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The Otter Lutra lutra in central Portugal
BiologicalConservation22 (1982) 207-215
THE OTTER L UTRA L UTRA IN CENTRAL PORTUGAL S. M. MACDONALD
The Vincent Wildli]e Trust, c/o Department of Zoology and Comparative Physiology, University o/ Birmingham, Birmingham, Great Britain & C. F. MASON
Department ol Biology, University of Essex, Wivenhoe Park, Colchester C04 35Q, Great Britain ABSTRACT
In a survey o f central Portugal during August 1980, otters w e r e / o u n d to be widespread with signs o/animals at 70 % o f the 90 stations visited. Otters were present on major rivers and their tributaries, on small streams connecting pools and at fetid pools in semi-dry river beds. The greatest densities of marking sites and signs were Jound aroundjetid pools. Snakes, as well as fish, were Jound to be important prey items. The survival, in good numbers, oj the otter in Portugal may be due inpart to the limited use of agricultural chemicals.
RESUMO
Durante uma inspecqdo no Centro de Portugal, no mds de Agosto de 1980, ver~'cou-se que as Iontras eram muito abundantes corn sinais de animais em 70°//o dos 90 locais visitados. As lontras encontravam-se nos rios principais e seus afluentes, nos ribeiros que ligavam a polos e n o s charcos no leito dos rios secos. Ver~'cou-se que a maior densidade de lugares marcados e de sitios se encontrava nos charcos. As presas preJeridas eram ndo so as cobras mas tambem os peixes. A sobrevivdncia, em elevado numero, das Iontras em Portugalpode ser devida em parle ao uso limitado de produtos quimicos na agricultura.
INTRODUCTION
In recent years the otter Lutra lutra L. has suffered a severe decline in numbers throughout large areas of Europe, e.g. in the Netherlands (Veen, 1975), Switzerland (MOiler etal., 1976), Spain (Aritio, 1978), England (Joint Otter Group, 1979) and 207
Biol. Conserr. 0006-3207/82/0022-0207/$02.75 ,i Applied Science Publishers Ltd, England, 1982 Printed in Great Britain
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S. M. MACDONALD, C. F. MASON
West Germany (Reuther, 1980). The overall situation and the possible reasons for this decline have been reviewed and discussed by von Miiflling (1977) and Reuther (1977). Portugal is one of the European countries for which there is no published information on the status of the otter (von Miiffling, 1977). Since the size of the country offered the possibility of covering a representative area within the time available, a survey of otters in central Portugal was undertaken. Because degradation I of habitat has been cited as a cause of the decline of the otter, distribution was also investigated in relation to different types of habitat.
THE STUDY AREA
Central Portugal comprises hills to the north of the River Tejo and rolling plains to the south. The north of the region is dominated by pine forests used for timber and for turpentine production and Eucalyptus is also widely grown. There are many vineyards and the alluvial soils in the narrow valleys are cultivated for maize, beans and other vegetables. The plains of the southern Alentejo are the main wheatproducing area of the country and this region is also rich in groves of olive and cork oak. In the lower reaches of the rivers, east of Lisboa, rice and tomatoes are extensively grown. The gross domestic product and agricultural yields in Portugal are the lowest in Western Europe (Keefe, 1977). While soils and climatic conditions are not ideal, the low yields may, in part, be due to the limited use of fertilizers and pesticides. Industry in Portugal is largely concentrated around Lisboa and Porto, but some mining of iron and tungsten occurs within the study area. The major rivers and their tributaries are harnessed for hydro-electric power and there are few natural lakes. The rivers are important for irrigation, with high demands for water from crops such as rice and tomatoes. All waters in central Portugal are also of great social importance for, since there is little piped water in rural areas, rivers, streams and pools are used very intensively for washing.
METHODS
The survey was carried out in central Portugal between 17 and 29 August 1980. In the short time available an attempt was made to cover as large an area as possible. Stations were selected for ease of access, e.g. bridges or places where a river ran close to a road, so that station selection was not random. At most stations a minimum of 200 m was searched for signs of otters, viz. spraints (faeces) and footprints, while notes were made on habitat at all stations. On a very few occasions spot checks only were made due to access difficulties. Signs were presumed to be absent from a station when a search of 600 m proved fruitless.
THE OTTER Lutra lutra IN CENTRAL PORTUGAL
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A sample o f spraints was collected from stations distributed throughout the study area. Spraints were washed through a fine sieve and air-dried for examination.
RESUL TS
Distribution and number
O~ I otter signs A total o f 90 stations, including rivers, streams and reservoirs was surveyed (see Fig. 1) and 63 (70 ~) produced evidence of otters. M a n y small rivers in central
PORTO
~rOOu.,
N 50 km
t
\
i R Z~zere
i
R Tejo
p._ :
L--. R Sorra~a
Fig, 1. The study area in central Portugal showing positive (0) and negative (O) stations. Asterisks indicate bisection lines (see text).
