Risto Sulkava Ecology of the otter (Lutra lutra) in central Finland and methods for estimating the densities of populations. - University of Joensuu, 2006, 128 pp. University of Joensuu, PhD Dissertations in Biology, No. 43, ISSN 1457-2486. ISBN 952-458-882-X

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.

Risto Sulkava, Department of Biology, University of Joensuu. Present address: 42800 Haapamäki, Finland.

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CONTENTS

LIST OF ORIGINAL PUBLICATIONS 6 1 INTRODUCTION 7 2 MATERIALS AND METHODS 9 2.1 Home ranges and activity 9 2.2 Snow-tracking methods and the estimates of otter population 11 2.2.1 Home range mapping method, segment method and one-visit census 11 2.2.2 Monitoring of otters 13 2.3 Breeding 14 2.4 Diet of otters 15 3 RESULTS AND DISCUSSION 15 3.1 Movements of otters within their home range 15 3.1.1 Home range size 18 3.2 Methods and the size of population 20 3.2.1 Snow-tracking versus other methods 20 3.2.2 Monitoring project 22 3.3 Density of otters 23 3.4 Reproduction and the carrying capacity of the study area 24 3.4.1 Reproduction 24 3.4.2 Density-dependence and carrying capacity 26 3.5. Source-sink dynamics and spatial organization of otters 27 3.6 Mortality 29 3.7 Diet 31 CONCLUDING REMARKS 32 ACKNOWLEDGEMENTS REFERENCES

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LIST OF ORIGINAL PUBLICATIONS

This thesis is based on the following articles, which are referred to in the text by their Roman numerals:

I Sulkava, R.T. 2006: Snow tracking - A reliable method for estimating otter (Lutra lutra) populations (accepted to Wildlife Biology).

II Sulkava, R.T. & Liukko, U-M. 2006: Use of snow-tracking methods to estimate the abundance of otter (Lutra lutra) in Finland with evaluation of one-visit census in monitoring purposes (manuscript).

III Sulkava, R.T., Sulkava, P.O. & Sulkava, P.E. 2006: Source and sink dynamics of density dependent otter (Lutra lutra) populations in rivers of Central Finland (manuscript).

IV Sulkava, R. 1996: Diet of otters (Lutra lutra) in Central Finland. Acta Theriologica 41: 395-408.

I designed all the studies (I-IV). Although I had many field and other assistants involved in different parts of the study, I was also responsible for the data collection and analyses. I wrote all the articles, I and IV myself, and articles II and III in collaboration with the co-authors.

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1 INTRODUCTION biology and distribution of otters was urgent. With very limited The distribution range of the knowledge of the otter, it was European otter (Lutra lutra) has difficult or even impossible to been extensive; from Portugal to plan the conservation and Japan and from North Africa to management of the species. arctic areas, including Finnish Only little is known world- Lapland. The populations wide about the numbers or declined in many areas between densities of otters. Techniques for the 1950s and the 1980s (e.g. estimating the size of populations Macdonald & Mason 1994). of otters are difficult to use and Distribution became patchy, time consuming. A basic field mostly in isolated areas on the survey method for estimating the periphery of Europe (Foster- presence of otters is based on Turley et al. 1990). However, in counts and observations of the 1990s and the 2000s, the spraints (faeces) and footprints on populations have been increasing, riverbanks. Studies based on this at least in some parts of Europe so-called “standard method” have (e.g. Sjöåsen et al. 1997, Kranz been carried out in many areas 2000, Roos et al. 2001, Conroy & (e.g. Jenkins & Burrows 1980, Chanin 2002, Reuther 2002). The Conroy & French 1987, Mason & otter was a threatened (V; Macdonald 1991, Sulkava & vulnerable) species in Europe Storrank 1993, Brzezinski et al. until 2004, when it was re- 1996, Trindade & Farinha 2002, evaluated as “near threatened” Chanin 2003). This field survey is (NT) (IUCN 2004). recommended as the best method At the beginning of this for investigating otter distribution work in 1980, the otter was rare in large areas (Macdonald & and listed as a threatened species Mason 1994). It gives knowledge in Finland (Rassi et al. 1985). about the distribution and relative Erlinge (e.g. 1967a, 1967b, 1968a status of otters (Mason & and 1968b) studied otters in Macdonald 1987, IUCN 2000), southern Sweden, but only a few but only little information about local studies were carried out in population size, density and the Finland in the 1980s (Skaren & vitality of populations, or the Kumpulainen 1986, Skaren & spatial organization of otters. Jäderholm 1987, Skaren 1988, Although all the currently Cronström 1989, Storrank 1989, used methods are difficult to use Sulkava & Sulkava 1989). In the and time consuming, the 1980s very little was known about monitoring of otter populations is the distribution, population very important for the purposes of dynamics or biology of otters in conservation and management Finland or other climatically (e.g. Foster-Turley et al. 1990, similar areas. The need to Rassi et al. 1992, Anon. 1994, improve our knowledge of the Stjernberg & Väisänen 1998, 8

IUCN 2000, Chanin 2003). For beginning of the 2000s, most the countries belonging to the Finnish otter study projects had European Union, it is a legal duty come to an end. However, much to collect data on the distribution material from the studies carried and population trends of out in the 1990s was still threatened species such as the unpublished, and some new otter (Habitat directive; articles were published (Ludwig 92/43/EEC,1992). The otter is et al. 2002, Storrank et al. 2002, listed in appendices II and IV in Hyvärinen et al. 2003, Hellsten the list of Interested Species of 2004, Sulkava 2006a and 2006b, Community. However, even Sulkava & Liukko 2006, I II, III). today, the monitoring of otters One of the most pleasing functions well only in England results of my studies was the (Crawford 2003). finding that the otter population Our knowledge and was rapidly increasing from the understanding of the otter’s 1980s to the 1990s (I, III). distribution, population dynamics Results of my study were among and biology in Finland is now those that contributed to the otter much better than it was at the being located in the new category, beginning of this study period. In “of least concern”, i.e. the species the 1990s, many otter studies is no longer threatened in Finland were carried out in Finland (Rassi et al. 2001). (Skaren 1990, 1992a and 1992b, The aim of this study was Skaren & Jäderholm 1990, to develop methods for Stjernberg & Hagner-Wahlsten investigation and monitoring of 1991 and 1994, Storrank 1993, otter populations, and to Sulkava 1993, Sulkava & investigate the distribution and Storrank 1993, Mäkelä & Rajala population density of otters in 1995), and the Finnish wildlife central Finland (Fig. 1), and to triangle scheme, which included study the biological factors the otter, began to produce a influencing otters’ reproduction snow-track index each year (e.g. and survival in the Finnish Helle et al. 1992, Lindén et al. environment, where there is long- 1996). At the end of the 1990s lasting ice cover on lakes and otter studies was perhaps at most rivers in winter. The main aims of active in Finland (e.g. Sulkava the study were: 1995, Sulkava & Storrank 1995, 1. To create a reliable Höglund 1996, Kauhala 1996, method for studying the density of Lindgren & Tornberg 1996, Luhta otter populations in northern 1996, Wikman 1996, Liukko Europe climate conditions, 1997, Cronström & Liukko 1999, 2. To find out whether there Helle & Wikman 1999, Liukko is trend in population density, and 1999, Rydbäck & Stjernberg 3. To find the essential 1999, Skaren 1999, Sulkava & factors like food and feeding Liukko 1999), (IV). At the 9

