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Distribution, habitat and diet of the otter Lutra lutra in the Drina catchment, Yugoslavia
Biological Conservation 45 (1988) 109-119
Distribution, Habitat and Diet of the Otter Lutra lutra in the Drina Catchment, Yugoslavia I. R. Taylor, M. J. Jeffries, S. G. A b b o t t , I. A. R. H u l b e r t & S. R. K. Virdee Department of Forestry & Natural Resources, University of Edinburgh, Darwin Building,The King's Buildings,Mayfield Road, Edinburgh EH9 3JU, UK (Received 13 October 1987; accepted 26 November 1987)
A BSTRA CT In a survey of the Drina catchment, Yugoslavia, during July and August 1986, otters were found to be widespread with sprain ts recorded at 41 (97.6 % ) of 42 sites investigated. Otters were present on all tributaries and streams down to 2 m wide. Throughout the area agriculture and industry were limited andmost rivers showed no signs o f pollution. Throughout there was abundant cover and suitable undisturbed sites for holts. It is suggested that there is little immediate threat to this apparently healthy otter population. Fish were the most important prey but crayfish were also important on small streams with silt deposits in slack water. A significant correlation was found between the number o f spraints per kilometre sample and the percentage occurrence of cra)fish in the spraints.
INTRODUCTION T h r o u g h o u t the Mediterranean region the otter L u t r a lutra has undergone substantial population declines in some countries, particularly Italy and France (Green & Green, 1981; Groupe Loutres, 1983; Macdonald & Mason, 1983), but is still widespread and numerous in others such as Greece, Albania and Portugal (Macdonald & Mason, 1982a,b; Prigioni et al., 1986). Information on the species' status in Yugoslavia is limited. The results of a preliminary survey in 1982 indicated that declines have occurred in intensively farmed areas but that some of the more remote river systems such 109 Biol. Conserv. 0006-3207/88/$03"50 © 1988 Elsevier Applied Science Publishers Ltd, England. Printed in Great Britain
110
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as the Bosna and Drina may still support good populations (Liles & Jenkins, 1984). This paper reports the results of a detailed survey of the Drina catchment.
STUDY A R E A The study covered the major part of the Drina catchment from Vigegrad southwards, an area of approximately 14 500 km 2 (Fig. 1). The entire region is mountainous with limestone bedrock and human settlement is restricted mostly to the valley bottoms. Cultivation is confined to narrow, broken strips along the valleys and to higher altitude plateaux such as the areas north of Nik~i6 and around Zabljak and Sjenica. Most of the farming units are small family holdings worked on a part-time or subsistence basis. Mechanisation is limited, most work being done by traditional methods. Little use is made of agricultural chemicals. Most of the cultivated area is devoted to grazing and hay production for sheep and dairy cattle. Crops include maize and vegetables and orchards, especially of plum, are common. Many of the tributaries (e.g. Tara, Komarnica and Sutjeska) (Fig. 2) run through precipitous limestone gorges where cultivation is impossible. Most of the higher ground is forested with deciduous and pine species at lower altitudes and spruce at high altitudes. On all rivers this woodland extends down to the streamside wherever the slope is too steep or rocky for cultivation. Some rivers (e.g. Komarnica, Uvac) have been harnessed for hydroelectric power. The effect in the narrow gorges is to produce deep, steep-sided reservoirs extending up to 25 km upstream of the dams.
METHODS The study area was surveyed from 10 July to 23 August 1986. Attempts were made to sample the Drina and all its major tributaries systematically at 20 km intervals. This sampling interval was considered adequate in relation to current estimates of otter home range lengths in riverine habitat (Melquist & Hornocker, 1983; Green et al., 1984; Green & Green, in press). In practice, access was determined by the limited road network and the precipitous nature of the terrain. Several areas were also closed to foreigners. Sampling was thus as regular and as close as possible to the 20 km intervals intended. At each site 1000 m of waterway were searched for signs of otters by the team of five workers. For streams less than about 15 m wide both banks were searched but for wider streams access could be obtained to only one bank. In all cases searches were made from the water as well as up to 5 m from the
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The otter in Yugoslavia
113
water's edge. For each sample stretch records were kept of the nature and extent of the bankside vegetation, the river width and depth, the nature of the substrate, the characteristics of the surrounding vegetation and land-use and any obvious signs of water pollution. In addition to the sample stretches, spot checks were made under bridges wherever possible. Samples of spraints were collected for diet analysis.