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S. M. MACDONALD, C. F. MASON
Portugal become dry during the summer and a further 16 such stations were visited, but since signs of otter were never found, they are not considered here as potential habitat for otters. The survey area can be equally bisected into northern and southern halves and into eastern and western halves, these bisections being indicated by asterisks in Fig. 1. In the north 75 ~o of 44 stations proved positive for otters, compared with 65.2 ~o of 46 stations in the south. Of the 43 stations to the east of a north-south line 72 ~ proved positive, compared with 68 ~o of 47 western stations. These differences are not significant using a X2 test, indicating a general distribution throughout the study area. To compare densities of sprainting sites and of numbers of individual spraints (signs) found at various stations the data have been standardized to numbers per 200 m. The six positive stations where spot checks only were made, together with one station where footprints only were found, have been omitted from these calculations. The number of otter signs include spraints and anal secretions, but not footprints. Over the entire study area an average of 3.4 marked sites and an average of 6.9 signs were found per 200 m. In the northern half of the area an average of 3.8 sites and 8.5 signs was found, while in the south there was an average of 3.0 sites with 5-2 signs. There was no significant difference in the number of sites (t = 0.78, p > 0.05) or the number of signs (t = 1.62, p > 0.05) between north and south. In the eastern half of the region we found an average of 4.8 sites per 200m, with 9.5 signs, compared with 2.0 sites and 4.2 signs in the west. The differences in the number of sites (t = 3.12, p < 0.01) and signs (t = 2.62, p < 0.05) between east and west are significant, the area nearer to the Spanish border therefore producing more evidence of otter activity. The results were considered in relation to types of habitat. At 27 stations no signs of otters were found. Of these, four were on major rivers, viz. the Guadiana, Tejo and Z6zere. A further six stations were hydro-electric dams or reservoirs which offered no shelter to otters. At five of the negative stations the level of pollution was clearly sufficient to affect food supplies but at the remaining 12 stations there was no obvious reason for the apparent absence of the otter. The negative stations were distributed throughout the study area. It is difficult to estimate the level of sprainting which indicates a healthy population of otters on Portuguese rivers, so we can only make comparisons with the situation in Britain. On the River Wye, Wales, for example, we surveyed a 5 km site in July 1980 which was considered to hold a viable population of otters and where signs of cubs were found. An average of two sites and four signs per 200 m was found and these data are here used as a yardstick for comparison with the Portuguese results. A total of 20 stations was found with less than two sites and less than four signs per 200 m and these, therefore, represent the poorest of the positive stations. Of these,
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one, on the Z6zere, was clearly polluted since no signs of water life were found in a 600 m search. At a further five stations there was little or no cover or potential for holt sites. However, at 14 of the stations there was no obvious reason for the dearth of otter signs. The habitats in which otter were found comprised four main categories: (1) large, deep rivers; (2) rivers of 2-10 m width; (3) streams of less than 2 m width, connecting pools and often flowing through agricultural plots and (4) fetid pools in partially dry river beds. Of the 10 most heavily marked stations found, i.e. with five or more sites and ten or more signs per 200 m, six fell within Category 4 and four in Category 3. The average number of marking sites and signs per 200 m is shown for each of the four habitat types in Table 1. Two stations, where holts were found with large accumulations of spraint at their entrances, have been omitted from the calculations. Analysis of variance showed that the number of sites and signs in the categories were significantly different (F = 5.07, p < 0-05 for sites; F = 3"81, p < 0'05 for signs). Comparing nearest neighbours, using Student's t, differences between fetid pools (Category 4) and running streams (Category 3) were significant for both sites (t = 2.25, p < 0.05) and signs (t = 5.26, p < 0.001). No other differences were significant. In view of these results it seemed possible that the apparent preference by otters for the eastern half of the study area may have been influenced by the number of pools surveyed in the east. Indeed, of the 13 stations where pools occurred, 12 were in the east. Comparisons were therefore made between eastern and western stations comprising small streams (Category 3), this habitat most closely resembling fetid pools (Category 4). There was no significant difference between the number of signs found (t = 1.6, p > 0-05) but the eastern streams produced a significantly greater number of marking sites (t = 2.39, p < 0.05). TABLE 1 THE NUMBER OF SITES AND SIGNS ( m e a n ___SE) OF OTTERS PER 200 m RECORDED ON FOUR CATEGORIESOF RIVER IN CENTRAL PORTUGAL
Category
1 Large rivers
2 Rivers 2-10m
3 Running streams <2m
Sites Signs No. stations
1.5 _+ 0.5 2.9 + 1.5 8
2,8 + 0.4 4.5 + 1.1 15
3.0 ± 0.7 6.1 + 1.5 19
4 Fetid pools 6.4 __+ 1.7 12.0 __+3.2 12
Observations on the Jbod oj the otter Of the 758 spraints found during the survey, 50 were collected from 16 stations distributed throughout the study area. Remains of snake, most probably Natrix maura, were found in 18 spraints (36 ~).collected from 11 stations. Remains of pond terrapin (Emys orbieularis and/or Mauremys caspica) were found in two spraints
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and amphibian bones in three. All 50 spraints contained fish bones or scales. Coleopteran and dipteran remains found in three of the spraints may have derived from prey species of otters. In most small rivers and streams and in unpolluted pools we observed an abundance of potential prey for otters. This comprised frogs (Discoglossus pictus, Rana ridibunda), pond terrapins (Emys orbicularis, Mauremys caspica) and water snakes (Natrix maura). Moorhens Gallinula chloropus were seen at only three stations. Feeding could pose no difficulties for otters in shallow waters since there was an abundance of coarse fish and, in pools, fishes, including carp Cyprinus carpio up to 30cm in length could be caught by hand.