habitats that affects the survival of 2 MATERIALS AND otters. METHODS The diet was predicted to be one of the essential factors in 2.1 Home ranges and activity survival. Because only limited information was available on this The spraint marking activity of factor in the 1980s, I studied the otters was studied over a period of diet first. At the same time, I three years (1989-91), from some started to develop new snow- 30 permanent study sites. All the tracking methods. Studying the study sites located near bridges or movement of otters in different other places easily reached by car. seasons and conditions, their The sites were in all kind of home ranges and their scent waters, from small streamlets to marking activity had to be figured large rivers and from oligotrophic out when the goal was to develop to mesotrophic waters. Each study a reliable method of estimating site was carefully examined at the the density of otter populations. end of every snow-free month Once the new reliable (from May to October). Otter snow-tracking method had been spraints were collected from all established, more detailed new sites, and therefore it was possible questions on the population to know exactly whether any dynamics of otters were raised. faeces had been deposited at a These were: particular site within one month. 4. What kind of The most important sprainting demographic process is going on sites were under spruce trees or in the local otter population? bridges. Since a large tree or 5. How high is the carrying bridge covered the place, the capacity for otters in my study spraints were not dissolved by area? rain. The differences between 6. Is there intraspecific monthly spraint marking activity competition and is population size were measured by the monthly limited in a density-dependent number of positive sprainting processes? sites. Spraints were also used in In addition, based on the summertime as an indicator of method developed in this study, moving of the otters in different these factors were applied to waters. estimate the size of the total The home ranges (Fig. 1) population of otters in Finland. were studied using snow-tracking (I, II). I studied the density of otters in every winter between 1985 and 2003, and followed all individuals or litters during each winter (I, III, Sulkava 2006b). I also studied the length of the daily cruising distance of otters inside 10

their home range in winter (Fig. 2) common, and also the number of (I). These extensive data on the otters in a single system is usually movements of otters gave a clear low. In such conditions, it is picture of their typical home possible to identify individuals by range. In 1990-1991 I also measuring size, direction of trail followed one otter family (a and age of tracks. This was easily female with one cub) every day accomplished, at least when the over a two-month period. There otters were followed every day, was only one litter in this and nearly all movements of particular river system, one male individuals could be seen in the and two other lone individuals in snow. that year. In central Finland lake and river systems of this kind are

Figure 1. The otter home range river systems of the study area. The number of area (1-16) is used later in the text and figures. 11

a) 2.2 Snow-tracking methods and Litter (n=79) 3.08 the estimates of otter population Single (n=84) 4.59 In Finland, the long winter with Male (n=31) 5.46 permanent snow cover and ice on Mean (n=194) 4.12 lakes, creates an excellent 0246 opportunity to study animal movements. Moving on snow is Daily movements (km) natural for otters when they are searching for new feeding areas. Between November and April b) only few rapids and some outlets of big lakes remain unfrozen. Snow >20cm 4.28 Between these ice-free areas, all Temp. > -1 C 5.09 lakes and ponds are totally Temp. < -20 C 3.64 covered by ice, and ice covers All (n=194) 4.12 most of the flowing rivers, too. It is therefore possible in these 0246conditions to see nearly all Daily movements (km) movements of an animal by their tracks in the snow.

2.2.1 Home range mapping Figure 2. The daily movements of method, segment method and otters (a) and the effects of some one-visit census weather conditions on the movement of otters (b) in central In the home range mapping Finland in winter (mean length method (later called HMM), the +SE). Single = female or young estimation of the total number of lone individual. The female with otters living in an area is started cubs moved less than other otters by identifying all separate river (ANOVA: F = 12.5, p < 0.001, and lake systems (I). All paired T-test, df = 1; litter-male: p waterfalls, rapids and other places = 0.0009 and litter-single: p < that are not covered by ice were 0.0001). Otters also moved more studied carefully, during suitable in mild than in very cold weather ice and snow conditions, as well (T-test, T = 2.1, p = 0.04). Only as all waters with potential tunnels loose snow, in which an otter below the ice. Different otter sinks, has been taken into account individuals were identified on the in the thickness of snow cover. N basis of footprint (or trail) size, = the number of days. Total age, direction of the trails and length of routes was 798.4 km. areas without tracks between trails Animals without known sex or (I, II, III). After the first survey age are included in the mean there were often signs of length of movements. 12

individuals that had not been estimated using snowfall, hoar counted in the first time, and frost and non-frozen scats. Only therefore open rapids and other fresh (≤ 24 h.) tracks were used in possible areas were studied again estimating the population. two or three days later, and again Individuals were distinguished some days later. After these two from one another by the age, size checks, it was highly likely that and direction of the tracks. The all otters along one river system population estimate is the mean had been found (I). It was possible number of animals or the mean to estimate the otter population of number of positive segments, a larger area when the census determined from observed fresh advanced systematically from one signs in the sampled segments, river system to another (I, II). The extrapolated across all segments same or nearly the same method in the study area (I). has been used in Sweden (Erlinge In the late 1980s I also 1967a and 1968a, Aronson 1995), started to develop a faster and Poland (Sidorowich et al. 1996), easier way to estimate the Belarus (Sidorovich 1997) and the population of otters. In this one- Czech Republic (Simek & visit census method (later OVC), Springer 1998). the entire study area can be In order to test the investigated in two or three days. reliability of a population estimate It is impossible to find all otters in carried out by HMM, the otter the area in such a short time, but if population was also estimated in the same proportion of the total 1998-99 by another method (I), study area was checked every previously used by Reid et al. time, the size of the total (1987). In this segment method population possibly can be (later SM), all shorelines of rivers estimated. and streamlets were divided into In the OVC method, 111 numbered, 500 metre-long permanent study sites were segments. I did not include the selected all over the study area (I, shorelines of lakes, because these II). All sites were combined into are totally covered by ice in three groups, and each group, i.e. winter, and lakes do not offer 30-40 study sites, was feeding areas for otters in my investigated in one day. The study area in winter (I, III, IV). A distance between the study sites total of 1522 segments were varied between one and five numbered. A random sample of kilometres in each watercourse. numbered segments, 205 At each study site 20-600 metres segments, was selected for the of river or lake banks were survey (I). Each segment was searched for otter tracks. The searched for the tracks of otters, length of the search depended on which were estimated to be either the ice cover and other physical less than or more than 24 hours characteristics of the watercourse old. The ages of the tracks were at the site. For example, a longer 13

