The European mink

The European mink (Mustela lutreola) is a semi-aquatic mustalid that is somewhat small in size: smaller than a marten and larger than a weasel.

Apart from its size, the European mink is best identified by its uniform chocolate brown colour all over its body except for two mall white marks, one on its upper lip and the other on its lower lip. 

The males are larger than the females, measuring from 50 - 55 cm in length and weighing some 800 grams, compared to the 38 - 46 cm and 400 grams for females.

The mink is an excellent swimmer, due to its webbed feet which allow it to move easily in the water. However, the mink performs best on land given the fact that its long skeleton, small ears and short tail allow it to move amongst the dense riverbank vegetation without hardly being noticed.

The European mink chooses to live in sections of small or medium sized rivers with little current, many small channels, islands, dykes and marshes permanently covered by water and with sloping banks. 

All these areas feature dense plant coverage of brambles, reeds and vegetation accumulated during flooding, used by the mink as a shelter and for breeding. This choice of habitat is very obvious in the case of the females and, particularly, during the breeding period from April to July.

The European mink is highly territorial, with considerable spatial requirements, despite its small size. Whilst the males occupy sections that are 10 kilometres in length (without invading territories inhabited by other males), the females tend to occupy smaller sections of around 4 kilometres. Therefore, several females come within a male's territory.

The European mink is generally carnivorous. Its diet is based on the most abundant and available prey at any given time, providing that it is to be found in the water or along the banks. It feeds on water rats, mice and moles, amphibians, fish and crabs in particular.

Minks are a crepuscular and nocturnal species. During the nightime and at twilight, they centre their activity on looking for food and changing their den, whilst their little daytime activity takes place within or close to these shelters.

The European mink mating season is from mid-March to the end of April, and, from mid-May to the end of June, the females restrict their movements to a small area around their den, where they give birth. 

During July, the females and their young (normally 1 -3 per litter) progressively extend their movements outside the small area around their den. The areas selected for giving birth and raising their young are located next to small streams, channels and islands, and wetlands located on the banks of the generally frequented water courses. In this way they appear to avoid any flooding and rises in river levels that generally take place in May after heavy storms. 

These areas offer a very dense plant coverage, comprising aquatic plants, large brambles and the accumulation of the remains of plants deposited by the floods, providing the minks with adequate shelter.

The main threats to the European mink are as follows:

1.   Invasion by the American mink: in Spain, this is the key threat.

The American mink is a larger, more aggressive species, which competes for food and space, and reproduces more successfully. This is leading to the disappearance of the European mink in those areas in which both species co-exist.

At first sight, both mink species are virtually identical, the only discernible difference being the muzzle: the European mink has a white upper and lower lip, whilst the American mink only has a white lower lip…

Although, for the time being, the presence of the American mink has not been detected in the rivers in Navarre, each year the Ministry of the Environment conducts captures in rivers in the neighbouring autonomous communities in order to control this invasive species.

2.    The destruction and degradation of its habitat is another factor contributing to the regression of the species. 

The flood plains have been occupied by agricultural crops and poplar plantations, invading the areas which should be reserved for the development of natural copses and wetlands, necessary to maintain a high level of biodiversity.

The Navarre Government has been working towards increasing the potential habitat of the minks, thereby reducing its risk of extinction. The projects implemented in this respect are focussed on the following actions:

• The elimination of structures preventing river movement and, therefore, the rejuvenation of the copses and the natural formation of habitats.
• The creation of favourable breeding areas.
• The restoration of river bank vegetation to facilitate mink movement and shelter.
• The publication of a technical document that sets out the guidelines to follow in order to minimise the negative impact of human activity on the species.

3. Death from being struck by a vehicle.

This is another cause of death, seen in the roads crossing the area coming within the European Natura Network, corresponding to the scope of this project.

During 2007, a total of eight black spots were improved and it was subsequently seen that all were used by the European mink and other carnivorous animals.

Currently the Government of Navarre is including fauna crossings that are adequate for this species, in new roads.

4. Drowning inside irrigation siphons. 

Irrigation siphons are used by minks and other carnivorous species for feeding. They jump inside for food, where they are then trapped and unable to get out, leading to death by drowning.

When upgrading irrigation systems, where minks are known to be present, siphons with ramps are constructed in order to allow the animals to get out.

5. Canine distemper virus.

Another great threat to the mink in Navarre is the disease produced by the canine distemper virus, which also affects other wild and domestic carnivorous species. This disease spreads rapidly in high density animal populations, such as the case of the population of European minks living in the lower reaches of the Arga and Aragón rivers.

Studies have been conducted since 2005, demonstrating a considerable drop in this population in addition to an increase in the number of mink specimens that are seropositive to the virus.

6. The great genetic resemblance between the minks at the Site.

Given the fact that this is a dense and isolated population, all specimens are genetically very similar (they could be said to be first cousins), which means that minks have fewer possibilities of adapting to environmental changes, lower defences in the face of diseases and, therefore, a greater risk of extinction.

If one family member falls ill, it is very likely that the rest catch the disease and all the family and neighbours die.

For this reason, there is an ongoing effort to monitor the health status of the population and to precisely determine the consequences and possible solutions to this low genetic diversity.