Autumn – winter diet of three carnivores , European mink ( Mustela lutreola ) , Eurasian otter ( Lutra lutra ) and small – spotted genet ( Genetta genetta ) , in northern Spain

Autumn–winter diet of three carnivores, European mink (Mustela lutreola), Eurasian otter (Lutra lutra) and small–spotted genet (Genetta genetta), in northern Spain.— This study describes the autumn–winter diet of three carnivores (Mustela lutreola, Lutra lutra and Genetta genetta) in northern Spain. Diet composition was analysed from 85 European mink, 156 otter and 564 spotted genet fecal samples The European mink diet was based on small mammals (relative frequency of occurrences 38.1%), fish (30.9%) and birds (16.7%). Spotted genet consumed mainly small mammals, birds and fruits, whilst otter predated practically only fish (95%). Using Levins’ index, trophic–niche widths in European mink, small–spotted genet and Eurasian otter were 3.76, 3.77 and 1.10, respectively. The trophic niche overlap by Pianka index for autumn–winter was 0.77 for European mink vs. small–spotted genet, and 0.60 for European mink vs. otter. The average size of brown trout taken by otter was larger than those consumed by European mink.


Introduction
Eurasian otter (Lutra lutra) and European mink (Mustela lutreola), both members of the family Mustelidae, are semi-aquatic mammals.The Eurasian otter is the most widely distributed otter species in the world, and it can be found in Europe, North Africa and Asia.European mink, however, is one of the most critically endangered mammals in the world (Schreiber et al., 1989).Furthermore, its distribution in Europe has declined dramatically since the 19th century (Youngman, 1982;Braun, 1990;Camby, 1990;Tumanov, 1999) and its range has been greatly fragmented.There remain only a few isolated populations in Eastern Europe and one population in south-west France / northern Spain, over 2,300 km away (Palazón & Ruiz-Olmo, 1997;Palazón, 1998;Palazón et al., 2003).
The small-spotted genet (Genetta genetta), family Viverridae, is widely distributed in Africa and the Middle East, but it is also found in the Iberian peninsula and southern France.It generally inhabits forested areas, but in northern Spain it lives in riparian habitats, sharing habitat with Eurasian otter and European mink.In this range the small-spotted genet feeds not only on terrestrial prey but also on aquatic prey (Ruiz-Olmo & López-Martín, 1993), potentially competing with the native European mink, that consumes both terrestrial and aquatic prey, and the otter, that feeds mainly on aquatic prey (Ruiz-Olmo & Palazón, 1997;Clavero et al., 2003).
The aim of this study was to show the diet of three species that share the same riparian habitat and whose ranges overlap.Understanding the factors that may affect European mink populations is specially important due to their endangered status.
In this study, we investigated the diets of the American mink, the spotted genet, and the Eurasian otter in an area of northern Spain in order to determine their trophic niche breadth and overlap, and examine the potential for these species to compete.

Material and methods
The study area is located in Navarra and La Rioja in Northern Spain (latitude: 42 o 10' N-43 o 14' N; longitude: 1 o 30' O-3 o 03' O) (fig.1), and the samples were collected from Mediterranean and Atlantic rivers.These Mediterranean rivers have a low flow during the summer.They are bordered by dense riparian cover, but surrounded by extensive dry and irrigated agricultural lands.In the Atlantic basin, the water flow is more constant year around.In both areas, the riparian vegetation is composed of black alders (Alnus glutinosa), white (Populus alba) and black poplars (P.nigra), white willows (Salix alba), ashes (Fraxinus angustifolia) and elms (Ulmus minor).
To study the autumn-winter diet of the three study species we used 85 European mink and 564 spotted genet faecal samples collected between 1992 and 1996 in the Ega River (Navarra), and 156 otter scats collected in the Ebro, Najerilla and Iregua (La Rioja) Rivers between 1996 and 1997 (fig.1).All the samples came from within the range of this isolated population of European mink.Samples were collected at different sites and times.Genet samples were gathered from latrines typically used for scent communication.Otter and European mink samples were easily differentiated by smell.Besides, most European mink samples were collected inside traps or from the dens of radio-tracked minks.
To quantitatively describe the diet of three species we used the index: relative frequency of occurrence of each food item Number of prey-items of prey taxa i RF i = Total prey-items Fork length and weight of three consumed fish species (Barbus sp., Salmo trutta, and Chondrostoma miegii) were estimated using the functional relationship found by Ruiz-Olmo (1995).To compare the food-niche breadth of three mammals species, the B index was calculated for the nine established food categories, according to Levins (1968): ranging from 1 to 9 prey groups, where p i is the relative frequency of prey-category i in the diet, and n is the number of prey-categories.Trophic niche overlap between the three species (European mink, spotted genet and otter) was calculated by means of the Pianka (1976) index: where p i is the proportion of prey-item i in the diet of the predator p, and q i is the proportion of prey item i in the diet of predator q.It ranges from 0 (exclusive niches) to 1 (complete overlap).Student t-test was used to compare the weight and length of different species of fish consumed by European mink and otter.