distance had to be examined at a tested in the early 1990s in central site that was situated in ice-free and eastern Finland (e.g. Sulkava rapids than at an inlet that was 1993 and 1995, Sulkava & Liukko totally covered by ice. 1999, Sulkava & Storrank Because field investigations unpublished, I, II). I acted as the were carried out within two to head of this project. The Finnish four days after the last snowfall, it monitoring pilot project was was possible to estimate the age designed to test whether such a of the tracks exactly. Individuals field method carried out during could be distinguished from each the winter is feasible, to assess other by identifying the empty how well it can be applied by areas between individuals, or by various organisations, and to see the age, size and direction of how useful it is in the monitoring tracks (I, II). For instance, if of Finland’s otter populations tracks were found in two streams (Sulkava 1995, Sulkava & Liukko separated by a lake without tracks, 1999, II). The survey was also the traces were most probably left expected to provide new data by two individuals. Because the about the status of the otter in distance travelled per 24 h. by an Finland. individual otter (excluding males) The Finnish monitoring did not usually exceed 6 km (Fig. system covers 16 study areas, 2a) (I), tracks were classified as comprising 37 000 km2 of Finnish being left by two different river and lake landscapes (about individuals if two positive sites 10% of the area of the country) were in different water systems, (Fig. 1 in II). About 100 or if the distance between sites permanent survey sites were with fresh tracks was more than established in each of the study 10 km. areas. During the three study Because at least one OVC winters, the total numbers of sites investigation had been carried out examined were 1466, 1589 and every winter after 1990 (I), and 1213 (II). All the study areas and the total population was known by sites were selected by uniform HMM, it was possible to compare criteria beforehand by myself how different conditions affect the (Sulkava & Liukko 1999, II). proportion of otters found in one Field investigations were carried sampling. out between November and April, and always two to four days after 2.2.2 Monitoring of otters a snowfall (II). All the observations were compiled in a In 1994, the environmental database hosted by the Finnish authorities in Finland decided to Environment Institute. Field carry out a pilot study of otter workers made the first estimates monitoring (II). The study was of the number of otters, and I carried out using the OVC made second critical examination, method, which was developed and and the final estimates of the 14

number of otters in all the areas known litters are born between (II). Maps of the survey areas with May and September in the study the positive and negative sites, area (Sulkava 2006b), and the and the field data with the female and cubs stay together information on the criteria used by over the following winter (III). the field workers in separating The situation is similar in other different otter individuals, were areas in the same latitudes, as in all of great importance in this Sweden (Erlinge 1967a) and in second examination (Sulkava & Shetland (Kruuk 1995). However, Liukko 1999, Storrank et al. 2002, identifying of a litter is also II). possible later in winter, if it is When the OVC method was possible to see a group or to tested, about 50% of otters were follow it for a couple of days (e.g. found in every survey conducted Sulkava 1993). In my study area in central Finland (Sulkava 1995, the number of litters and cubs was Sulkava & Liukko 1999, I, II). studied by snow-tracking in Based on this probability of autumn and early winter (I, III). finding otters, the total population It was not possible to know size could be estimated. When the the actual date of birth in this total number of otters in Finland study. I calculated the date of was evaluated, the abundance of birth using the first snow tracks otters outside the study areas was measured in snow in autumn or estimated with help of data from a early winter (Sulkava 2006b). I Finnish wildlife triangle scheme had one reference collection of (e.g. Lindén et al. 1996, Helle et footprint size made by a young al. 1998). The wildlife triangle captive female otter at different scheme gives the relative ages and size. I also compared this abundance of otters throughout data with larger data for weight the country. It thus enabled us to and length of otter cubs in evaluate that how large area captivity (Reuther 1999), and the around of our OVC study areas size of feet measured from dead the density of otters was individuals (Hellsten 2004, approximately the same as inside Sulkava, unpublished). However, of our study areas (I). it is uncertain how reliable this method was, as otters in the wild 2.3 Breeding may not develop as rapidly as in captivity, and measurements made The size of snow tracks of a on dead animals are not exactly female (wide of a footprint in the same as footprints in nature. I most often 55-65 mm) and cubs therefore did not try to find out (40-54 mm) are different at the exact date of birth. beginning of winter, between Thanks to the exact data I October and December (Sulkava had on the number of otters living 2006a and 2006b) in central in one lake and river system, and Finland. This is because all the number of cubs in these well 15

defined living areas, it was item, expressed as a percentage of possible to study differences in the total number of occurrences of cub production between river all items in the samples (sum is systems. Because the population 100). Bulk percentage is the growth curve was S-shaped, i.e. percentage where the importance first the density increased and of a particular item in a spraint then become asymptotic, I could has been scored visually. The find out the local carrying value for each item in each spraint capacity, and analyse possible is summed and expressed as the density-dependence in the bulk percentage (see, Wise et reproduction of the otter al.1981, Mason & Macdonald population. 1986). The diet of otters in different seasons and in different 2.4 Diet of otters areas was compared.

A total of 1506 spraints (scats) 3 RESULTS AND was collected in different parts of DISCUSSION the study area from 1988 to 1993 (IV). The spraints were collected 3.1 Movements of otters within every month throughout these their home range years. The year was divided into four seasons: spring (April and In summer the spraint marking May); summer (June to August); intensity of otters increased autumn (September and October); towards autumn in all kinds of and winter (November to March). waters in central Finland The spraints were dried and the (Kruskal-Wallis test, χ = 53.9, df remains were identified according = 5, p < 0.0001) (Fig. 3). Most to the keys of Eloranta (1975), possibly the increasing sprainting Webb (1976), Steimmetz & activity towards autumn was due Muller (1988), and using the to the greater need of individuals reference collections of skeletons to scent mark (i.e. reserve) their and scales of fish and other taxa. home range for winter, a period of The size of the fish was estimated very limited food resources (III, according to the method of IV). Between June and August Jenkins et al. (1979) and Wise there were no statistical (1980). differences in the monthly The composition of the sprainting activity of otters otter diet has been presented (IV) between different waters, i.e. in as percentage frequency (PF), as small streamlets, rivers and areas relative frequency (RF) and as near lakes (measured by positive bulk percentages (BP). Percentage sprainting sites) (Kruskal-Wallis frequency records the percentage test, in all cases p > 0.1). This of spraints containing a particular means, for example, that otters prey item. Relative frequency is moved in all kinds of waters in the number of occurrences of an their home range in summer. 16