RESULTS
Distribution A total of 42 one-kilometre sample sites was surveyed and otter spraints were found on 41 (97.6%). The single exception was a shallow stream of only about 1 m width which may simply have been too small (Fig. 2). The smallest streams on which spraints were found were about 2 m wide. Spraints were found on all of 9 kilometre samples on streams between 2 and 5 m wide and at 11 out of 14 spot sites. These were distributed throughout the catchment, suggesting that otters made widespread regular use of streams of this size. The mean number of spraints found per 200 m was 4.6 + 1-3 for streams less than 15m wide (searched on both banks) and 2.84 + 0-51 for larger streams up to 50 m wide (searched on one bank). The lowest number of spraints found, one per 1000 m in each case, occurred on the two sample sites on the R. Lim north of Bijelo Polje, per.haps indicating a reduced use of this area by otters. There was abundant woodland cover on both stretches but the river was heavily polluted (see below). Thirty-six spot sites, mostly at bridges, were also inspected primarily to obtain additional samples for diet analysis. Spraints were found at 24 (66%) of these butabsence of spraints at such sites cannot be taken as evidence of the absence of otters.
Habitat Most of the streams examined had clear unpolluted water. The main exception was the R. Lim north of Ivangrad where a large pulp mill discharged chemical waste into the river. Further inputs of pollutants occurred around Bijelo Polje and high sediment loads resulted from a number of gravel extraction sites. In all about 90 km of this river was affected by these inputs. Water pH was measured at 23 river sites, all of which were within the range of 7-0-8.2, as would be expected in a limestone area. However, hydroelectric reservoirs had considerably higher levels; those on the R. Uvac registered a pH of 9.2. Most reservoirs had algal blooms and
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dead fish were seen floating on several of them. They also produced large downstream fluctuations in water flow. In all small streams we observed an abundance of potential prey, particularly fish and crayfish where there was fine silt sediment. Snakes, mostly grass snake Natrix natrix, and frogs were also extremely common along river banks. From the number of anglers and their success, fish seemed numerous in most of the large rivers. All streams were flowing; there was no evidence of dried-up streams or disconnected pools holding high prey densities, as have been reported in Greece and Portugal during the summer (Macdonald & Mason, 1982a,b). T h r o u g h o u t the entire catchment there was extensive bankside vegetation. Forty (95.2%) of the kilometre samples had almost continuous tree cover along both banks. Eleven of these had entirely natural forest on both banks and a further 17 had forest along part of their length. All had highly broken ground on the slopes down to the stream with numerous large boulders offering abundant suitable sites for holts. The remaining 12 sample stretches flowed through more heavily cultivated areas but still had tree cover along their banks and extensive forest within 5 km at most along the stream. Diet
A total of 836 spraints were collected throughout the catchment and from them the frequency of occurrence of the major taxonomic groups of prey was determined. All spraints collected were fresh. Spring flood levels were always well above the positions where spraints were collected and this, along with known disappearance rates of spraints (Jenkins & Burrows, 1980; Mason & Macdonald, 1986), would suggest that the information obtained refers to the otters' diet between June and August. When data for the whole catchment are pooled it is clear that fish were the most frequently occurring item, remains being found in 64.1% of spraints. The only other significant prey type was crustacea, all crayfish Austropotamobius pallipes (Lereboullet), which occurred in 36-6% of spraints. Bird, amphibian and snake remains were found in only 0.3, 0.2 and 0.1% of the spraints respectively (Table 1). The relative occurrence offish and crayfish in spraints varied greatly from stream to stream. Comparing individual kilometre samples, the occurrence of crayfish ranged from 0 to 100%. These differences were analysed in relation to the stream substrate and width. The most important substrate characteristic was the presence or absence of fine silt in areas of slack water. For streams less than 20m wide crayfish occurred significantly more frequently where silt deposits were present than where they were not (Mann
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Whitney Test: U -- 10-5, n~ = 18, n 2 = 14, p < 0-002). When comparing all streams with silt present, crayfish occurred more frequently in spraints from small streams (less than 20 m wide) than from larger streams (Mann Whitney Test: U = 19, n~ = 18, n 2 = 14, p < 0-002). A notable feature of the spraints examined was the very low frequency (1.1%) of those containing both fish and crayfish remains. This is probably slightly exaggerated as small quantities of fine fish bones and small scales were difficult to detect amongst the compacted parts of crayfish exoskeleton in spraints and some may have been missed. However, it is possible that individual otters spent some of their time hunting where crayfish were the only prey available or that they concentrated on crayfish for short periods. Compared with fish, eating crayfish includes the ingestion of greater quantities of indigestible material in the form of exoskeleton. This should result in the production of higher volumes of faecal material manifest in either larger or greater numbers of spraints. If the latter held it would have implications for the use of spraint numbers as an index of otter abundance. For individual kilometre samples where we searched both banks we found a highly significant correlation between the total number of spraints and the percentage of these containing crayfish (Spearman rank correlation, r s = 0-65, n = 19, p < 0.01).
