipelago/British Indian Ocean Territory as a no-take marine reserve.

On 1st April 2010, the British Government announced designation of the British Indian Ocean Territory--or Chagos Archipelago--as the world's largest marine protected area (MPA). This near pristine ocean ecosystem now represents 16% of the worlds fully protected coral reef, 60% of the world's no-take protected areas and an uncontaminated reference site for ecological studies. In addition these gains for biodiversity conservation, the Chagos/BIOT MPA also offers subsidiary opportunities to act as a fisheries management tool for the western Indian Ocean, considering its size and location. While the benefits of MPAs for coral-reef dwelling species are established, there is uncertainty about their effects on pelagic migratory species. This paper reviews the increasing body of evidence to demonstrate that positive, measurable reserve effects exist for pelagic populations and that migratory species can benefit from no-take marine reserves.

Physiological responses to acute temperature increase in European eels Anguilla anguilla infected with Anguillicola crassus.

The swimbladder parasite, Anguillicola crassus has infected, and spread rapidly, through European eel Anguilla anguilla (L.) populations over the past 20 to 25 yr. Our aim in the present studies was to elucidate whether the presence of A. crassus in these eels alters their rapid physiological responses to an acute temperature increase, compared to the response of uninfected fish. Both infected and uninfected fish showed significant increases in plasma cortisol after 2 h at a raised environmental temperature with increased plasma glucose after 6 h. However, infected eels exhibited a slight lag in glucose mobilisation, which may be due to the metabolic cost of harbouring a sanguiverous parasite. Both infected and uninfected fish showed a significant increase in haematocrit after 6 h of temperature elevation, but only uninfected fish exhibited a significant increase in haemoglobin at this point. However, there were no significant changes in mean erythrocyte haemoglobin concentration in either group. Our results suggest that acute temperature increase alone is unlikely to cause significant mortality of A. crassus-infected European eels; however, the effects of chronic increases in temperature in combination with other factors such as toxicants and hypoxia requires examination.