However, the areas near lakes, i.e. (Kruskal-Wallis test, χ2 = 11.76, estuaries and outflows, were the df = 5, p = 0.04) (Fig. 4a). Today, most intensively marked in spring however, it seems to decrease (in May, Kruskal-Wallis test, χ2 = towards spring (Kruskal-Wallis 9.23, df = 2, p = 0.01) and in test, χ2 = 18.65, df = 5, p = 0.002) autumn (September, χ2 = 6.75, df (Fig. 4b). However, there is no = 2, p = 0.03). The reason for this decrease before February was most probably the same as in (November to January), and the case of increasing activity between February and April (both, towards autumn, the greater need p > 0.05). The possible decrease to reserve a home range for could be due to mortality and lack winter. Estuaries and outflows are of births in the population of places where all individuals enter otters in the study area in winter. an area, and most probably find If mortality rate is at the same the markings of other individuals. level every month in my study A different pattern of area as it was in south and central seasonal sprainting activity has been found in many other areas, too. Most spraints were found Near lake between winter and spring in 1 Big river Scotland and Shetland (Mason & Streamlet Macdonald 1986, Conroy & French 1987, Macdonald & 0.8 Mason 1987, Kruuk 1992). On the other hand, in Spain most spraints found during winter or summer, 0.6 depending on the area studied (Lopez-Nieves & Hernando 1984, 0.4 IUCN 2000). In Austria, the Czech Republic and Germany, two peaks were found, one in 0.2

autumn and another in spring sites of positive Proportion (IUCN 2000, Roche 2001). At least in some parts of Portugal, 0 most spraints were found in 5678910 autumn (IUCN 2000). Erlinge Month (1968b and 1969) did not find any seasonality in the sprainting of Figure 3. Proportion of positive otters in southern Sweden. sprainting sites in different At the beginning of the months and areas. The number of study period, in the period of sites studied: May 151, June 101, snow cover, between November July 123, August 89, September and April, the probability of 85, October 85, and total 634. finding fresh otter tracks increased towards spring 17

a) Finland in studies of otters found dead (Rudbäck & Stjernberg 1999, Hellsten 2004), mortality 0.8 could explain about half of this decline. However, individuals 0.6 killed by traffic accidents or other accidents caused by humans, are 0.4 found more easily and fall into the hands of museums more often 0.2 than individuals that have died of

Proportion of positive sites positive of Proportion starvation or predation. 0 Because the food shortage 11121234 is much more serious in winter Month than in other seasons, nowadays

the actual death rate probably is higher in winter than in other b) Near lake seasons (see also Fig. 9). Higher 0.3 River mortality rate in winter could explain most of the observed decrease in monthly probability of 0.2 ositive sites ositive

p finding tracks in the field (Fig. 4b). 0.1 The otters moved about in ortion of of ortion

p their home range throughout the

Pro year in central Finland (I, III, IV, 0 this thesis). Spraints and tracks 11 12 1 2 3 4 Month were found in all kinds of waters the year round. Even the smallest streamlets (less than 2 metres Figure 4. Monthly proportion of wide) had been utilized in all areas with otter tracks in all snow- seasons. trackings during the years 1986- In studies using the OVC 1993 (a), and in one-visit censuses method, most of the otters were (OVC) during the years 1990- usually found in small rivers and 2004 (b). The total number of streamlets in winter, but the sites studied in the years 1986- frequency of observed otters per 1993 was 2216 (monthly: study sites was higher in larger November 363, December 427, rivers (Fig. 5). However, this is January 797, February 508, March most probably due to the large 610 and April 155), and in 1990- amount of small watercourses. 2004 n = 2860.

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In winter, spraints were distance of six to ten kilometres in recorded more often in areas near winter. Correspondingly, a typical lakes (Fig. 6). Otters clearly spend large home range consisted of more time in areas near lakes, i.e. only smaller waters, and included near possible good feeding areas. very few open waters in winter. Males moved longer distances, and often in more than one lake and river system. Typical home range size was 20-50 km of main Total number of otter sites watercourse (Fig. 7a) (I, III). Positive study sites / 10 sites 80 4 The size of home ranges varied greatly also in Sweden 60 3 (Erlinge 1967a), as in my study area. Erlinge (1967a) found that the diameter of the home range of 40 2 an otter family group was about 7 km, and for an adult male about

Total number of positive ottersites 20 1 Positive sites study / 10 sites 0 0 <2 m <2 2-5 m 5-10 m >10 m Strait 0.8

Width of the rivers 0.6

0.4 Figure 5. The number of observed positive otter sites in different habitats during the 0.2

Finnish otter monitoring pilot sites positive of Proportion project in 1995-98 (Sulkava & 0 Liukko 1999, II).

River

Near lake 3.1.1 Home range size Streamlet

There was great variation in the sizes of home ranges in central Figure 6. The proportion of Finland, and the demand for positive otter sites in different forage determined the size of waters between November and home range. For example, one April in the years 1990-2004 (n = small home range lay in a large 2860) (Kruskal Wallis test, χ = estuary, where one otter or one 14.02, df = 2, p < 0.0001). family group lived almost the whole winter. They only moved a 19

Figure 7. The movement of the otter litter (female and one cub) based on snow-tracking in winter 1990-91 (a), and movements of other otters (b) in the home range of studied litter at the same time (area number 1 in Fig. 1). In a: = moving of the litter, = exact site of the litter (date), ○ = litter near the site (impossible to know exact site). In b: = moving of the one male otter, and = other single otters. 20

15 km in southern Sweden. Later but the core area of the home the home ranges of otters have range was usually the biggest been studied using radio tracking. river (Fig. 7a). Only few otters The mean length of rivers and lived in the same lake and river streams used by otters in central system in wintertime in central (Jenkins 1980, Green et al. 1984) Finland (Fig. 7b) (I, III). To find and northern Scotland (Kruuk et two litters or two males in the al. 1993, Durbin 1996 and 1998), same river was extremely unusual. was between 19 and 85 km for Probably the heaviest competition males and 16 to 25 km for takes place in summer and females. The mean area of water autumn, when the boundaries of utilized by males was 63 ha, and home ranges are established. by females 20 ha in Scotland, and Otters divide the resources for the average number of otters in winter, and high marking intensity streams was approximately one in autumn indicates another individual per 15 km (Kruuk et al. individual that the river is already 1993). There were great variations occupied. This system could between the sizes of the areas notably reduce the need for utilized. The water area in the aggression in winter, when the smallest home ranges was only demand for food is greater and its 6.2 ha and in the biggest 78.7 ha availability lower than in summer. (Durbin 1996). In marine coastal conditions the home ranges of 3.2 Methods and the size of otters were smaller than those in population inland waters, and social behaviour is also different. Most 3.2.1 Snow-tracking versus probably the large amount of prey other methods in marine habitats allows very different social behaviour in these Snow-tracking was found to give areas. For example, on the coast more accurate estimates than of Shetland, the otters live in other large-scale field methods for groups, and the group utilizes an the density of otter populations. area between 4 and 14 km of The main problems involved in seacoast (Kruuk & Moorhause the standard method include the 1991, Kruuk 1995). Coastal otters seasonality of sprainting activity. use a wider strip of water than is In central Finland I found distinct possible for otters in most rivers seasonality of summertime or tributaries. Kruuk et al. (1989) sprainting activity, with its peak found that otters use water as far in autumn (Fig. 3). Together with as 100 metres offshore, and the short summer and other estimated that there is about one possible errors, this does not animal per km living on the provide a good opportunity to seacoast of Shetland. study otter populations by the Otters utilized all kinds of standard method in Finland. waters inside their home range, 21