Additional observations Otters in central Portugal do not appear to favour bridges as sprainting sites as they do in Britain. Although many bridges had ledges, boulders or washing stones beneath them, few spraints were found there. Occasional spraints were found on sand or on logs but most of the sprainting sites were on boulders. At many stations piles of boulders offered the only visible potential holt sites. Two probable holts were found in piles of large rocks above high-water level and were heavily marked with spraints at the entrances. No bankside trees were found with root systems suitable for holt sites. The majority of trees lining rivers in the study area were alder A Inus glutinosa, which were frequently coppiced, and aspen Populus tremula. At many sites the bankside scrub of brambles Rubus spp., A rundo donax and Typha latijolia was dense enough to offer refuge to otters.
DISCUSSION
The otter appears to be widespread in central Portugal although slightly less evidence of the animal was found in the west, It is also known to occur, and may even be more numerous, in the rivers of northern Portugal (V. Oliveira, pets. comm.). Otters are also present in the coastal region south of Lisboa (e.g. Rio Sado; V. Oliveira, pets. comm.), but we have no information for the coastal stretch between Lisboa and Porto, or for the south of the country (Algarve). In central Portugal, all types of freshwater habitat are utilized by otters, with the exception of H E P reservoirs, which are very deep and steep-sided, offering little bank-side cover or potential holt sites, and with fluctuating water levels. The depth of some of the major rivers, e.g. the Tejo, Mondego and Z6zere, may also limit the otters ability to feed and in some stations otter signs were absent or sparse. However, these rivers possess numerous backwaters and tributary streams where feeding would be easier. At 12 stations visited and found to be negative and at a further 14 localities where relatively few signs were found, we could find no clear reason for the unsuitability of
THE OTTER
Lulra lulra
IN CENTRAL PORTUGAL
213
the habitat. It may be that, locally, otter numbers are being reduced through trapping for, while the animal is protected in Portugal, otters are still taken for their pelts (V. Oliviera, pers. comm.). Alternatively the otter may have begun to decline in Portugal, as appears to be happening in Spain (Aritio, 1978). The relative abundance of the otter, however, contrasts with the situation in much of the rest of western Europe. Chanin & Jefferies (1978) suggested that, in England, the initial decline could be attributed to the widespread introduction of the persistent pesticide dieldrin and it may well be significant that little use is made of pesticides in Portugal. Food supplies could not be limiting to otters since an abundance of potential prey items was observed at most stations. The frequency of snake and paucity of amphibian remains in spraints may reflect the relative ease of capture of different species since frogs were present in high numbers at most localities. Fetid pools provided greatest ease of feeding and it was here that the highest density of marking sites and spraints was found. It has been implied elsewhere (Green & Green, 1980) that a higher number of signs indicates a greater number of otters present. This may not hold true in some Portuguese habitats. Fetid pools hold, temporarily, a high number of 'captive' prey items, bat with advancing summer, desiccation reduces the number of pools. We found pools where fishes were dying due to deoxygenation of the water. The extraction of water for irrigation, which was especially intense in the rice-growing areas of the west, may exacerbate this situation. It seemed unlikely that such habitat could hold high numbers of otters, although otters may be forced to concentrate at a dwindling number of pools. Heavy marking at such places may, therefore, indicate the defence of a resource which is becoming increasingly limited. Over-exploitation of pools in summer may also lead to poorer prey densities the following winter. It is possible that the high number of spraints found at pools simply indicated that otters spent longer at such sites or that faeces were, of necessity, deposited on land. However, at the many stations where a tiny stream connected pools, the marking level was significantly lower, although little additional water was available. Since pools with a throughflow of water were less liable to desiccation, the need for defence of resources may have been less acute. Clearly the territories of many otters must alter dramatically with season. Dry river beds, with scattered pools, become torrents in the winter and such changes in the habitat and in the availability of resources may well affect the animal's behaviour. Encounters between individuals may be more frequent in the summer when only a limited number of feeding sites can be visited or, alternatively, the seasonal increase in water may enhance movement. Potential holt sites, in the form of piles of boulders, were found in all types of utilized habitat and the two holts found were reminiscent of holts on the Scottish west coast (Macdonald & Mason, 1980). We found no suitable holt sites in the root systems of trees, a frequently used habitat in English rivers (Macdonald et al., 1978),