DISCUSSION Otters occurred throughout the entire Drina catchment, utilising rivers of up to 50 m width (Drina) and streams down to about 2 m wide. The total length of suitable waterway available within the catchment was thus of the order of about 1400 km. We were unable to inspect reservoirs for signs of otters as stopping was not permitted, but their nature was similar to those in central Portugal which were not used by otters (Macdonald & Mason, 1982b). High water pH, algal blooms, very deep water, steep banks and the presence of dead and dying fish all suggest that reservoirs within the Drina system would also be unsuitable. Potential prey appeared to be abundant on all rivers, with the probable exception of the Lira north of Bijelo Polje. Also cover and suitable undisturbed sites for holts were plentiful throughout. On all rivers a range size of as little as 6 km would have encompassed areas of natural forest on boulder-strewn slopes. Current levels and methods of farming offer no obvious threat to otters and it is unlikely that this situation will change in the foreseeable future. There is thus every indication that this area will continue to support a population of otters. Adjacent areas such as the Bosna catchment, which were not surveyed in the present study, seem to offer
The otter in Yugoslavia
! !7
similar habitat so it seems likely that a substantial area of the Yugoslavian uplands may still have widespread otter populations. From our data it is not possible to deduce values for the actual abundance of otters within the Drina. The use of signs to determine abundance is under considerable debate and clearly in need of further research (Jefferies, 1986; Kruuk et al., 1986; Kruuk & Conroy, 1987; Mason & Macdonald, 1987). However, the values given here for sprainting intensity may prove useful as a baseline for future monitoring of the population and it may prove possible to determine population levels from them once the relationship between abundance and sprainting has been clarified. The analysis of diet from spraints indicated fish to be the most important item but also showed crayfish to be of significance, particularly on small streams with silt substrate. Similar utilisation of crayfish has been reported from other limestone areas (McFadden & Fairley, 1984; Callejo Rey & Delibes, in press). Crayfish occurred less frequently in the diet on larger rivers. This probably arose from a greater availability of fish on these rivers rather than a shortage of crayfish. Erlinge (1968) found that captive otters preferred fish to crayfish and McFadden & Fairley (1984) found a seasonal drop in the consumption of crayfish on the R. Abbert, Ireland, when migratory salmonids were abundant. Both snakes and amphibia were abundant along most fiver banks but featured insignificantly in spraints. This contrasts strongly with the results of other studies in southern Europe (e.g. Macdonald & Mason, 1982b; Simoes Graca & Ferrand de Almeida, 1983; Callejo Rey & Delibes, in press). These studies were in considerably drier areas than the Drina, with some streams drying up or being reduced to small pools during summer months. It is possible that snakes are not preferred prey but are taken when fish become less widely available. In most areas amphibians are taken less often during summer months than at other times. A significant correlation was demonstrated between the number of spraints found within kilometre samples and the percentage occurrence of crayfish in the spraints. It is suggested that this arose, at least in part, from the greater amount of indigestible material in crayfish. It can be argued that spraints deposited along a particular stretch need not necessarily derive from prey taken in that stretch. However, three streams that were sampled for spraints at frequent intervals showed consistency in spraint contents for distances of up to 20 km. Alternative explanations for the relationship are that otters made greater use of areas where crayfish were more abundant or that such areas were marked more frequently. From our data we cannot differentiate between these and our suggestion regarding the greater amount of indigestible material. However, all factors that affect spraint production need to be understood to clarify the relationship between sprainting
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intensity and otter numbers. The effect o f diet composition on spraint production seems not to be known and is clearly an area in need o f further research which should be possible using captive animals.