Methodological problems (Sidorovich 1991 and 1997, of the standard and other methods Sidorovich & Lauzhel 1992), have been widely discussed Germany (Klenke 1996 and 2002, earlier (e.g. Jenkins & Burrows Hertweck et al. 2002), Poland 1980, Kruuk et al. 1986, Kruuk & (Sidorovich et al. 1996), the Conroy 1987, Mason & Czech Republic (Simek 1996 and Macdonald 1987, Conroy & 1997, Simek & Springer 1998), French 1991, Mason & Austria (Kranz & Knollseisen Macdonald 1991, Romanowski et 1998, Kranz et al. 2002) and al. 1996, Romanowski & Slovakia (Kadlecik & Urban Brezezinski 1997, Carss et al. 2002). 1998, IUCN 2000, Ruiz-Olmo et All techniques for al. 2001b). It is evident that there estimating otter populations are is much variation in sprainting time consuming and difficult, activity between seasons and involving highly specialized areas, and possibly also between investigators. In snow-tracking different age and sex groups (e.g. methods, for example, the Mason & Macdonald 1986, investigators may miss otters that Sulkava & Sulkava 1989, Kruuk have not left tracks at a study site. 1995, IUCN 2000). Other possible It is also possible that methodological problems include distinguishing different differences between field workers individuals from one another may (e.g. Sulkava & Storrank 1993), cause problems. However, the bank-side vegetation (e.g. general visibility of snow tracks is Elmeros & Bussenius 2002) and good, and compared to spraints the presence of spraintable they are very easy to identify. bridges (IUCN 2000). Comparison of two Snow-tracking gives more different snow-tracking methods, information on the populations of HMM and SM in 1998-99, gave a otters than spraints and footprints similar picture of the local otter in summer (Sulkava & Storrank population (I). In the SM method, 1993, Sulkava 2006a, I, II, III). twelve otter individuals and eight For example, it is possible to positive segments (with fresh estimate the total population of tracks) were found in the studied otters in the study area (I, II, III). 0.5-km intervals. This means Snow-tracking has been used in either 0.059 or 0.039 otters per otter studies earlier in Sweden segment respectively, and a total (e.g. Erlinge 1968a, Kjellander & population estimate of 89 or 59 Mortensen 1985, Aronson 1995), otters. The population estimate in Canada (Reid et al. 1987), made by the number of positive Finland (Skaren & Kumpulainen segments (59 otters) was almost 1986, Skaren & Jäderholm 1987, the same as the estimate by the Sulkava & Sulkava 1989, Kauhala HMM method, in which I 1996, Sulkava & Liukko 1999, estimate that 51 otters lived in the Storrank et al. 2002), Belarus study area (I). 22

The results of the OVC the central part of the country (II). method were weakly dependent This result is supported by other on weather or the thickness of studies as well (e.g. Stjernberg & snow cover. Only very cold Hagner-Wahlsten 1991 and 1994, weather affected negatively the Wikman 1996, Helle et al. 1998). moving activity of otters (Fig. 2b, The evaluated OVC method I). During the period of high was developed for the monitoring population level (after the year of otter populations (I, II). 1993 in the study area), the high Because of the short monitoring death rate in winter most probably period, only three years, it was not leads to decreasing probability of possible to detect any significant finding otter tracks towards spring change in the size and distribution (Fig. 4b). It seems that the OVC of the otter population. The pilot method (I) is a reliable way of study produced new information estimating the total number of on the status of the otter otters. However, it is important to populations, and this can be used do the field work as early as if the survey is repeated in the possible, i.e. when the population future. The track index (the is at its highest at the beginning of number of positive sites per 100 winter. When the OVC-method is study sites) or otter index (the used to monitor the population, number of otters per 100 sites) of the field work should be done at the areas can be used as the same time in winter every abundance index in monitoring year. (II). These results also provided an opportunity to estimate the 3.2.2 Monitoring project abundance of otter populations in the study areas and to generalize it A national OVC study on the for the whole country. monitoring of otters was carried In the OVC pilot project the out in 1995-98 (Liukko 1999). studied areas together made up Otter tracks were found in all about 10% of the area of Finland, study areas during all the field and the estimated minimum periods (II). Fresh otter tracks number of otters was 1116 were found at 169 sites in the first individuals in the years of the year (11.5% of the sites were monitoring project. The OVC positive), at 141 sites in the method was developed and tested second year (8.9%) and at 120 in central Finland (I, II, III), sites in the third year (9.9%). where about 50% of all otters The estimated minimum were found in all samplings number of otters in the study areas (Sulkava 1993, 1995 and 2006a, was 121 in the first winter (8.3 Sulkava & Liukko 1999, I, II). individuals per 100 sites), 112 in Based only on these results, the the second winter (7.0) and 101 in population might be about 2200 the third winter (8.3). The otter individuals. However, it was population was most abundant in possible to estimate this result 23

more accurately with help of the 3.3 Density of otters indices of relative abundance of otters produced by the Finnish The otter population in the study wildlife triangle scheme (Lindén area increased significantly et al. 1996, Helle et al. 1998). The between 1985 (20 animals) and final estimate of the Finnish otter 2002 (I, III). In winter 2002-2003 population was 2000-2550. there were 52 otters living in the Knowledge from Finnish study area, and at least 20 other wildlife triangle scheme (Lindén individuals in the nearest et al. 1996), regional studies (see surrounding area. The mean citations in Sulkava & Liukko density of otters was 1.2 per 100 1999 and II) and from this study km2 (in area of all river systems, (II), indicate that probably more in Fig. 1) in 1985-86 and 3.2 per than 50% of Finnish otters lived 100 km2 in 2002-03. In rivers and in a central region of Finland, and streamlets, the density of otters more than 80% in the area where was 0.3 otters per 10 km of river the OVC method works its best (0.11 individuals per river (area with enough snow and ice in hectare) in 1985-86 and 0.7 per 10 winter). For this reason I believe km (0.28 per ha) in 2002-03. The that the estimated densities of the otter population increased rapidly Finnish otter population during from 1985 to 1993. Since then, the years of the project were the population has varied between reliable. 40 and 50 individuals. The pilot project indicated Growth of the otter that the OVC method was reliable population in the study area and its costs were relatively low, followed the sigmoidal model of at least compared to the standard population increase (III). method (Sulkava & Liukko 1999, Population growth at its most IUCN 2000, II). The otter's rapid phase (1985-93) was 143%. behaviour and the prevailing This represents on average 17.9% climatic conditions make the per year (III). Population growth monitoring of otters by snow- was equal in all kind of waters, tracking possible in Finland (I, II, when calculated by individuals III, Sulkava 2006a). The per river hectare or per river conclusion drawn from the study kilometre. was that the OVC method works At present, in central well on a national scale and gives Finland, the otter population is useful results. It could therefore dense and probably fully saturated also be a useful tool for at a low local carrying capacity. monitoring otter populations in The density of otters (0.7 other large northerly areas (II). individuals per 10 km of river) was more or less equal to that in the same latitudes in Scotland (0.7) (Kruuk et al. 1993) and in Russian Karelia (0.6-1.3) 24