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but V. Oliveira (pers. comm.) has found possible tree bolts in other parts of the country. Despite an abundance of potential holt sites on some of the major rivers e.g. the Mondego, otter signs were few. Industrial or mining pollution may pose problems. While there was little visible evidence of industrial pollution, many smaller streams were clearly affected by domestic sewage, although signs of otters were found at such places. After heavy rain, some streams became overloaded with raw sewage and pond terrapins were observed to be under stress. While breakdown of sewage may be rapid in high temperatures, the slow rate of flow of many such streams suggested that local mortalities of fishes may occur. H u m a n disturbance is difficult to evaluate but in Portuguese rivers it was undoubtedly greater than on most British rural waterways. It was common to find up to 40 people at one place using the river for washing and bathing. Such activities led to heavy loads of detergents being released and these accumulated in streams and pools. Sand and gravel extraction direct from dried river beds also caused day-time disturbance and local pollution at many localities. Gypsy encampments, frequently extensive, were often encountered on river banks and these would cause disturbance at night as well as by day. However, high numbers of signs of otters were found at many stations where human disturbance was great. Feral dogs are abundant in Portugal, were often seen on rivers and, by competition or predation could disturb otters. It is likely that otters in central Portugal are entirely nocturnal. Most sources of disturbance to otters are not, however, of recent origin and their importance should not be magnified. A greater potential threat lies in the possible entry of Portugal to the European Economic Community. The yield of grains in Portugal is at present only 25 ~o of the average for EEC countries (Keefe, 1977) and if outputs are to be increased a much greater use of pesticides is likely. Management of waterways will conform to statutory standards and land use may alter. Such changes in Portugal could bode ill for the otter population, which is of considerable importance in view of the widespread decline in numbers elsewhere in Europe. ACKNOWLEDGEMENTS
We wish to thank Dr Jofio Bugalho and Dr Vasco Oliveira of the Serviqo de Inspecq~o da Caqa e Pesca in Lisboa for their help and advice. Dr Nigel Collar kindly helped us with contacts in Portugal and we are indebted to the Hon. Vincent Weir for financial aid.
REFERENCES ARITIO, L. B. (1978), Informesobre la situacion de la nutria en Espafia. In Otters." Proceedings oj thefirst working meeting oj the otter specialist group, ed. by N. Duplaix, 140-2. Morges, IUCN. CHANIN, P. R. F. 8£JEFFERIES,D. J. (1978). The decline of the otter Lutra lutra L. in Britain: an analysis of hunting records and discussion of causes. Biol. J. Lin. Soc., 10, 305-28.
THE OTTER L u l r a l u l r a IN CENTRAL PORTUGAL
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GREEN, J. & GREEN, R. (1980). Otter survey of Scotland 1977-79. London, Vincent Wildlife Trust. JOINT OTTER GROUP (1979). Otters, 1979, the second report of the Joint Otter Group. Lincoln, Society for the Promotion of Nature Conservation. KEEFE, E. K. (1977). Area handbook for Portugal. Washington, The American University. MACDONALD, S. & MASON,C. F. (1980). Observations on the marking behaviour of a coastal population of otters. Acta Theriol., 25, 245-53. MACDONALD, S., MASON,C. F. & COG,ILL, I. S. (1978). The otter and its conservation in the River Teme catchment. J. appl. Ecol., 15, 373-84. MULLER, H., MARTIN, C. & DIETHELM, P. (1976). La loutre. Sa presence, ses conditions d'existence, sa conservation en Suisse. Zurich, University of Zurich. REUTHER,C. (1977). Der Fischotter, Lutra lutra (Linne, 1758). Biologic, Status und Schutz an Beispiel des Harzes. Mitt. Erganzungsstudium Okol. Umwehsicherung, 3, 1-180. REUTHER, C. (1980). Der Fischotter, Lutra lutra L. in Niedersachsen. Naturschutz undLandscha/lspflege in Niedersachsen, 11, 1-182. VEEN, J. (1975). Het voorkomen en enige gedragsverschijnselen van de visotter, Lutra lutra (Linnaeus, 1753) in Noord-Holland. Lutra, 17, 21-37. YON MfJEELIrqG,S. F. (1977). Fischotter in Europa. Loizenkirchen, Internationaler Jagdrat zur Erhaltung des Wildes.