ACKNOWLEDGEMENT We thank the Vincent Wildlife Trust for funding this work.
REFERENCES Callejo Rey, A. & Delibes, M. (in press). L'alimentation de la Ioutre (Lutra lutra) en Espagne. In Proceedings of the 3rd International Otter Colloquium, Strasbourg, November 1983, ed. N. Duplaix & C. Kempf. Erlinge, S. (1968). Food studies on captive otters (Lutra lutra L.). Oikos, 19, 259-70. Green, J. & Green, R. (1981). The otter (Lutra lutra L.) in western France. Mammal Rev., 11, 181-7. Green, J. & Green, R. (in press). Territoriality and home range in Scotland. In Proceedings of the 3rd International Otter Colloquium, Strasbourg, November 1983, ed. N. Duplaix & C. Kempf. Green, J., Green, R. & Jefferies, D. J. (1984). A radio-tracking survey of otters Lutra lutra on a Perthshire river system. Lutra, 27, 85-145. Group Loutres (1983). Repartition de la loutre en France. Bull. Groupe Loutres, 15, 17-21.
Jefferies, D. J. (1986). The value of otter Lutra lutra surveying using spraints: An analysis of its success and problems in Britain. J. Otter Trust, 1, 25-32. Jenkins, D. & Burrows, G. O. (1980). Ecology of otters in northern Scotland, III. The use of faeces as indicators of otter (Lutra lutra) density and distribution. J. Anim. EcoL, 49, 755-74. Kruuk, H. & Conroy, J. W. H. (1987). Surveying otter Lutra lutra populations: A discussion of problems with spraints. Biol. Conserv., 41, 179-83. Kruuk, H., Conroy, J. W. H., Glimmerveen, U. & Ouwerkerk, E. J. (1986). The use of spraints to survey populations of otters Lutra lutra. Biol. Conserv., 35, 187-94. Liles, G. & Jenkins, L. (1984). A field survey for otters (Lutra lutra) in Yugoslavia. J. ZooL, London, 203, 282-4. Macdonald, S. M. & Mason, C. F. (1982a). Otters in Greece. Oryx, 16, 240-4. Macdonald, S. M. & Mason, C, F. (1982b). The otter Lutra lutra in central Portugal. Biol. Conserv., 22, 207-15. Macdonald, S. M. & Mason, C. F. (1983). The otter Lutra lutra in southern Italy. Biol. Conserv., 25, 95-101. McFadden, Y. M. T. & Fairley, J. S. (1984). Food of otters Lutra lutra (L.) in an Irish limestone river system with special reference to the crayfish Austropotamobius pallipes (Lereboullet). J. Life Sci. R. Dubl. Soc., 5, 65-76. Mason, C. F. & Macdonald, S. M. (1986). Otters: Ecology and conservation. Cambridge University Press, Cambridge. Mason, C. F. & Macdonald, S. M. (1987). The use of spraints for surveying otter Lutra lutra populations: An evaluation. Biol. Conserv., 41, 167-77.
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Melquist, W. E. & Hornocker, M. G. (1983). Ecology of fiver otters in west central Idaho. Wild. Monogr., 83, 1-60. Prigioni, C., Bogliani, G. & Barbieri, F. (1986). The otter Lutra lutra in Albania. Biol. Conserv., 36, 375-83. Simoes Graca, M. A. & Ferrand de Almeida, F. X. (1983). Contribuicao para o conhecimento da Iontra (Lutra lutra L.) num secto da bacia do Rio Mondego. Cienc. Biol. Ecol. Syst., 5, 33-42.