(Tumanov 2002), but the carrying where all waters are ice-free most capacity of those areas is not or all of the year. In central known. In southern latitudes or on Finland nearly all waters (also marine coasts, most waters are most of the flowing rivers) are more productive, and there is totally covered by ice in winter. more food available for otters than For this reason the differences in in northern rivers and lakes. For food availability between southern this reason the southern and and northern latitudes are marine otter populations can be extremely pronounced in winter. denser. For example, in Belarus the otter density varied between 3.4 Reproduction and the 1.7 and 5.9 individuals per 10 km carrying capacity of the study of watercourse (3.7-10.7 area individuals per 100km2), depending on the size and type of 3.4.1 Reproduction river (Sidorovich 1997, Sidorovich & Pikulik 2002). The It was not possible to know the density of otters was 2.2 actual date of birth in this study. individuals per 10 km in Poland Determination of birth seasons (Sidorovich et al. 1996) and 1.5- was, however, possible. 3.1 in Latvia (Ozolins & Rantins Occasionally, it was also possible 1992a and 1992b). In a fishpond to determine the time of birth at area in the Czech Republic the one or two month's intervals (Fig. density was as high as 48-62 8). individual per 100 km2 (Simek & Seasonality in the timing of Springer 1998). On the coast of otter reproduction increases Shetland, one animal per towards the northern and kilometre of shoreline was found continental areas, and non- (Kruuk et al. 1989, Kruuk & seasonal reproduction has been Moorhause 1991). In the 1960s, found only in marine populations. the density of otters was one adult Seasonal breeding was typical for per 4-6 km of river in southern otter populations in Sweden Sweden (Erlinge 1967a and (Erlinge 1967a), Shetland (Kruuk 1968a). et al. 1993, Kruuk 1995), Norway Comparing the density of (Heggberget & Christensen 1994), otters in different areas is Denmark (Elmeros & Madsen complicated. The otter population 1999) and Russia (Tumanov in central Finland was low when 2002). Some breeding seasonality all shores of lakes and rivers were has been found also in the included (0.24 individuals per 10 Netherlands (Wijngaarden & km of shoreline), while the Peppel 1970), Germany density per available feeding areas (Reuther& Festetics 1980), in winter (5.2 individuals per 10 Portugal (Beja 1996) and Belarus km) was, in fact, even higher than (Sidorovich 1997). In England, in more southerly populations, Mason & Macdonald (1986) did 25

not find seasonality in the The female and cubs stayed breeding of otters. This was together for more than eight probably due to the marine months; at least over most of climate conditions and the small following winter in central differences between the seasons in Finland (Sulkava 2006a and England. However, later surveys 2006b, I, III). The time offspring have found possible seasonality in is dependent on their mother England, too (Liles 2003). There seems to be approximately of the are more than enough good same length as in other areas in feeding habitats and food for the same latitudes. In Shetland the otters in summer in central cubs stay with their mother for 9- Finland. However, only those 13 months (Kruuk et al. 1991), in cubs that were born in summer Sweden for about one year and had grown sufficiently before (Erlinge 1967a) and in Scotland winter were able to survive the for about 16 months (Kruuk winter following their birth, and 1995). How long the offspring is all known litters were born dependent on their mother in more between May and September (Fig. southern populations is not 8) (Sulkava 2006b). known. Several litters were born in most years in my study area, indicating a healthy and vital otter 20 population (I, III). In central 18 Finland the mean number of cubs 16 with a female was 1.51 (III). This 14 is a typical fecundity for 12 populations in similar latitudes, 10 but lower than that of more 8 southerly populations. On the sea coast of Scotland the number of Numberlitters of 6 cubs was 1.55 and in Shetland 4 1.86 (Kruuk et al. 1987 and 1991). 2 In lower latitudes the number of 0 cubs seems to be higher; in May- June-Aug Aug-Sept Holland 2.8 (Wijngaarden & June Peppel 1970), in Poland 2.4 Months (Wlodek 1980) and in England 2.5 (Mason & Macdonald 1986). Figure 8. Approximately birth- In Belarus Sidorovich (1991 and time of 50 litters in Central 1997) found the litter size to be Finland in 1985-2002. All litters 2.7 from embryos, 2.6 at the age (total of 119 litters), found in the of one month, and 2.1 cubs after study area in 1985-2002, were leaving the den. On the sea coast born between the beginning of of Norway the mean litter size of May and the end of September. otters was 2.5 during pregnancy, 26

but only 2.0 after the cubs became indicating intraspecific mobile outside the breeding den competition between otters (Heggberget & Christensen 1994). (Royama 1992, III). Although The mortality of cubs was 0.63 there is abundant water in the (total of 2.75 embryos per study area, the carrying capacity pregnancy) before the young left is low. The limiting factor is the the den (2.12 cubs with a female) time when waters are open, i.e. in Germany (Ansorge et al. 1996). competition for food intensifies Most probably the small when nearly all waters are number of cubs and clear covered by ice in winter. When seasonality of breeding in central the population density increased Finland, as in other northern shortage of winter food became areas, is an adaptation to the evident, and the litter size highly seasonal availability of decreased. The shortage of food prey (food). The cubs were born could also be seen in cub in summer, but the mating season production in different river was in spring. In the spring the systems. Only females living in female otters may not have been the river systems with large open in very good condition, due to the waters in winter could produce harsh climate conditions and many offspring (III). The shortage of food and feeding areas increased competition could also in winter. The fact that only be seen in the number of death females overwintering in river otters as the population density systems with large lakes (which increased (Fig. 9). maintain ice-free outflows) could Kruuk et al. (1991) and produce cubs (III), also points out Kruuk & Conroy (1991) found a to the conclusion of food shortage strong correlation between during winter. numbers of cubs and numbers of prey in Shetland. They also 3.4.2 Density-dependence and observed that females can carrying capacity deliberately reduce litter size by neglecting their offspring. A The mean litter size decreased in female will only abandon a cub if the study period with increasing there is shortage of food (Kruuk density of the population (III). 1995). Also in Spain, a clear The density-dependence of connection between fish offspring production indicates abundance and otter populations intraspecific competition in the was found (Ruiz-Olmo et al. population of otters. In 1993 the 2001a). This may be the case in otter population most probably central Finland, too. I counted the reached the local carrying number of cubs in late autumn or capacity of the study area (Fig. 3 early winter, but I do not have in I, and Fig. 3 in III). After 1993 data on the litter size at birth. population size oscillated up and Hypothetically litter size might be down (was typically "saw-like"), the same, but the number of cubs 27

