Hong Kong is one of the busiest metropolises, and the Eurasian otter Lutra lutra is one of its most threatened species. We collected published data for 1890–2020 to document changes in local otter abundance and distribution over time. The 108 records revealed new distribution data and showed that decline began as early as the 1930s. The local Eurasian otter is strongly associated with coastal and alluvial wetlands, and its last refugium, the Yuen Long floodplain draining the Mai Po Inner Deep Bay Ramsar Site, has always been a critical habitat. Our analysis suggests that hunting is unlikely to be a major cause of the otter's decline. Rather, dependence on lowland wetlands makes it particularly susceptible to habitat loss caused by increasing urbanization. Auxiliary infrastructures and water pollution have also exacerbated wetland degradation to the detriment of otters and their prey. There is a need for greater cooperation amongst government agencies and landowners to guarantee otter survival: priority steps include the establishment of additional and interconnected wetland reserves, better stakeholder engagement and enforcement efficiency to tackle entrenched pollution problems, and flood prevention schemes that preserve or restore functional riverine ecosystems within critical otter habitats. The current otter distribution range has been earmarked for development in a government-led mega urbanization plan; the plight of the local Eurasian otter needs to be widely publicized to garner stakeholders' support and galvanize immediate conservation actions across society.

As part of a translocation project, 28 Eurasian otters (Lutra lutra) were captured from the wild and transported to the Barcelona Zoo for veterinary evaluation, quarantine and intraperitoneal implantation of telemetry devices. Eleven animals were injected with the long-acting neuroleptic (LAN) perphenazine enanthate at the time of capture and the remaining animals served as a control group. During their time in captivity, which averaged 23 days, all of the animals were bled three times. Haematological and biochemical parameters were evaluated, including red blood cell count (RBC), haemoglobin (Hb), white blood cell count (WBC), blood urea, aspartate aminotransferase (AST), alanine aminotransferase (ALT), alkaline phosphatase (AP), lactate dehydrogenase (LDH), creatine kinase (CK), albumin, and serum cortisol. No significant differences were found between treated and control otters except for monocyte count, which was higher in treated animals. Time after capture had an effect on many parameters. RBC and Hb decreased at first and then increased, while WBC and segmented neutrophils decreased over time. Most of the biochemical parameters considered to vary in relation to stress, including AST, ALT, CK, AP and LDH, decreased over time, suggesting that the stress responses of the animals decreased throughout the period of captivity. However, no significant change in serum cortisol levels was noted. The lack of effect of perphenazine treatment on haematological parameters should encourage further research on other stress indicators applicable to wild animals, such as behaviour or faecal cortisol concentration. Finally, the results obtained in this study suggest that, when captive conditions are adequate, keeping wild-caught animals in human care for a period of time prior to their release into the wild can be beneficial. However, further studies taking into account other welfare indicators would be useful.

On the River Ticino in northern Italy, a small number of captive Eurasian otters Lutra lutra, belonging to the European breeding programme for self-sustaining captive populations, were reintroduced in 1997, after the species had been declared locally extinct in the 1980s. We surveyed for otter signs in 2008, 2010, 2016–2017 and 2018, confirming the presence of what is probably a small population. To assess the abundance and viability of the population, we genotyped fresh spraints collected during the last two surveys, using 11 microsatellite markers, and modelled the population trend using Vortex. A minimum of six individuals were identified from 25 faecal samples. The analysis of mitochondrial DNA determined that the reintroduced otters share a transversion that is characteristic of the Asiatic subspecies Lutra lutra barang, confirming the contribution of the Asiatic subspecies to the genetic pool of the captive-bred founder population. Population size was consistent with the release of three pairs of otters and all models implied that the number of founders was too small to ensure the long-term survival of the population. Stochastic factors are therefore likely to threaten the success of this reintroduction.