THE OTTER L UTRA L UTRA IN CENTRAL PORTUGAL S. M. MACDONALD
The Vincent Wildli]e Trust, c/o Department of Zoology and Comparative Physiology, University o/ Birmingham, Birmingham, Great Britain & C. F. MASON
Department ol Biology, University of Essex, Wivenhoe Park, Colchester C04 35Q, Great Britain ABSTRACT
In a survey o f central Portugal during August 1980, otters w e r e / o u n d to be widespread with signs o/animals at 70 % o f the 90 stations visited. Otters were present on major rivers and their tributaries, on small streams connecting pools and at fetid pools in semi-dry river beds. The greatest densities of marking sites and signs were Jound aroundjetid pools. Snakes, as well as fish, were Jound to be important prey items. The survival, in good numbers, oj the otter in Portugal may be due inpart to the limited use of agricultural chemicals.
RESUMO
Durante uma inspecqdo no Centro de Portugal, no mds de Agosto de 1980, ver~'cou-se que as Iontras eram muito abundantes corn sinais de animais em 70°//o dos 90 locais visitados. As lontras encontravam-se nos rios principais e seus afluentes, nos ribeiros que ligavam a polos e n o s charcos no leito dos rios secos. Ver~'cou-se que a maior densidade de lugares marcados e de sitios se encontrava nos charcos. As presas preJeridas eram ndo so as cobras mas tambem os peixes. A sobrevivdncia, em elevado numero, das Iontras em Portugalpode ser devida em parle ao uso limitado de produtos quimicos na agricultura.
INTRODUCTION
In recent years the otter Lutra lutra L. has suffered a severe decline in numbers throughout large areas of Europe, e.g. in the Netherlands (Veen, 1975), Switzerland (MOiler etal., 1976), Spain (Aritio, 1978), England (Joint Otter Group, 1979) and 207
Biol. Conserr. 0006-3207/82/0022-0207/$02.75 ,i Applied Science Publishers Ltd, England, 1982 Printed in Great Britain
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S. M. MACDONALD, C. F. MASON
West Germany (Reuther, 1980). The overall situation and the possible reasons for this decline have been reviewed and discussed by von Miiflling (1977) and Reuther (1977). Portugal is one of the European countries for which there is no published information on the status of the otter (von Miiffling, 1977). Since the size of the country offered the possibility of covering a representative area within the time available, a survey of otters in central Portugal was undertaken. Because degradation I of habitat has been cited as a cause of the decline of the otter, distribution was also investigated in relation to different types of habitat.
THE STUDY AREA
Central Portugal comprises hills to the north of the River Tejo and rolling plains to the south. The north of the region is dominated by pine forests used for timber and for turpentine production and Eucalyptus is also widely grown. There are many vineyards and the alluvial soils in the narrow valleys are cultivated for maize, beans and other vegetables. The plains of the southern Alentejo are the main wheatproducing area of the country and this region is also rich in groves of olive and cork oak. In the lower reaches of the rivers, east of Lisboa, rice and tomatoes are extensively grown. The gross domestic product and agricultural yields in Portugal are the lowest in Western Europe (Keefe, 1977). While soils and climatic conditions are not ideal, the low yields may, in part, be due to the limited use of fertilizers and pesticides. Industry in Portugal is largely concentrated around Lisboa and Porto, but some mining of iron and tungsten occurs within the study area. The major rivers and their tributaries are harnessed for hydro-electric power and there are few natural lakes. The rivers are important for irrigation, with high demands for water from crops such as rice and tomatoes. All waters in central Portugal are also of great social importance for, since there is little piped water in rural areas, rivers, streams and pools are used very intensively for washing.
METHODS
The survey was carried out in central Portugal between 17 and 29 August 1980. In the short time available an attempt was made to cover as large an area as possible. Stations were selected for ease of access, e.g. bridges or places where a river ran close to a road, so that station selection was not random. At most stations a minimum of 200 m was searched for signs of otters, viz. spraints (faeces) and footprints, while notes were made on habitat at all stations. On a very few occasions spot checks only were made due to access difficulties. Signs were presumed to be absent from a station when a search of 600 m proved fruitless.
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A sample o f spraints was collected from stations distributed throughout the study area. Spraints were washed through a fine sieve and air-dried for examination.
RESUL TS
Distribution and number
O~ I otter signs A total o f 90 stations, including rivers, streams and reservoirs was surveyed (see Fig. 1) and 63 (70 ~) produced evidence of otters. M a n y small rivers in central
PORTO
~rOOu.,
N 50 km
t
\
i R Z~zere
i
R Tejo
p._ :
L--. R Sorra~a
Fig, 1. The study area in central Portugal showing positive (0) and negative (O) stations. Asterisks indicate bisection lines (see text).