in winter decreased during the the population growth in sink study period. river systems was due to births in source river systems close by. 3.5. Source-sink dynamics and This phenomenon was not known spatial organization of otters earlier in otters. To find out why some river There was clear source-sink systems were more productive dynamics between the habitats than others, I studied the size of occupied by otter populations. rivers, riverbed and lakeshore Otters in a few river systems lengths, lake areas of otters and produced most cubs, which then possible feeding areas in winter, occupied the secondary habitats, and examined the relationships allowing the total otter density to between the population density or increase in all the river systems in cub production and the the study area. The five most characteristics of habitats. productive river systems More litters and cubs were (altogether 16 river systems) born in river systems with large produced 72% (111 cubs) of all lake surface area than those of the juveniles in the study area small lake area (Table 1) (III). during the years 1985-2003 (total Seven river systems with large of 154 cubs) (III). Otters in the lake surface areas produced 84% secondary habitats did not (129 cubs) of the juveniles (areas reproduce (Table 1) (III). 1, 3, 4, 6, 10, 11 and 15 in Fig. 1). It was also likely that Three of these river systems were immigration was not an important large and four medium-sized, factor for population growth in the classified by the mean width of study area. Earlier studies the main river. There were also revealed that population density more large and medium-sized increased simultaneously both lakes in the river systems that inside the study area and its produced most of the litters and surroundings (personal cubs (Table 1) (III). observations, Mäkelä & Rajala Riverbed length or size of 1995, Helle et al. 1998, Sulkava & water areas in rivers (ha) did not Liukko 1999, Ludvig et al. 2002). correlate with the number of cubs The birth rate was also higher (III). Neither did riverbed length than population growth rate (Fig. correlate with the number of 9b), and so there were more litters. Large lakes were important young otters than could be for otters in winter even though sustained inside the area. This all the lakes themselves were could be seen also in the totally ice covered, since large increasing number of death lakes maintain ice-free outflows, individuals (Fig. 9a). The high and these were important feeding birth rate and limited possibilities areas for otters (Fig. 6). for finding a free home range outside the study area indicate that 28

Table 1. The number of cubs born in river systems with large (first seven) and small lakes in the study area in 1985-2003, and the number and surface area of lakes in all river systems. In a few cases it was not clear whether there were one or two cubs in one litter. These cases were measured as 1.5 cubs per litter.

River Total Cubs / Number Number of Total Total system production 100 ha of large medium surface number (numbers of cubs in of lake lakes size lakes area of of lakes in Fig. 1) 1985-2003 surface (more (20-99.9 lakes (ha) and than ha) ponds 100 ha) Large lakes 1 23 2.47 1 10 931 40 3 23.5 1.34 4 7 1752 29 4 14 0.44 4 4 3156 24 6 15 0.75 6 11 2013 42 10 17.5 1.50 2 10 1166 27 11 32 0.95 2 0 3353 9 15 4 0.28 3 6 1428 22 Subtotal 129 0.93 22 48 13799 193

Small lakes 2 3 0.86 1 2 347 22 5 1 0.31 1 1 327 27 7 3 0.58 1 3 515 10 8 7 3.54 0 4 198 6 9 3 1.63 0 3 184 10 12 1 0.19 1 3 526 27 13 2 0.38 3 6 526 33 14 3 0.90 1 2 334 20 16 2 0.65 1 2 310 5 Subtotal 29 0.89 9 26 3267 160

Total 154 0.90 31 74 17066 353

29

Although the litter size of my study area than the decreased in the study period, the surrounding areas, it is not likely annual number of litters increased that many individuals emigrated at the same time. This was (or immigrated) from the study especially clear in the river area to surrounding areas. The systems with small lake surface fluctuating population ("saw-like" areas (ANOVAR, df=1, F=19.00, shape of dynamics) (Fig 9a) (III), p=0.002) (Table 1). Most also indicate that possible probably the reason was that all migration was not an important river systems with large lake areas factor in population dynamics. (good habitats for overwintering) Therefore I calculated the number were occupied, and when young of dying individuals by individuals came to reproductive subtraction: age, they had to breed in Number of dying = Total number secondary habitats. There was less of otters in year t ─ the number of food available in these habitats, adult otters in year t+1 and females were younger than This calculation is, of course, other areas. For these two reasons confused by possible immigration litters were smaller in secondary and emigration, but is after all habitats, and also the possibilities highly indicative. of cubs surviving over the next Birth rate is calculated: winter diminished. Birth rate = Number of cubs in year t / number of adults in year t 3.6 Mortality There were less dying individuals in the period of increasing A decreasing birth rate and/or an population than after it (Fig. 9a). increasing death rate could Before the year 1993, the mean explain the S-shaped growth growth rate of the population was curve of the population (Becon et 0.11 per year, and after that 0.02 al. 1990). Mortality for otters is per year (Fig. 9b). The calculated difficult to estimate. In the study annual number of deaths (4.43) area, dead otters are found was lower than the number of occasionally in the field, but I do births (7.75) before the year 1993. not know how many otters After that, population growth actually died per year. However, stopped, and the number of births since the density of the population (10.00) and deaths (11.00) seems was at about the same level inside to be equal (Mann-Whitney Test, p > 0.05) (Fig. 9a).

30

a)

Adults 45 Cubs Number of deaths 40

35 30

25 20

15 Number of otters 10

5 0 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 00 01 02 03 04 Year

b)

0.5 Birth rate Growth rate (r) 0.4

0.3

0.2

0.1

Rate 0 86 87 88 89 90 91 92 93 94 95 96 97 98 99 00 01 02 03 04 -0.1

-0.2

-0.3

-0.4

Figure 9. The annual number of cubs and adult otters, and the estimated number of dead otters in the study area (a). There were more deaths after saturation of the population in 1993 (Mann-Whitney Test, U = 18.5, p = 0.03). The birth rate and the growth rate (r) of the population (b).