The status of Asian populations of the Eurasian otter Lutra lutra is largely unknown. Since its designation as a Natural Monument (in 1983) and as Endangered (in 1997) in South Korea the authorities there have been trying to conserve and recover the species. We conducted a national otter survey by standard methods in 2017 and compared the current otter distribution to those recorded in a previous survey (2010). We found otter signs in 84.5% of 1,105 10 × 10 km grid cells, with the highest sprainting intensity in the south-west in the Yeongsan River Basin and on the south coast, where we recorded 7.05 and 6.26 spraints/site, respectively. Despite relatively low spraint densities, the otter has expanded its range since 2010 by colonizing urban areas. This trend suggests that South Korea could be a source area for the recovery of the Eurasian otter in East Asia.

Three species of otters are known from China; the Eurasian otter Lutra lutra is widespread throughout the country and the smooth-coated Lutrogale perspicillata and Asian small-clawed otters Aonyx cinereus occur in tropical and subtropical regions. We summarize the past status and distribution of otters in China, and provide an update based on a literature review, interviews and field surveys. Otter populations have undergone a dramatic countrywide decline, and are extirpated over much of their former ranges. Relict populations persist, however, in well-protected nature reserves, in sparsely populated headwaters of the Qinghai–Tibetan Plateau, at remote sites along international borders, and in densely populated deltas and floodplains. Recent records were mostly of the Eurasian otter, and we could find no confirmed recent record of the smooth-coated otter. The otters that survive in certain well-protected sites could act as source populations for recolonization if adequate conservation interventions are implemented. Urgent, focused action is needed to protect the remaining populations, and to study the taxonomy and ecology of China's otters.

In countries with emerging and developing economies the need to promote development and the lack of information on the status of the Near Threatened Eurasian otter Lutra lutra have given rise for concern about the conservation status of the species. In Albania information about the distribution of this otter dates from 1985. In 2013 we resurveyed 31 sites previously surveyed in 1985, and a further 42 sites throughout the country. At each site nine habitat variables of potential importance to otters were recorded and analysed. Overall the distribution pattern in 2013 did not differ from that recorded in 1985, although a reduction in marking intensity suggested a possible decline in otter numbers. Distribution of the otter has been influenced by land use and human density, suggesting man-induced habitat changes since the fall of communism may have affected the quality and fragmentation of otter habitats.

Although the Near Threatened Eurasian otter Lutra lutra has been recovering in Europe since the 1980s nothing is known about population trends of the species in northern Africa. Ninety sites was searched for signs of otters in northern and western Morocco in 1983 and we repeated this survey in 2011. At each site we searched for otter spraints (faeces) or clear footprints along a maximum of 600 m of river bank, ending the search when the first sign was found. Overall results were strikingly similar in 1983 and 2011, with 36 positive sites and the same general distribution pattern of the species. Healthy otter populations appear to remain in the foothills of the Middle and High Atlas but the trend of populations disappearing from the relatively flat Atlantic slope has increased. The results are in clear contrast with the recovery of otter populations in Europe, probably because of differences in the implementation of environmental policies, especially regarding water pollution.

To trace the local extinction of the Eurasian otter Lutra lutra in Latium, central Italy, and examine the causes of the species’ disappearance, we reviewed and classified information from both the scientific and grey literature according to the reliability and geographical accuracy of the records. The temporal and spatial patterns of 160 records from 23 geographical subunits from 1832 to 2006 suggest that the species collapsed between 1960 and 1975; two different extinction patterns were revealed by a set of multivariate analyses. In northern Latium the species collapsed because of several independent local threats. In central and southern Latium the species collapsed because of catastrophic habitat alteration (land reclamation during the 1930s) that negatively affected the source population. After this event the species went extinct in hilly and mountainous areas, where several population sinks occurred. We presume that this latter process drove the remnant otter subpopulations to extinction in central Italy, emphasizing the role of an extinction vortex in causing the collapse of this metapopulation rather than the classical threats recognized for this species. The value of the grey literature for a posteriori historical analysis of local extinction dynamics is highlighted by this research.