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S. M. MACDONALD, C. F. MASON
Portugal become dry during the summer and a further 16 such stations were visited, but since signs of otter were never found, they are not considered here as potential habitat for otters. The survey area can be equally bisected into northern and southern halves and into eastern and western halves, these bisections being indicated by asterisks in Fig. 1. In the north 75 ~o of 44 stations proved positive for otters, compared with 65.2 ~o of 46 stations in the south. Of the 43 stations to the east of a north-south line 72 ~ proved positive, compared with 68 ~o of 47 western stations. These differences are not significant using a X2 test, indicating a general distribution throughout the study area. To compare densities of sprainting sites and of numbers of individual spraints (signs) found at various stations the data have been standardized to numbers per 200 m. The six positive stations where spot checks only were made, together with one station where footprints only were found, have been omitted from these calculations. The number of otter signs include spraints and anal secretions, but not footprints. Over the entire study area an average of 3.4 marked sites and an average of 6.9 signs were found per 200 m. In the northern half of the area an average of 3.8 sites and 8.5 signs was found, while in the south there was an average of 3.0 sites with 5-2 signs. There was no significant difference in the number of sites (t = 0.78, p > 0.05) or the number of signs (t = 1.62, p > 0.05) between north and south. In the eastern half of the region we found an average of 4.8 sites per 200m, with 9.5 signs, compared with 2.0 sites and 4.2 signs in the west. The differences in the number of sites (t = 3.12, p < 0.01) and signs (t = 2.62, p < 0.05) between east and west are significant, the area nearer to the Spanish border therefore producing more evidence of otter activity. The results were considered in relation to types of habitat. At 27 stations no signs of otters were found. Of these, four were on major rivers, viz. the Guadiana, Tejo and Z6zere. A further six stations were hydro-electric dams or reservoirs which offered no shelter to otters. At five of the negative stations the level of pollution was clearly sufficient to affect food supplies but at the remaining 12 stations there was no obvious reason for the apparent absence of the otter. The negative stations were distributed throughout the study area. It is difficult to estimate the level of sprainting which indicates a healthy population of otters on Portuguese rivers, so we can only make comparisons with the situation in Britain. On the River Wye, Wales, for example, we surveyed a 5 km site in July 1980 which was considered to hold a viable population of otters and where signs of cubs were found. An average of two sites and four signs per 200 m was found and these data are here used as a yardstick for comparison with the Portuguese results. A total of 20 stations was found with less than two sites and less than four signs per 200 m and these, therefore, represent the poorest of the positive stations. Of these,
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one, on the Z6zere, was clearly polluted since no signs of water life were found in a 600 m search. At a further five stations there was little or no cover or potential for holt sites. However, at 14 of the stations there was no obvious reason for the dearth of otter signs. The habitats in which otter were found comprised four main categories: (1) large, deep rivers; (2) rivers of 2-10 m width; (3) streams of less than 2 m width, connecting pools and often flowing through agricultural plots and (4) fetid pools in partially dry river beds. Of the 10 most heavily marked stations found, i.e. with five or more sites and ten or more signs per 200 m, six fell within Category 4 and four in Category 3. The average number of marking sites and signs per 200 m is shown for each of the four habitat types in Table 1. Two stations, where holts were found with large accumulations of spraint at their entrances, have been omitted from the calculations. Analysis of variance showed that the number of sites and signs in the categories were significantly different (F = 5.07, p < 0-05 for sites; F = 3"81, p < 0'05 for signs). Comparing nearest neighbours, using Student's t, differences between fetid pools (Category 4) and running streams (Category 3) were significant for both sites (t = 2.25, p < 0.05) and signs (t = 5.26, p < 0.001). No other differences were significant. In view of these results it seemed possible that the apparent preference by otters for the eastern half of the study area may have been influenced by the number of pools surveyed in the east. Indeed, of the 13 stations where pools occurred, 12 were in the east. Comparisons were therefore made between eastern and western stations comprising small streams (Category 3), this habitat most closely resembling fetid pools (Category 4). There was no significant difference between the number of signs found (t = 1.6, p > 0-05) but the eastern streams produced a significantly greater number of marking sites (t = 2.39, p < 0.05). TABLE 1 THE NUMBER OF SITES AND SIGNS ( m e a n ___SE) OF OTTERS PER 200 m RECORDED ON FOUR CATEGORIESOF RIVER IN CENTRAL PORTUGAL
Category
1 Large rivers
2 Rivers 2-10m
3 Running streams <2m
Sites Signs No. stations
1.5 _+ 0.5 2.9 + 1.5 8
2,8 + 0.4 4.5 + 1.1 15
3.0 ± 0.7 6.1 + 1.5 19
4 Fetid pools 6.4 __+ 1.7 12.0 __+3.2 12
Observations on the Jbod oj the otter Of the 758 spraints found during the survey, 50 were collected from 16 stations distributed throughout the study area. Remains of snake, most probably Natrix maura, were found in 18 spraints (36 ~).collected from 11 stations. Remains of pond terrapin (Emys orbieularis and/or Mauremys caspica) were found in two spraints
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and amphibian bones in three. All 50 spraints contained fish bones or scales. Coleopteran and dipteran remains found in three of the spraints may have derived from prey species of otters. In most small rivers and streams and in unpolluted pools we observed an abundance of potential prey for otters. This comprised frogs (Discoglossus pictus, Rana ridibunda), pond terrapins (Emys orbicularis, Mauremys caspica) and water snakes (Natrix maura). Moorhens Gallinula chloropus were seen at only three stations. Feeding could pose no difficulties for otters in shallow waters since there was an abundance of coarse fish and, in pools, fishes, including carp Cyprinus carpio up to 30cm in length could be caught by hand.