31

3.7 Diet like the mink (Bonesi & Macdonald 2004), it is obvious Fish and amphibians were the that otters take the most abundant main components in the diet of food and prey easiest to catch on otters in central Finland (IV). most foraging occasions (e.g. Small cyprinids (mainly Rutilus Clavero et al. 2004, IV). In rutilus, Alburnus alburnus, Finnish conditions there are few Leuciscus idus and Acerina possibilities to favour certain prey cernua), perch (Perca fluviatilis items over others in winter (I, III, and Acerina cernua) and pike IV). Several studies have shown (Esox lucius) were the most that fish dominate the diet of important fish for otters in central otters, but there was also Finland (IV). The food significant temporal and spatial composition varied seasonally. variation in the diet (e.g. Erlinge Amphibians (Rana sp. and Bufo 1969, Fairley 1972, Jenkins et al. bufo) were the most important 1979, Chanin 1981, Kemenes prey in winter (BP used in all 1989, Mortensen 1989, Carss et percents), (33.2% of food). Perch, al. 1990, Brzezinski et al. 1993, pike and cyprinids were important Kucerova 1998, Roche 1998, in all seasons, but in winter small Ludwig et al. 2002, Clavero et al. sculpins (Cottus cottus) (9.8% of 2004). Variation in the diet of food) and burbots (Lota lota) different areas also indicates that (8.9%) were also important food the foraging habits and diet of sources for otters. Cyprinids were otters is still incompletely known. the most important prey in spring The diet of otters in central (38.7% of food). Amphibians Finland appears to be very were important especially in the different from that found in most smallest streamlets (in winter; other areas (e.g. Erlinge 1967b 68.4% of food). The otters also and 1972, Webb 1975, Kruuk & ate toads. In some river systems, Hewson 1978, Jenkins & Harper crayfish (Astacus sp.) and 1980, Wise et al. 1981, Kruuk & mammals were commonly Moorhouse 1990, Beja 1991, consumed, but salmonids, birds, Bodner 1998, Taaström & reptiles, insects, molluscs and Jakobsen 1999). Most of the otters plants were used only lived in small rivers or streamlets occasionally by most otters in the in central Finland in winter, and study area. Most of the otters fed mainly on amphibians. A few lived in small forest rivers, and earlier studies have reported that therefore amphibians and sculpins otters eat remarkable amounts of played a critical role in the diets amphibians (Fairley 1972, of most otters in winter (IV). Macdonald & Mason 1982, These results are also supported Skaren & Kumpulainen 1986, by later studies (Ludwig et al. Adrian & Delibes 1987, Laanetu 2002). Although the otter is 1989, Sulkava & Sulkava 1989, more a specialist than a generalist Weber 1990, Ozolins & Rantins 32

1992a, Skaren 1992a, Sidorovich the reason for competition, lack of 1997). food, is evident in winter, when most feeding areas are covered by CONCLUDING REMARKS ice (I, III, IV). Local characteristics, such In this study the long-term trend as the presence of amphibians in and clear increase in otter the otter's diet in Finland, have population were documented for now been discovered (IV). Other the first time by total annual typical features such as the very counting of otters (I, III). Finding large home ranges and long a clear sigmoidal model for the distances moved by the otters are increasing densities of otter now also known (I, III, this population made it possible to thesis). Evaluating the monitoring determine the local environmental of otter numbers using the new carrying capacity for the study OVC method developed, shown area (III). Otters in a few river that the method works well in systems produced most of the Finnish conditions, and that the cubs, creating a source-sink Finnish otter population in the habitat structure in studied river years 1995-1998 comprised 2000- systems (III). The density- 2250 individuals (I, II). dependent offspring production of More studies are needed to the population of otters was also investigate the role of different documented first time (III). The water systems and possible density-dependence of offspring seasonal territoriality of otters production, together with typical during winter. Home range studies the saw-like shape of the growth of male otters are still needed. curve and increasing number of Possible changes in diet after the deaths when the population saturation point of the otter density increased, indicate that population have not yet been intraspecific competition in the studied. The lack of real otter population was the main monitoring system of otters in factor regulating population size. Finland needs to be urgently Aggression between individuals fixed. may occur in summer, although

33

ACKNOWLEDGEMENTS

I should like to thank all those people who worked with me and made this study possible. I am especially grateful to my father Pertti Sulkava, who has done more work in the field than others. I should also like to thank all the other field assistants for their great help in the field investigations. I thank you Ari, Esa and Matti Aalto, Pauli Arppe, Eero Heinonen, Jouni and Teijo Kaijanmäki, Jouni Kalmari, Juhani Kiltinen, Lauri Laitinen, Jouni Lamminmäki, Kari and Tarmo Myntti, Antero Mäkelä, Marko Mäkinen, Ossi Nokelainen, Jouko Pihlainen, Ari Sillanpää, Kimmo Sipiläinen, Bo Storrank, Jaana, Pekka and Raija Sulkava and Tuomas Syrjä. I hope that I have not forgotten any of you. Many other peoples have also informed me of otter tracks, otters, litters, dead otters or other otter-related matters during all these years. My thanks go to them! I wish to thank Prof. Heikki Hyvärinen, Prof. Heikki Roininen and Prof. Jorma Tahvanainen, my supervisors from the University of Joensuu. Jaana Höglund, Ulla-Maija Liukko, Gilbert Ludwig, Torsten Stjernberg, Bo Storrank, Pekka Sulkava and Hannu Ylönen, I thank you, too, for your excellent co-operation. My thanks are also due to Prof. Seppo Sulkava for his comments and inspiration, and Pekka Sulkava, Heikki Roininen and Hannu Huuskonen for their help with the statistical analyses. The Finnish Environment Institute, the Finnish Ministry of the Environment, The Regional Environment Centres, the regional administration of the Forest and Park Service, the Finnish Museum of Natural History, the Finnish Game and Fisheries Research Institute, the Memoranda Society, WWF Finland, the Finnish Association for Nature Conservation, Natur och Miljö rf, Keurusseudun Luonnonystävät ry and Ranua Zoo, have all supported some part of this study either financially or in some other way. My thanks also go to those 60 people who worked in the field or with data during the pilot study of the Finnish otter monitoring. Rosemary Mackenzie corrected the English language of this paper. I also wish to thank my friends Tuula & Ari, Päivi & Hannu, Airi & Jouni and others: some moments of life are happy. Thanks also go to all my friends in the Finnish Association for Nature Conservation, BirdLife Finland, Suomenselän Lintutieteellinen Yhdistys, Keurusseudun Luonnonystävät, the Finnish Flying Squirrel Association and other co-operating conservationists: It sometimes seems hard work, but is sometimes also rewarding. Finally I should like to thank and intensively kiss my wife Jaana: perhaps it is a miracle, but after these twenty years of study we still love each other. Our children, Erika and Carita, have been splendid, too.

34

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