Mink trapping and radiotracking

One trapping session was conducted each year with a total of 588 trap-nights per each session in 2003 and 2004. The sessions were set each year between October and December in both rivers. Animals were live-trapped in single cage traps (15 x 15 x 60 cm) located on both river sides with a separation of 300–400m between them and checked daily. After immobilization with 0.15 ml of ketamine (Imalgéne, Rhone Merieux, Lyon, France) and 0.03 ml of medetomidine (Domtor, Pfizer SA, Madrid, Spain), manipulated animals were released in the capture area once fully recovered. Trapping was made during the post breeding season (September to February) when there are no juvenile mink (all animals older than 5 months). Animals were sexed and aged (subadults 5–8 months old; and adults > 8 months old) based on a combination of teeth condition and weight (Maran & Robinson 1996).

The captured animals were marked with a transponder (Trovan Ltd., Madrid, Spain) in order to distinguish between new captures and recaptures. Some of them were fitted with waterproof radiotransmitters collar necks (frequency 150 and 151 MHz, Bio Track, Ltd., Wareham, Dorset, UK and Tinyloc Ltd., Mataró, Spain). Radiocollars weighted approximately 15 g, < 3% of the lightest adult mink captured during the study (510 g). All procedures were approved by the Scientific Ethical Committee of the Department of Environment and Housing of the Catalonian Government (Spain). A receiver TR4 (Telonics Ltd., Mesa, Arizona, USA) was used connected to a multidirectional or a bidirectional antenna. Radiolocations were recorded by homing to the animals (U.T.M. positions, using a Garmin GPS, Romsey, Hampshire, UK) without triangulation as mink movements follow the river course (Gerell 1970, Birks & Linn 1982, Ireland 1990, Bonesi 1996). For six-ten hours daily shifts animals were radiotracked every hour unless it took longer to find them. Radiotracking shifts changed every three-four days. Additionally, male 2, male 3, male 6 and female 4 were 24h period-radiotracked. Hour time was set as Universal Time Coordinates (UTC).

Transmitters used had a sensor of activity. This sensor allows detecting differences in signal pulse rate and differences in signal strength. Differences in pulse rate inform if animals have an active or an inactive behaviour. Active behaviour can imply spatial displacement (locomotion) or local movement without locomotion. Differences in signal strength detect active behaviour implying locomotion (Kenward 1987). Thus, animals were classified as either active or inactive depending on changes in the signal pulse rate. Simultaneously, active behaviour was set as local movement or locomotion. Overall, classification was as follows: local activity, locomotion activity and inactivity. A total of 903 radiolocations were gathered between October and February (Table 1).

Activity analysis

We distinguished between local activity, locomotion activity and inactivity. Local activity was defined as active behaviour without displacement, locomotion activity as active locomotion behaviour while foraging or travelling (active behaviour with displacement), and inactivity as the lack of active behaviour. Diurnal time was set between dawn and dusk for each day and nocturnal between dusk and dawn.

Activity versus inactivity behaviour

Total activity (all local and locomotion activity radiolocations pooled together, n = 376) versus inactivity (inactivity radiolocations, n = 527) was studied, analysing the influence of sex, age, daylight (diurnal and nocturnal), month, river flow and average rainfall per day using GLMM fitted to a binary distribution. Factors were set as fixed effects. Individual and river (Llobregat or Gavarresa) were set as random effects. By considering individual as random effect, the problem of different number of repeated measures per individual was avoided.

Table 1.

Data of captured individuals during the trapping sessions 2003 and 2004. Age refers to subadult (SA) or adult (A).

Locomotion activity versus local activity

Finally, we also studied locomotion activity (n = 187) versus local activity (n = 189). The influence on locomotion activity of sex, age, daylight, month, river flow and average rainfall was analysed using GLMM under the same conditions as before.

Fig. 1.

Daily local activity and daily locomotion activity of American mink. Values are given as percentage of activity locations per hour.

River flow (m³/s) was measured as daily mean flow for both rivers Llobregat and Gavarresa in the study area. Rainfall (mm) was measured as daily mean rainfall in the study area.