Additional observations Otters in central Portugal do not appear to favour bridges as sprainting sites as they do in Britain. Although many bridges had ledges, boulders or washing stones beneath them, few spraints were found there. Occasional spraints were found on sand or on logs but most of the sprainting sites were on boulders. At many stations piles of boulders offered the only visible potential holt sites. Two probable holts were found in piles of large rocks above high-water level and were heavily marked with spraints at the entrances. No bankside trees were found with root systems suitable for holt sites. The majority of trees lining rivers in the study area were alder A Inus glutinosa, which were frequently coppiced, and aspen Populus tremula. At many sites the bankside scrub of brambles Rubus spp., A rundo donax and Typha latijolia was dense enough to offer refuge to otters.
DISCUSSION
The otter appears to be widespread in central Portugal although slightly less evidence of the animal was found in the west, It is also known to occur, and may even be more numerous, in the rivers of northern Portugal (V. Oliveira, pets. comm.). Otters are also present in the coastal region south of Lisboa (e.g. Rio Sado; V. Oliveira, pets. comm.), but we have no information for the coastal stretch between Lisboa and Porto, or for the south of the country (Algarve). In central Portugal, all types of freshwater habitat are utilized by otters, with the exception of H E P reservoirs, which are very deep and steep-sided, offering little bank-side cover or potential holt sites, and with fluctuating water levels. The depth of some of the major rivers, e.g. the Tejo, Mondego and Z6zere, may also limit the otters ability to feed and in some stations otter signs were absent or sparse. However, these rivers possess numerous backwaters and tributary streams where feeding would be easier. At 12 stations visited and found to be negative and at a further 14 localities where relatively few signs were found, we could find no clear reason for the unsuitability of
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Lulra lulra
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the habitat. It may be that, locally, otter numbers are being reduced through trapping for, while the animal is protected in Portugal, otters are still taken for their pelts (V. Oliviera, pers. comm.). Alternatively the otter may have begun to decline in Portugal, as appears to be happening in Spain (Aritio, 1978). The relative abundance of the otter, however, contrasts with the situation in much of the rest of western Europe. Chanin & Jefferies (1978) suggested that, in England, the initial decline could be attributed to the widespread introduction of the persistent pesticide dieldrin and it may well be significant that little use is made of pesticides in Portugal. Food supplies could not be limiting to otters since an abundance of potential prey items was observed at most stations. The frequency of snake and paucity of amphibian remains in spraints may reflect the relative ease of capture of different species since frogs were present in high numbers at most localities. Fetid pools provided greatest ease of feeding and it was here that the highest density of marking sites and spraints was found. It has been implied elsewhere (Green & Green, 1980) that a higher number of signs indicates a greater number of otters present. This may not hold true in some Portuguese habitats. Fetid pools hold, temporarily, a high number of 'captive' prey items, bat with advancing summer, desiccation reduces the number of pools. We found pools where fishes were dying due to deoxygenation of the water. The extraction of water for irrigation, which was especially intense in the rice-growing areas of the west, may exacerbate this situation. It seemed unlikely that such habitat could hold high numbers of otters, although otters may be forced to concentrate at a dwindling number of pools. Heavy marking at such places may, therefore, indicate the defence of a resource which is becoming increasingly limited. Over-exploitation of pools in summer may also lead to poorer prey densities the following winter. It is possible that the high number of spraints found at pools simply indicated that otters spent longer at such sites or that faeces were, of necessity, deposited on land. However, at the many stations where a tiny stream connected pools, the marking level was significantly lower, although little additional water was available. Since pools with a throughflow of water were less liable to desiccation, the need for defence of resources may have been less acute. Clearly the territories of many otters must alter dramatically with season. Dry river beds, with scattered pools, become torrents in the winter and such changes in the habitat and in the availability of resources may well affect the animal's behaviour. Encounters between individuals may be more frequent in the summer when only a limited number of feeding sites can be visited or, alternatively, the seasonal increase in water may enhance movement. Potential holt sites, in the form of piles of boulders, were found in all types of utilized habitat and the two holts found were reminiscent of holts on the Scottish west coast (Macdonald & Mason, 1980). We found no suitable holt sites in the root systems of trees, a frequently used habitat in English rivers (Macdonald et al., 1978),
