Sulkava, Risto
Ecology of the otter (Lutra lutra) in central Finland with methods for estimating the densities of populations
Abstract
Keywords: activity, breeding, carrying capacity, competition, density, density-dependence, diet, distribution, food, habitat quality, home range, juveniles, litter, monitoring, population, S-shaped growth curve, snow-tracking, source-sink.
The otter (Lutra lutra) is a semiaquatic Mustelid with wide distribution. However, its populations decreased in many areas in the 20th century. Otters are well adapted to life in the water. Their body is elongated, the tail flattened and the fur is thick and waterproof. The main food is fish all over the otter’s range. However, amphibians have also been important food for otters in Finland. Finding suitable feeding areas is problematic in the North, due to the thick ice layer in winter, and otters have to move over large home ranges searching for food and possible new feeding areas.
In this long-term study, the ecology of otters was studied extensively in central Finland. The diet of otters was studied by analysing spraints (faeces). The activity of otters was investigated by following sprainting activity in summer and by snow-tracking in winter. Snow-tracking was also used to study the home ranges, breeding and density of otters. To estimate population densities two new snow-tracking methods were developed. The home range mapping method (HMM) is very intensive, but gives an exact estimate of the number of otters. The one-visit census (OVC) is a sampling method that provides a faster way to estimate the population densities.
The one-visit census method was used in a large project for monitoring the Finnish otter population in 1995-1998. In this study the possibility of monitoring Finnish otter populations by snow-tracking was evaluated. The results were found to be promising and the method applicable for nation-wide studies. The number of otters in Finland was approximately 2000-2550 individuals in the above-mentioned years.
In the study area (1650 km2) in central Finland, the otter population increased from about 20 individuals in 1985 to 50 individuals in 2002. It was possible to estimate the density of otter populations in different river systems by exact annual counting of all otter individuals by HMM. Finding an S-shaped growth curve made it possible to determine the local carrying capacity for the otter population in the study area. The density of otters depends on used method. It was 0.7 individuals per 10 km of riverbed in summertime (2002), but increased up to 5.2 individuals per 10 km if only available winter feeding areas was used. The length of the home range for female otters or litters was typically 20-40 km of watercourse. Otters used all the available feeding areas in winter, but in summer there was abundant free space and food available.
The density-dependent offspring production for the population of otters was documented for the first time in this study. Density-dependent reproduction indicates the existence of intraspecific competition in otter populations in central Finland. Food was scarce and competition took place only in the winter, when most of the feeding sites were covered by ice. The ice cover limits the availability of food resources enormously, and creates a "bottleneck" for the otter populations in central Finland.
In the study area most of the otter cubs produced in a few river systems, resulting in a source-sink structure between the network of habitats or local populations. The increase of local populations was equal in all river systems. Highly productive source populations rendered population growth possible in other river systems. In central Finland the mean number of cubs per female was 1.51 in autumn. All known litters were born between May and September.
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