Distribution, Habitat and Diet of the Otter Lutra lutra in the Drina Catchment, Yugoslavia I. R. Taylor, M. J. Jeffries, S. G. A b b o t t , I. A. R. H u l b e r t & S. R. K. Virdee Department of Forestry & Natural Resources, University of Edinburgh, Darwin Building,The King's Buildings,Mayfield Road, Edinburgh EH9 3JU, UK (Received 13 October 1987; accepted 26 November 1987)
A BSTRA CT In a survey of the Drina catchment, Yugoslavia, during July and August 1986, otters were found to be widespread with sprain ts recorded at 41 (97.6 % ) of 42 sites investigated. Otters were present on all tributaries and streams down to 2 m wide. Throughout the area agriculture and industry were limited andmost rivers showed no signs o f pollution. Throughout there was abundant cover and suitable undisturbed sites for holts. It is suggested that there is little immediate threat to this apparently healthy otter population. Fish were the most important prey but crayfish were also important on small streams with silt deposits in slack water. A significant correlation was found between the number o f spraints per kilometre sample and the percentage occurrence of cra)fish in the spraints.
INTRODUCTION T h r o u g h o u t the Mediterranean region the otter L u t r a lutra has undergone substantial population declines in some countries, particularly Italy and France (Green & Green, 1981; Groupe Loutres, 1983; Macdonald & Mason, 1983), but is still widespread and numerous in others such as Greece, Albania and Portugal (Macdonald & Mason, 1982a,b; Prigioni et al., 1986). Information on the species' status in Yugoslavia is limited. The results of a preliminary survey in 1982 indicated that declines have occurred in intensively farmed areas but that some of the more remote river systems such 109 Biol. Conserv. 0006-3207/88/$03"50 © 1988 Elsevier Applied Science Publishers Ltd, England. Printed in Great Britain
110
I.R. Taylor et al.
as the Bosna and Drina may still support good populations (Liles & Jenkins, 1984). This paper reports the results of a detailed survey of the Drina catchment.
STUDY A R E A The study covered the major part of the Drina catchment from Vigegrad southwards, an area of approximately 14 500 km 2 (Fig. 1). The entire region is mountainous with limestone bedrock and human settlement is restricted mostly to the valley bottoms. Cultivation is confined to narrow, broken strips along the valleys and to higher altitude plateaux such as the areas north of Nik~i6 and around Zabljak and Sjenica. Most of the farming units are small family holdings worked on a part-time or subsistence basis. Mechanisation is limited, most work being done by traditional methods. Little use is made of agricultural chemicals. Most of the cultivated area is devoted to grazing and hay production for sheep and dairy cattle. Crops include maize and vegetables and orchards, especially of plum, are common. Many of the tributaries (e.g. Tara, Komarnica and Sutjeska) (Fig. 2) run through precipitous limestone gorges where cultivation is impossible. Most of the higher ground is forested with deciduous and pine species at lower altitudes and spruce at high altitudes. On all rivers this woodland extends down to the streamside wherever the slope is too steep or rocky for cultivation. Some rivers (e.g. Komarnica, Uvac) have been harnessed for hydroelectric power. The effect in the narrow gorges is to produce deep, steep-sided reservoirs extending up to 25 km upstream of the dams.
METHODS The study area was surveyed from 10 July to 23 August 1986. Attempts were made to sample the Drina and all its major tributaries systematically at 20 km intervals. This sampling interval was considered adequate in relation to current estimates of otter home range lengths in riverine habitat (Melquist & Hornocker, 1983; Green et al., 1984; Green & Green, in press). In practice, access was determined by the limited road network and the precipitous nature of the terrain. Several areas were also closed to foreigners. Sampling was thus as regular and as close as possible to the 20 km intervals intended. At each site 1000 m of waterway were searched for signs of otters by the team of five workers. For streams less than about 15 m wide both banks were searched but for wider streams access could be obtained to only one bank. In all cases searches were made from the water as well as up to 5 m from the
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111
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20
30
~0
50
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Fig. 2. Map of the Drina catchment showing locations of sample plots and the recorded distribution of otters.
The otter in Yugoslavia
113
water's edge. For each sample stretch records were kept of the nature and extent of the bankside vegetation, the river width and depth, the nature of the substrate, the characteristics of the surrounding vegetation and land-use and any obvious signs of water pollution. In addition to the sample stretches, spot checks were made under bridges wherever possible. Samples of spraints were collected for diet analysis.