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but V. Oliveira (pers. comm.) has found possible tree bolts in other parts of the country. Despite an abundance of potential holt sites on some of the major rivers e.g. the Mondego, otter signs were few. Industrial or mining pollution may pose problems. While there was little visible evidence of industrial pollution, many smaller streams were clearly affected by domestic sewage, although signs of otters were found at such places. After heavy rain, some streams became overloaded with raw sewage and pond terrapins were observed to be under stress. While breakdown of sewage may be rapid in high temperatures, the slow rate of flow of many such streams suggested that local mortalities of fishes may occur. H u m a n disturbance is difficult to evaluate but in Portuguese rivers it was undoubtedly greater than on most British rural waterways. It was common to find up to 40 people at one place using the river for washing and bathing. Such activities led to heavy loads of detergents being released and these accumulated in streams and pools. Sand and gravel extraction direct from dried river beds also caused day-time disturbance and local pollution at many localities. Gypsy encampments, frequently extensive, were often encountered on river banks and these would cause disturbance at night as well as by day. However, high numbers of signs of otters were found at many stations where human disturbance was great. Feral dogs are abundant in Portugal, were often seen on rivers and, by competition or predation could disturb otters. It is likely that otters in central Portugal are entirely nocturnal. Most sources of disturbance to otters are not, however, of recent origin and their importance should not be magnified. A greater potential threat lies in the possible entry of Portugal to the European Economic Community. The yield of grains in Portugal is at present only 25 ~o of the average for EEC countries (Keefe, 1977) and if outputs are to be increased a much greater use of pesticides is likely. Management of waterways will conform to statutory standards and land use may alter. Such changes in Portugal could bode ill for the otter population, which is of considerable importance in view of the widespread decline in numbers elsewhere in Europe. ACKNOWLEDGEMENTS
We wish to thank Dr Jofio Bugalho and Dr Vasco Oliveira of the Serviqo de Inspecq~o da Caqa e Pesca in Lisboa for their help and advice. Dr Nigel Collar kindly helped us with contacts in Portugal and we are indebted to the Hon. Vincent Weir for financial aid.
REFERENCES ARITIO, L. B. (1978), Informesobre la situacion de la nutria en Espafia. In Otters." Proceedings oj thefirst working meeting oj the otter specialist group, ed. by N. Duplaix, 140-2. Morges, IUCN. CHANIN, P. R. F. 8£JEFFERIES,D. J. (1978). The decline of the otter Lutra lutra L. in Britain: an analysis of hunting records and discussion of causes. Biol. J. Lin. Soc., 10, 305-28.
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GREEN, J. & GREEN, R. (1980). Otter survey of Scotland 1977-79. London, Vincent Wildlife Trust. JOINT OTTER GROUP (1979). Otters, 1979, the second report of the Joint Otter Group. Lincoln, Society for the Promotion of Nature Conservation. KEEFE, E. K. (1977). Area handbook for Portugal. Washington, The American University. MACDONALD, S. & MASON,C. F. (1980). Observations on the marking behaviour of a coastal population of otters. Acta Theriol., 25, 245-53. MACDONALD, S., MASON,C. F. & COG,ILL, I. S. (1978). The otter and its conservation in the River Teme catchment. J. appl. Ecol., 15, 373-84. MULLER, H., MARTIN, C. & DIETHELM, P. (1976). La loutre. Sa presence, ses conditions d'existence, sa conservation en Suisse. Zurich, University of Zurich. REUTHER,C. (1977). Der Fischotter, Lutra lutra (Linne, 1758). Biologic, Status und Schutz an Beispiel des Harzes. Mitt. Erganzungsstudium Okol. Umwehsicherung, 3, 1-180. REUTHER, C. (1980). Der Fischotter, Lutra lutra L. in Niedersachsen. Naturschutz undLandscha/lspflege in Niedersachsen, 11, 1-182. VEEN, J. (1975). Het voorkomen en enige gedragsverschijnselen van de visotter, Lutra lutra (Linnaeus, 1753) in Noord-Holland. Lutra, 17, 21-37. YON MfJEELIrqG,S. F. (1977). Fischotter in Europa. Loizenkirchen, Internationaler Jagdrat zur Erhaltung des Wildes.