Gall bladders from 273 otter carcasses, collected throughout England and Wales, were screened to assess the status of gall bladder parasites in the Eurasian otter, Lutra lutra. The digenean Pseudamphistomum truncatum had previously been found in UK otters collected between 2000 and 2007. The parasite was established in Somerset and Dorset but its distribution elsewhere in the UK was largely unknown. In the current study, P. truncatum was also found to be abundant in south Wales, with occasional cases elsewhere, but appears to be absent from the north of England. Overall, 11·7% of otters were infected with 1–238 P. truncatum. A second digenean, Metorchis albidus, previously unreported in British otters, was found in the biliary system of 6·6% of otters. M. albidus appears well established in Suffolk, Norfolk and north Essex but was recorded elsewhere rarely. Both parasites are associated with pathological damage to the otter gall bladder. The recent discovery of these two non-native parasites provides a unique opportunity to assess their impact on native British fauna.

In this study the diet of otters Lutra lutra was examined following the establishment of a population of non-native ruffe Gymnocephalus cernuus in Loch Lomond, Scotland. Otter spraints (n=453) were collected at monthly intervals from four sites during 2002. Overall, 98% of spraints contained at least one species of fish, invertebrates were found in 49%, 17% contained amphibians, while mammals and birds were found in 9%. The most frequent prey from all sites were ruffe, which were found in 69% of spraints. Eel Anguilla anguilla, cyprinids (roach Rutilus rutilus, dace Leuciscus leuciscus and minnow Phoxinus phoxinus) and Salmo spp. were also relatively common and were recovered in 60%, 35% and 33% of spraints, respectively. Northern pike Esox lucius (14%), perch Perca fluviatilis (12%), stickleback Gasterosteus aculeatus (9%), powan Coregonus lavaretus (14%) and lampreys Lampetra spp. (2%) were less common. The fact that ruffe were the most frequent prey in otter spraints in all regions and in all seasons of the year indicated that they are one of the most important prey for otters in Loch Lomond at the present time.

Direct observation of Eurasian otters was used to demonstrate the characteristics and interannual use of small patches of habitats by females and litters in stretches of two rivers in north-east Spain. During censuses and other vigils, 215 otter sightings (involving 265 otters) were obtained, 14–18% of which were observations on females and litters. Thirty-five additional sightings were obtained in stretches used by litters of large cubs (47 individuals). Six small cub (2- to 6-month-old) and 15 large cub (>7-month-old) different family units (litters) were detected in both rivers as a whole. Average sizes of litters of small and large cubs were significantly similar, being respectively 1.17 (SD=0.41, range 1–2, n=6) and 1.27 (SD=0.46, range 1–2, n=15), so mortality of cubs was low at the period before dispersal. Litters were detected in significantly different patches from other otters, and litters of small and large cubs were also found in significantly different river sections. Litters of large cubs used small home ranges and core areas, and selected sections at reservoir tails, reservoirs, wider stretches, rough waters and refuges. Small cubs were found in narrow river stretches with calm waters. Selected patches had sheltered zones with abundant fish. This reduced use of space may result in the low rate of mortality.

Oil spills can have direct effects on organisms (mortality or morbidity), indirect effects (through alteration of lower trophic levels) or be associated to exposure to trace elements contained in oil (markers). An effect of the “Erika” oil spill was investigated by spatio-temporally comparing mortality, population structures, diets and concentrations of vanadium, nickel and porphyrines in small delphinids, seals and otters from the French Atlantic coasts. These species might differ in their vulnerability to oil. Changes in mortality and its demographic structures were within previously observed ranges. The diet of the common dolphin showed an extended period of high inter-individual variability in the year 2000. Vanadium concentrations in delphinids were chronically high, but did not increase significantly after the oil spill. Porphyrins concentrations in seals and otters were low suggesting a limited exposure to contaminants, but the ratio between proto- and copro-porphyrins in otter spraints from oiled vs. unoiled sites varied significantly. No measurable effect of the “Erika” oil spill was found in dolphins and seals.