Results
Autumn-winter diet overlap between European mink and spotted genet was 0.77, between mink and otter 0.60, and between spotted genet and Eurasian otter 0.09.The autumn-winter food-niche breadth index (B) was 3.76 for European mink, 3.77 for spotted genet, and 1.10 for Eurasian otter (table 1).The three main prey categories in the European mink diet were small mammals (38%), fish (31%) and birds (17%) (table 1).
All three together reached a relative frequency of 85.3%.Like European mink, small-spotted genet based their diet on three main items: small mammals (39%), birds (18%) and fruits (26%) (table 1).In contrast, Eurasian otter almost exclusively consumed fish (95%), a prey-item category which only represented a third of the prey-items consumed by European mink.
In the case of the otter, the remaining prey-items were of minor importance.Small-spotted genet (Genetta genetta)

Discussion
Our results have shown that the American mink and the small-spotted genet tend to have a similar diet, partitioned between aquatic and terrestrial prey, while the otter fed almost exclusively on aquatic prey.These findings are similar to those in Belarus, where the food-niche breadth was higher for the European mink than for the otter (Sidorovich, 2000).In Spain the diet overlap between Eurasian otter and European mink was similar (Sidorovich, 2000), with otter consuming more than 50% fish, and European mink consuming more than 50% frogs.The diet overlap between European mink and spotted genet was slightly higher.
The study of diet overlap should not be confused with competition in the diet.Otters have a higher rate of fish capture than mink.This is likely because they are better adapted to an aquatic life-style due to their larger surface of interdigital membranes, greater diving capacity, better under-water vision, higher percentage of milk fat, and the vibrissae on muzzle and elbows that help them detect fish in murky waters.(Dunstone, 1993;Conroy & Jenkins, 1986;Estes, 1989;Kemenes & Nechay, 1990;Brzezinski et al., 1993).There are no studies on the swimming behaviour of European mink, although it is likely similar to American mink (Dunstone, 1993).Due to their larger size, and their adaptations to aquatic life, otter can prey on fish of different species and sizes.European mink, however, have a limited capacity for swimming and can only prey on small fish.Otter and European mink share some fish.When they concur on the same river, could there be a competitive relationship between the two species?Could otter attack or predate on European mink?Since the 1990s Spanish otter populations are increasing and spreading (Ruiz-Olmo & Delibes, 1998).Could this lead to a decrease in European mink populations in a similar way to the American mink in England (Strachan & Jefferies, 1996;Crawford, 2003)?We consider it would be interesting to study the ecological relations between European mink, otter and small-spotted genet in the Mediterranean areas of sympatry in depth.Such studies should also be extended to the European polecat and American mink in Spain as in the near future, this invasive species could become a major competitor of European mink not only in Spain but also in other European countries (Maran et al., 1998;Sidorovich et al., 1998Sidorovich et al., , 2000Sidorovich et al., , 2001)).

Table 2
en el norte de España.