RESULTS
Distribution A total of 42 one-kilometre sample sites was surveyed and otter spraints were found on 41 (97.6%). The single exception was a shallow stream of only about 1 m width which may simply have been too small (Fig. 2). The smallest streams on which spraints were found were about 2 m wide. Spraints were found on all of 9 kilometre samples on streams between 2 and 5 m wide and at 11 out of 14 spot sites. These were distributed throughout the catchment, suggesting that otters made widespread regular use of streams of this size. The mean number of spraints found per 200 m was 4.6 + 1-3 for streams less than 15m wide (searched on both banks) and 2.84 + 0-51 for larger streams up to 50 m wide (searched on one bank). The lowest number of spraints found, one per 1000 m in each case, occurred on the two sample sites on the R. Lim north of Bijelo Polje, per.haps indicating a reduced use of this area by otters. There was abundant woodland cover on both stretches but the river was heavily polluted (see below). Thirty-six spot sites, mostly at bridges, were also inspected primarily to obtain additional samples for diet analysis. Spraints were found at 24 (66%) of these butabsence of spraints at such sites cannot be taken as evidence of the absence of otters.
Habitat Most of the streams examined had clear unpolluted water. The main exception was the R. Lim north of Ivangrad where a large pulp mill discharged chemical waste into the river. Further inputs of pollutants occurred around Bijelo Polje and high sediment loads resulted from a number of gravel extraction sites. In all about 90 km of this river was affected by these inputs. Water pH was measured at 23 river sites, all of which were within the range of 7-0-8.2, as would be expected in a limestone area. However, hydroelectric reservoirs had considerably higher levels; those on the R. Uvac registered a pH of 9.2. Most reservoirs had algal blooms and
114
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dead fish were seen floating on several of them. They also produced large downstream fluctuations in water flow. In all small streams we observed an abundance of potential prey, particularly fish and crayfish where there was fine silt sediment. Snakes, mostly grass snake Natrix natrix, and frogs were also extremely common along river banks. From the number of anglers and their success, fish seemed numerous in most of the large rivers. All streams were flowing; there was no evidence of dried-up streams or disconnected pools holding high prey densities, as have been reported in Greece and Portugal during the summer (Macdonald & Mason, 1982a,b). T h r o u g h o u t the entire catchment there was extensive bankside vegetation. Forty (95.2%) of the kilometre samples had almost continuous tree cover along both banks. Eleven of these had entirely natural forest on both banks and a further 17 had forest along part of their length. All had highly broken ground on the slopes down to the stream with numerous large boulders offering abundant suitable sites for holts. The remaining 12 sample stretches flowed through more heavily cultivated areas but still had tree cover along their banks and extensive forest within 5 km at most along the stream. Diet
A total of 836 spraints were collected throughout the catchment and from them the frequency of occurrence of the major taxonomic groups of prey was determined. All spraints collected were fresh. Spring flood levels were always well above the positions where spraints were collected and this, along with known disappearance rates of spraints (Jenkins & Burrows, 1980; Mason & Macdonald, 1986), would suggest that the information obtained refers to the otters' diet between June and August. When data for the whole catchment are pooled it is clear that fish were the most frequently occurring item, remains being found in 64.1% of spraints. The only other significant prey type was crustacea, all crayfish Austropotamobius pallipes (Lereboullet), which occurred in 36-6% of spraints. Bird, amphibian and snake remains were found in only 0.3, 0.2 and 0.1% of the spraints respectively (Table 1). The relative occurrence offish and crayfish in spraints varied greatly from stream to stream. Comparing individual kilometre samples, the occurrence of crayfish ranged from 0 to 100%. These differences were analysed in relation to the stream substrate and width. The most important substrate characteristic was the presence or absence of fine silt in areas of slack water. For streams less than 20m wide crayfish occurred significantly more frequently where silt deposits were present than where they were not (Mann
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Whitney Test: U -- 10-5, n~ = 18, n 2 = 14, p < 0-002). When comparing all streams with silt present, crayfish occurred more frequently in spraints from small streams (less than 20 m wide) than from larger streams (Mann Whitney Test: U = 19, n~ = 18, n 2 = 14, p < 0-002). A notable feature of the spraints examined was the very low frequency (1.1%) of those containing both fish and crayfish remains. This is probably slightly exaggerated as small quantities of fine fish bones and small scales were difficult to detect amongst the compacted parts of crayfish exoskeleton in spraints and some may have been missed. However, it is possible that individual otters spent some of their time hunting where crayfish were the only prey available or that they concentrated on crayfish for short periods. Compared with fish, eating crayfish includes the ingestion of greater quantities of indigestible material in the form of exoskeleton. This should result in the production of higher volumes of faecal material manifest in either larger or greater numbers of spraints. If the latter held it would have implications for the use of spraint numbers as an index of otter abundance. For individual kilometre samples where we searched both banks we found a highly significant correlation between the total number of spraints and the percentage of these containing crayfish (Spearman rank correlation, r s = 0-65, n = 19, p < 0.01).