In this study the predation and fish prey selection of otters Lutra lutra L. living by eutrophic fish ponds on agricultural land and in a protected area of temperate climate in Hungary were investigated. The correlations between fish in the diet of the otters (by spraint analysis involved 1942 and 1280 samples for the two habitats) and the fish stock available (by harvest and sample fishing) were generally close. Prey selection was significantly related to selected species of a particular size range (P<0.01). The preference calculations were performed with Ivlev's index of preference (Ei, minimum −1, maximum +1). Regardless of species, the otters avoided (Ei=−0.51) fish heavier than 1000 g, with a preference for individuals weighing between 500 and 1000 g (Ei=0.79). No substantial or clear preference was observed in the weight range below 500 g (Ei=−0.02–0.38). The preference for fish in accordance with their characteristic sites of occurrence within the body of water was also significant (P<0.01). They avoided fish living primarily in open water (Ei=−0.64) and to a lesser degree those occurring near the pond bed (Ei=−0.22). They favoured fish inhabiting the area with a covering of aquatic plants (Ei=0.46), and showed a preference to a lesser degree for fish living in the shallow littoral regions (Ei=0.14). With the cessation of fish farming and the effect of the drastic changes which occurred in the vegetation, the otters fed substantially on the stock of alternative sources of prey such as amphibians and water insects as well as terrestrial animals, and at such times, depending on season, fish became a secondary source of prey.

Recent studies show that the availability of prey influences the ecology of otters by regulating breeding, carrying capacity and mortality. Mediterranean habitats experience extreme seasonal variation in water flow – a stress period can occur in summer when water flows and levels are low with long periods of drought, while torrential floods usually occur in autumn and spring. Reservoirs can affect this situation by further influencing water flow regimes and acting as a barrier. Our data show the close relationship between the amount of food (fish) available to otters and their use of a stretch of water and the otter density there. The adjustment of the number of individuals (and carrying capacity) in an area can be seen from their mortality, a decrease in their reproductive success, or migration. Our results are consistent with the hypothesis that otters are food-limited. Torrential rains, summer droughts and dams for water management have repercussions for otter populations in as much as they affect their prey-species. Barbels Barbus spp. and, secondarily, nases Chondrostoma spp. are shown to be important to otter ecology and breeding in Mediterranean Iberian rivers. The conservation and management of the otter in these habitats must not ignore the management of food sources. In order not to affect otter populations, the effect of the reservoirs and their dams also must be considered. The release and cessation of floodwater from reservoirs must be progressive and mimic more closely the flow regime of natural rivers.

At the end of the 1970s otter surveys were developed to study the distribution of the Eurasian otter Lutra lutra. The method was quickly adopted and expanded to include other species of otter. Different methods to estimate otter density were also used. There is some uncertainty, however, as to the significance and precision of the results. Otter surveys, radio-tracking studies and otter censuses (tracks and visual) were conducted in the same areas simultaneously. Comparison of the home ranges of otters tagged with transmitters with the results from otter surveys carried out in the same areas showed very similar otter distribution. Some 71% of those sites examined in which only one otter lived were positive, and 97% of those sites with two or more otters. Differences were found between different watercourses. In small or average sites with waterways, one single otter was capable of marking many kilometres so that it could be detected in most of the conventional 600 m sites (here 71% of sites). The data obtained by radio-tracking showed that almost all otters can be seen by experienced observers, confirming the validity of visual censuses. A high correlation was found between the number of otters seen during visual censuses and the number of otters detected by means of the length of their footprints. Using new and old tracks (2-days-old or more) the number of otters was overestimated.

The extent to which anti-predator mobbing can be modified in response to new predators was investigated by examining the response of terns (Sterna hirundo and S. paradisaea) and gulls (Larus fuscus, L. argentatus and L. canus) to introduced mink Mustela vison, native otter Lutra lutra and rabbit Oryctolagus cuniculus models in areas inhabited by mink and areas uninhabited by mink. Gulls and terns typically flock over predators of adult birds (a fleeing response), whilst diving at predators of chicks and eggs (an attacking response). Terns in mink-inhabited areas flocked over a model of a mink, but did not dive at it as much as they dived at otter and rabbit models. A stronger fleeing rather than attacking response suggests that adult terns perceive mink as a threat to themselves as well as to their chicks and eggs. Gulls, in comparison, dived at the mink model, indicating that mink are not perceived as a threat to the adults of these considerably larger species. Both gulls and terns showed a much lower level of response to mink in mink-free areas, suggesting that the type and extent of mobbing is altered through experience with predators and the level of risk associated with mobbing the predator.

