Impact of released Eurasian otters on a population of American mink: a test using an experimental approach

Summary

American mink Mustela vison were introduced to the UK at the beginning of the 20th century, and are now considered an invasive species. They predate heavily on native animals and are strongly implicated in dramatic declines in water vole Arvicola terrestris populations, a species now considered endangered over much of the country. During re-introduction of Eurasian otters Lutra lutra into the Upper Thames catchment (Southern England) in 1999, an investigation was undertaken to assess if there was evidence of competition between otters and American mink, and to see if mink densities declined in response to otter presence.

The otter releases took place in an area previously populated by mink alone, thus making this experiment possible. Fluctuations in the mink population in this area were compared with those of mink in a nearby area used as a control, where no otters were present.

Study site: The study took place in two riverine areas (covering 1,353 km²) near the city of Oxford (southern England). Characterized by slow-flowing lowland rivers and surrounded by agricultural land, these areas provide suitable habitat for both American mink Mustela vison and otter Lutra lutra. Prior to otter re-introduction, no otters were present at either site, but both had populations of mink. During the study, otters were released into one area (as selected by the Otter Trust, the organization that captive-bred and released the otters), the second area acted as a control.

The study areas were close enough to minimize any spatial and temporal variation, but far enough apart to prevent colonization of the control site by released otters within the study period - no signs of otters were found in the control area during the study.

Using information on otter and mink distribution provided by 'The National Otter Survey' dating back to 1977, enabled the results of the study to be framed over a longer timescale.

Monitoring: Control and release areas were surveyed before otter releases, in March 1999, and then every three months for one year after release; July 1999, October 1999, January 2000 and March 2000. These months were chosen to correspond with distinct stages in the yearly cycle of mink. March and October represent periods of high activity in the mink population, with breeding occurring in March. Juveniles born in April/May disperse in October. The mink population is relatively stable in July and January, with no major movements occurring.

In each area, 44 sites, 500 m in length, were surveyed along stretches of river. Sites were 3 km apart in order to prevent overlap in mink home ranges (home range size estimated to be 2.8 km) and therefore recording the same individual twice. Otter and mink presence was established using methods developed by The National Otter Survey, i.e. through dropping and footprint sightings.

Supplementary data, collected by the Environment Agency in 2002, which corresponded to the release area, was used to compare the percentage of sites occupied by mink with the results from the March 2000 survey.

In order to estimate mink population size in the otter release area, data on mink densities from a population adjacent to the study area were used, and the number of mink estimated by multiplying mink density estimates by the length of river that they occupied at different times. The exact number of otters was known, from surveys using molecular marking and records of otters found dead by roads, which could be identified from tags.

Live-trapping: In order to supplement sighting data, mink and otters were trapped in the release area using live-traps which did not harm the animals. Traps were left open for one night, with an average of 259 trapping nights per month, and carried out in March, April, May, June, September, October and December 1999 and March 2000. Traps were located at eight locations in the main otter activity area, with between five and 18 traps at each site. Both released otters and any captured mink were fitted with individual identification microchips.

In total, 17 otters (11 female, 6 male), between 1.5 and 2 years old, were released at three sites, which were aproximately 7 km apart. Releases took place between April and October 1999. Range expansion quickly occurred, with otters occupying most of the release area by March 2000. During the study, three of the released otters died, and a further two died after the study's completion (March 2000).

Mink densities: In the year following otter releases, mink declined rapidly, the number of sites occupied by mink falling from 77% to 23%. In contrast, mink in the control area slightly increased, from 50% to 55%, but overall, remained stable.

Data collected by the Environment Agency, two years after the study was concluded, revealed that the number of sites occupied by mink, in the otter release area, had almost doubled from 23% in March 2000 to 41% in March 2002. However, mink range had still greatly decreased from the original 77% of sites occupied prior to otter release.

Mink population size: Estimates of mink numbers, based on the length of river occupied and a population density of 0.3 mink per km, resulted in an estimate of around 60 mink in the otter release area in March 1999, with 203 km of rivers occupied. This estimate fell in March 2000, with only 87 km of rivers occupied, and an estimated population size of 26 mink. At the same time otters increased from a population of 0 to 17, and occupied around 125 km of rivers, with a density of 0.1 otter per km. Thus, around 30 mink were lost in one year, which is beyond the levels expected through normal population fluctuations.

Trapping results: Results from the mink trapping efforts in the otter release area confirmed the survey data. When taking into account trapping effort, nine mink in 1,000 trap nights were caught between March and May 1999, and four mink in 1,000 trap nights between June 1999 and March 2000.

Mink distribution: Prior to otter release, mink were widely distributed throughout the study area. Four months after the release of nine otters in July 1999, mink had almost disappeared from river stretches used by the newly established otters. Mink then re-gained some of their lost range by October 1999. In January 2000, mink range was becoming increasingly fragmented, and they were not present in the core otter area, by which time 17 otters had been successfully released. By March 2000, the mink population was still fragmented but they had re-colonized some of the areas lost in January 2000. These data suggest that otter presence had not only caused a decrease in mink population size, but had also restricted their range. Two years after the study (March 2002), mink co-occurred with otters in only 24% of sites.

Implications for water voles: During the mink dispersal season (August to November), mink were able to temporarily re-colonize areas which they had been previously excluded from due to the presence of otters. Water vole predation, by mink, may occur during this time period. In respect to the effects this may have on water vole populations at this time of year, implications for their long-term survival are not known. There may be a possibility that water voles could co-exist with mink when their levels are suppressed by the presence of otters, but this needs further investigation.

Conclusions: This study provides evidence that otters can have a dramatic impact on American mink population density and distribution. The initial rapid decline in mink density after otter re-introduction may, in part, be due to the relatively high density of otters (0.1 otters/km) compared to other UK populations (0.06 otter/km). However, when otter range had increased and the population stabilized, the number of mink-occupied sites was still much lower than those prior to otter releases.