DISCUSSION Otters occurred throughout the entire Drina catchment, utilising rivers of up to 50 m width (Drina) and streams down to about 2 m wide. The total length of suitable waterway available within the catchment was thus of the order of about 1400 km. We were unable to inspect reservoirs for signs of otters as stopping was not permitted, but their nature was similar to those in central Portugal which were not used by otters (Macdonald & Mason, 1982b). High water pH, algal blooms, very deep water, steep banks and the presence of dead and dying fish all suggest that reservoirs within the Drina system would also be unsuitable. Potential prey appeared to be abundant on all rivers, with the probable exception of the Lira north of Bijelo Polje. Also cover and suitable undisturbed sites for holts were plentiful throughout. On all rivers a range size of as little as 6 km would have encompassed areas of natural forest on boulder-strewn slopes. Current levels and methods of farming offer no obvious threat to otters and it is unlikely that this situation will change in the foreseeable future. There is thus every indication that this area will continue to support a population of otters. Adjacent areas such as the Bosna catchment, which were not surveyed in the present study, seem to offer
The otter in Yugoslavia
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similar habitat so it seems likely that a substantial area of the Yugoslavian uplands may still have widespread otter populations. From our data it is not possible to deduce values for the actual abundance of otters within the Drina. The use of signs to determine abundance is under considerable debate and clearly in need of further research (Jefferies, 1986; Kruuk et al., 1986; Kruuk & Conroy, 1987; Mason & Macdonald, 1987). However, the values given here for sprainting intensity may prove useful as a baseline for future monitoring of the population and it may prove possible to determine population levels from them once the relationship between abundance and sprainting has been clarified. The analysis of diet from spraints indicated fish to be the most important item but also showed crayfish to be of significance, particularly on small streams with silt substrate. Similar utilisation of crayfish has been reported from other limestone areas (McFadden & Fairley, 1984; Callejo Rey & Delibes, in press). Crayfish occurred less frequently in the diet on larger rivers. This probably arose from a greater availability of fish on these rivers rather than a shortage of crayfish. Erlinge (1968) found that captive otters preferred fish to crayfish and McFadden & Fairley (1984) found a seasonal drop in the consumption of crayfish on the R. Abbert, Ireland, when migratory salmonids were abundant. Both snakes and amphibia were abundant along most fiver banks but featured insignificantly in spraints. This contrasts strongly with the results of other studies in southern Europe (e.g. Macdonald & Mason, 1982b; Simoes Graca & Ferrand de Almeida, 1983; Callejo Rey & Delibes, in press). These studies were in considerably drier areas than the Drina, with some streams drying up or being reduced to small pools during summer months. It is possible that snakes are not preferred prey but are taken when fish become less widely available. In most areas amphibians are taken less often during summer months than at other times. A significant correlation was demonstrated between the number of spraints found within kilometre samples and the percentage occurrence of crayfish in the spraints. It is suggested that this arose, at least in part, from the greater amount of indigestible material in crayfish. It can be argued that spraints deposited along a particular stretch need not necessarily derive from prey taken in that stretch. However, three streams that were sampled for spraints at frequent intervals showed consistency in spraint contents for distances of up to 20 km. Alternative explanations for the relationship are that otters made greater use of areas where crayfish were more abundant or that such areas were marked more frequently. From our data we cannot differentiate between these and our suggestion regarding the greater amount of indigestible material. However, all factors that affect spraint production need to be understood to clarify the relationship between sprainting
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intensity and otter numbers. The effect o f diet composition on spraint production seems not to be known and is clearly an area in need o f further research which should be possible using captive animals.
ACKNOWLEDGEMENT We thank the Vincent Wildlife Trust for funding this work.
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