Otter spraints from five Danish freshwater localities were analysed. In all localities fish was the main prey (76–99% of estimated bulk), especially in winter. Depending on locality, the prey fish mainly consisted of cyprinids (Cyprinidae), percids (Percidae) or salmonids (Salmonidae). Seasonal variations were found in the diet corresponding to the availability of prey. Non-fish prey categories, such as frogs, birds, mammals and invertebrates, were most frequently taken in spring and summer, but only frogs made an important contribution to the diet (0–21%). The results of analysing 978 otter spraints were compared with prey fish populations as estimated by electrofishing. It was concluded that the fish species composition in the otter diet generally reflected that of the foraging area, however, with the exception of a negative preference for trout (Salmo sp.) and a preference for sticklebacks (Gasterosteidae) in some of the study areas. Otters generally took all available sizes of fish with a tendency for selecting individuals between 9–21 cm, though the preferred size of eels was 18–36 cm. A principal component analysis showed that the variation in diet between the localities was associated with differences in fish communities between free-flowing rivers and lake outflows.

The abundance of mink, otter, and polecat may be inferred from records of their scats, and the feeding biology of the species is often studied by analysis of skeletal remains and other hard parts in faeces. However, in some situations it is difficult to distinguish between faeces from these three mustelid species. A method is described for assigning faeces to these three mustelid species, based on analysis of DNA extracted from their scats. Mustelid-specific primers were developed for PCR amplification of a part of the mitochondrial cytochrome b gene, and two restriction enzymes were found to detect species-specific sequence variation. Analysis of DNA from different faecal samples showed that the results were reproducible and that the approach provided an efficient method of species identification.

Cysts of a Sarcocystis species were found in large numbers in skeletal muscle of an otter (Lutra lutra) which was raised in Norway and died in captivity in Sweden. This is the first report of Sarcocystis infection in the otter. The sarcocysts were 0·3–2·3 mm long and 0·06–0·25 mm wide. As judged by light microscopy the sarcocyst walls were thin (<3 μm) with a serrated surface but without visible projections. By transmission electron microscopy, the sarcocyst wall measured 0·6–1·8 μm and had minute undulations covering the entire sarcocyst surface giving the wall a wavy appearance. Septa were indistinct. The sarcocysts contained few metrocytes and numerous bradyzoites. Sarcocysts were not found in 69 other otters subjected to necropsy in Sweden.

This study is an investigation into the influence of activity on otter (Lutra lutra) spraint production and composition, based on feeding trials involving captive, tame otters. Trials were of two types: ‘active’ where animals were encouraged to run, swim and dive for at least 33% of the time during the first six hours after each meal, and ‘inactive’ where activity varied between zero and 18% of this period. Trial types were balanced for number of fish, meal volume and fish size. Activity was found to have a large effect on spraint production and composition. For active trials, median minimum gut transit time was 67 minutes and median number of spraints per trial was 23. Corresponding figures for inactive trials were 170 minutes and 14, respectively. Of ingested salmonid (Salmo spp.) atlas vertebrae, 60% were recovered in spraints after active trials, while only 32% were recovered after inactive ones. Analyses of these effects showed that our previous models for estimating prey size distribution from bones recovered in spraints gave a better fit for inactive than for active trials. As otters in the wild are likely to spraint mostly during periods of activity, especially foraging, further feeding trials were undertaken to refine an earlier model better to mimic the field situation. Trials, involving active otters, were undertaken to: (i) produce in vivo bone to fish length equations for salmonid atlases and eel (Anguilla anguilla) thoracic vertebrae; and (ii) model the size-related differential recovery of these bones. These refined equations are more soundly based to provide estimates of prey size distribution than the earlier ones and their use in future studies of size-selection is recommended. The present study also highlights the potential limitations of extrapolating from the captive situation to the wild.