Activity analysis

Individuals tracked spent almost half of their time active with a total of 27.6% of local activity and 13.9% of locomotion activity (Fig. 1). During the daylight period minks spent 29.4% of their time with local activity and 13.3% with locomotion activity while during the night they spent 15.4% with local activity and 20.3% with locomotion activity (Fig. 1 and 2).

Activity versus inactivity behaviour

There was no significant effect of any factor on total activity (Table 2 and Fig. 1).

Locomotion activity versus local activity

The GLMM for locomotion versus local activity enabled us to detect a positive effect of sex and age (Table 2 and Fig. 3a and 3b) and a negative effect of average rainfall (Table 2). Daylight was not a significant factor for locomotion activity, although mink presented slightly more locomotion activity during the night (Table 2 and Fig. 3c and 1).

Activity

Based on the results, American mink in the study area, and during the study period (October–February), used little time in foraging and travelling, and spent most of the time inside or close to their dens active or inactive what is in concordance with Dunstone & Birks (1983) in the United Kingdom.

Table 2.

Results of the GLMM for total activity (active vs inactive) and locomotion activity (locomotion vs local activity). Individual and river were introduced as random effects in the models. d.f.: degrees of freedom. Significant factors are in cursive.

Daily rhythm did not show any general pattern as it has happened with other mink populations (Zuberogoitia et al. 2006, Zschille et al. 2010) in contrast with other studies that found an activity pattern (Harringtond & Macdonald 2008, García et al. 2009). Irregular patterns with a similar percentage of active nocturnal and diurnal radiolocations, as we found, have also been observed in other studies (Gerell 1969, Birks & Linn 1982, Niemimaa 1995, Marcelli et al. 2003).

Fig. 3.

Probability of (a) Locomotion and local activity according to sex, (b) Locomotion and local activity according to age, and (c) Locomotion and local activity according to daylight. Values are given as mean and their standard error calculated from the GLMM all else being equal.

Activity of carnivores is known to coincide with that of their most common prey (Laundré & Keller 1981, Zielinski et al. 1983, Ferguson et al. 1988) and the same pattern is typical for American mink (Gerell 1969, Bonesi et al. 2000). Scats collected in the study area during autumn and winter showed that crayfish, fish and small mammals are the main food sources for both sexes (Melero et al. 2008). Fish and crayfish can be hunted either in daylight or at night. Only small mammals are predominantly, although not exclusively, active at night or twilight (Halle & Stenseth 2000, Brown 2009). Thus, American mink in the studied area can hunt at both diurnal and nocturnal periods since it seems there was no constrain on prey availability at day or night. This can explain the low difference between diurnal and nocturnal activity found in our study.

Other possible causes for activity patters are the avoidance of intra-specific competition and potential predators (Gerell 1969, Lodé 1995, Lodé 1999, Zuberogoitia et al. 2006, García et al. 2009, Zschille et al. 2010), environmental factors, physiological constraints and energy requirements (Marcelli et al. 2003). However, none of the studied factors were significant for the total activity analysis.

Activity versus inactivity behaviour and locomotion activity versus local activity

The biological factors sex and age affected only the locomotion activity of the studied mink. Once again this correlates with most mustelids locomotion activity pattern as males tend to be more active than females and subadults use to be also more active than adults (Ireland 1990, Dunstone 1993, Palazón & Ruiz-Olmo 1997, Garin et al. 2002a, b, Revilla & Palomares 2002, Zschille et al. 2010). This result may imply that there is sexual segregation in feeding strategies between mink, since feeding segregation might involve differences in activity behaviour (Birks & Linn 1982, Dunstone & Birks 1983, Zschille et al. 2010).

River flow did not affect either total activity or locomotion activity, as it probably does not affect foraging or displacement. Mink are good swimmers and changes in flow are not big enough to imply a decrease in hunting efficiency. However, average rainfall per day affected locomotion activity. Although this factor was not important for other populations, the Mediterranean population was negatively affected by rain since Mediterranean areas have irregular rain patterns with infiequent but strong autumn storms that may disturb individual behaviour. Zschille et al. (2008) and Fournier et al. (2001) did not recommend the use of radiocollars from the point of view of the animal welfare. Recapture minks presented a good physical condition but some of them presented neck injuries caused by collars. This is a common problem with radiotracking animals such as mink or polecat with radiocollars that can influence behaviour of the animals. Therefore, we want to express our support for the research on alternative methodologies.