The otter is found in freshwater habitats from coast to upland, and is capable of long overland journeys between watersheds. It also exploits marine environments, particularly rocky coasts where there is high food supply, but it is dependent on the availability of fresh water for cleaning salt from its fur (Kruuk et al., 1989). Adult females are highly territorial and defend large home ranges that are overlapped by one or more males. The size of the home range varies from 4km to 50km in length, and depends on the availability of prey and denning resources, as well as on the spatial configuration of aquatic habitats. It is challenging to estimate population densities accurately because the otter is difficult to observe directly, its holts are difficult to find, and spraint abundance (faecal markings) has complex relationships with the numbers of individuals, varying according to sex, season and other factors (Kruuk and Conroy, 1987; Chanin, 2003). Although some new insights are being brought by genetic analysis of non-invasive samples, otter faecal DNA amplifies very poorly compared with many other species (Dallas et al., 2003; O'Neill et al., 2013).
Dependence on water, and aquatic prey, makes the otter vulnerable to river management and to agricultural pollution. Persistent organic pollutants are likely to have caused the historic declines in otter populations. The species has recolonised most of its former range in Great Britain following the banning of these compounds (Chanin, 2003; Kean et al., 2013).
IUCN Red List (GB: LC; England: [LC]; Scotland: [VU]; Wales: [VU]; Global: NT.). • National Conservation Status (Article 17 overall assessment 2013. UK: Favourable; England: Favourable; Scotland: Favourable; Wales: Favourable).
The length of total riparian habitat within the geographical range of the otter in each country was derived by multiplying the data on riparian lengths given in Table 4 of Harris et al. (1995) by the proportion of each country included in the species’ distribution. The length of potentially suitable coastline was derived from the report by Jefferies et al. 10.3 for England and Wales; Table 10.6 for Scotland). These values excluded areas unlikely to be included within the home ranges of otters (e.g., long lengths of sheer cliffs), whereas all riparian habitat was included. Population size was adjusted using the most recent occupancy values for each country. For Scotland, the mean population density values for coastlines in mainland Scotland, the Inner Hebrides, Shetland and Orkney were taken from Table 10.6 of Jefferies et al. (2003). No population density estimates or occupancy values were available for coastlines in England and Wales, so the values for inland populations were applied. This method will provide a conservative estimate of the number of coastal otters in England and Wales, but was judged preferable to applying Scottish coastal values, which are likely to be much higher than those found in England and Wales.
Twelve papers were identified by the literature search. Of these, one reported pre-breeding population density (Jefferies et al., 2003; originally surveyed by Green and Green (1987)), three contained occupancy values (Crawford, 2010; Findlay et al., 2015; Strachan, 2015b), and the remainder reported small-scale surveys (no density estimates) and distribution surveys.
The most recent occupancy values for each country were obtained from surveys in 20092010 for England (Crawford, 2010), 2009-2010 for Wales (Strachan, 2015b) and 2011-2012 for Scotland (Findlay et al., 2015).
Population size for each country is based on a single country-specific population density estimate for riparian habitats (and coastlines in England and Wales), and four population density estimates for coastlines in Scotland. These density estimates are applied to all occupied riparian habitats and coastlines, so no account is taken of habitat heterogeneity. This is a particular problem for coastlines in England and Wales, as the application of riparian density estimates to coastal areas is highly likely to be inaccurate.
Percentage occupancy for Scotland was taken from Findlay et al. (2015). Field conditions during the survey were poor, with high rainfall, which may have increased the chance of obtaining false negatives. Percentage occupancy may, therefore, be higher than estimated in Scotland.
The total pre-breeding population of 7,350 individuals was estimated by Harris et al. (1995) for the mid-1980s, comprised of 350 in England, 6600 in Scotland (3,600 on the mainland and 3,000 on the islands), and 400 in Wales. The method of calculating total population size was based on calculations from D. J. Jefferies, which were later published in Jefferies et al. (2003), and were used as the basis for the 2013 Article 17 Report (Joint Nature Conservation Committee, 2013a).
The current review employs the same density estimates as Harris et al. (1995). The 49% increase in population size is therefore the consequence changes in occupancy and geographical range compared with Arnold (1993). The geographical range (surface area) is very similar to the values given in the Article 17 Report (Joint Nature Conservation Committee, 2013a).
A series of national surveys have been conducted to detect the rate of change in the otter’s area of occurrence. These surveys were not, however, designed to provide information on population trends. There has been an increase in the number of occupied 10km squares in all three countries, with an increase from 5.8% in 1977-1979 to 58.8% in 2009-2010 in England (Crawford, 2010); from 38% in 1977-1989 to 72% in 2002-2003 in Wales (Strachan, 2015b); and from 57% in 1977-1979 to 80% in 2003-2004 in Scotland (Findlay et al., 2015).
Futrue trends.---Population and range Increasing, Habitat stable.
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