Riverbank assessment and management for the Eurasian otter in the north of Luxembourg

© Koninklijke Brill NV, Leiden, 2008 DOI 10.1163/157075608X383728

Animal Biology 58 (2008) 473–490 www.brill.nl/ab

Riverbank assessment and management for the Eurasian otter in the north of Luxembourg

Gérardt Schmidt* , Lionel L’Hoste , Alain Dohet , Aurore Boscher , Henry-Michel Cauchie , and Lucien Hoff mann

EVA-Département Environnement et Agro-biotechnologies, Centre de Recherche Public - Gabriel Lippmann, rue du Brill, 41, 4422 Luxembourg

Abstract Th e presence of habitats such as burrows and dense willows along the rivers is very important for the survival of the European otter in terms of structures allowing quiet living conditions and possibilities for reproduction. To prevent the extinction of the otter in Luxembourg and nearby areas, an assessment of otter habitats and populations was made in 1994. Th is fi rst evaluation has demonstrated that only rivers in the north of Luxembourg were suitable for otter populations in terms of quality of habitats and that only a few individuals were still present in this part of the country. In 2007, a new habitat assessment was carried out to defi ne priorities for the management of habitats and to build or rebuild corridors to support otter migration and allow re-establishment of stable otter populations in the Benelux Countries. Two similar methods were used to assess quality of habitats in 1994 and 2007. Th e results of the habitat assess-ment are diff erent for the Mid Sûre River and similar for the River Wiltz, the River Wark, the River Our and the Upper Sûre River. Th is variation is probably due to assessment complexity and observer subjectiv-ity, but also to the diffi culty to have a practical methodology for management priorities. From this point of view it would be useful to have a standard methodology at the European level to assess otter habitats. © Koninklijke Brill NV, Leiden, 2008

Keywords Habitat assessment, habitat management, European otter, Lutra lutra , otter conservation, Luxembourg

Introduction

Th e Eurasian otter ( Lutra lutra L., Carnivora, Mustelidae) is an endangered mammal, especially in the west of Europe and in particular in Luxembourg and in the south of Belgium (Walloon Region). Th e otter is a mammal adapted to water and represents a fl agship species for wetland recovery and undisturbed rivers (Kruuk, 2006 ). Protection of otters indeed generally benefi ts also other species.

* ) Corresponding author; e-mail: [email protected]

474 G. Schmidt et al. / Animal Biology 58 (2008) 473–490

Since the beginning of the 1990s otter populations in France, Germany, England, and other European countries have recolonized a part of the previous distribution area from which it had disappeared during the fi rst half of the twentieth century (for example: River Allier and River Dordogne in France) (Bouchardy, 2001 ). Th is is partly a consequence of its protection status by the Berne Convention (1982). In the process of their recolonization, otters are relying on large rivers as migration corridors. It thus appears very important to improve otter habitat quality to assist its dispersal.

For the moment, the otter is considered very rare in Luxembourg and in the Walloon Region (Reuther et al., 2004 ) ( fi g. 1 ). Only a few individuals appear to be present on the River Sûre, the River Our, the two Rivers Ourthes, and the River Semois. Th ese individuals seem to be so separated from each other that they are no longer urged to spraint and mark their territory (Groupe Loutre luxembourgeois, 1994 ).

In 1994, the Otter Group in Luxembourg assessed the quality of otter habitats. Th e results of the this study demonstrated that only 40% of the rivers in Luxembourg are suitable for otter populations. Furthermore, contamination levels of PCBs in fi sh, an important food resource for the otter, were considered as critical (more than 150 ng PCBs/ng fresh body weight) in many river stretches (Groupe Loutre Luxembourg, 1997 ), on the basis of the critical threshold level defi ned by Leonards et al. ( 1994 ).

Figure 1. Distribution map for the Eurasian otter referring to the ISOS (Information System for Otter Surveys) databank system (UTM grid 10 km, period 1999-2003) (Reuther et al., 2004 ).

G. Schmidt et al. / Animal Biology 58 (2008) 473–490 475

Th erefore, all these habitats were considered as potential habitats with risks to otters. In order to improve the situation, some actions were undertaken along the Upper Sûre River to improve water quality: planting of indigenous trees (willows and alders) along river banks and setting-up conventions named “otter havens” to involve private owners in the protection of natural structures of the river and to avoid human disturbance.

Ten years later, seven partners (Natural Parks of the Upper Sûre, the Upper Sûre and Anlier Forest, “Hautes-Fagnes-Eifel”, the Our, the Two Ourthes, foundation “Hëllef fi r Natur”, Centre de Recherche Public - Gabriel Lippmann) have agreed to restore otter habitats in the north of Luxembourg and in the southeast of the Walloon Region within the framework of a LIFE Nature Project ( fi g. 2 ). Th e study area comprises the upper part and the middle part of the River Sûre, the basins of the River Two-Ourthes and of the River Our.

Th e main objectives of this cross-border project are to:

1. Assess the quality of otter habitats (water quality, refuges…) and compare the results with those of 1994.

2. Improve otter habitats with concrete management measures such as the installation of fences along riverbanks, of drinking troughs and mitigation measures near bridges.

Figure 2. Map of the study area. White surfaces represent Natura 2000 areas.


3. Protect the last otters in Belgium and in Luxembourg. 4. Facilitate otter migration and try to restore corridors between the two main western

European otter populations located in Germany and France.

Th ese actions are aimed to protect the otter at diff erent levels. Th e improvement of the habitat quality is the fi rst level of management but is clearly correlated with many other factors like habitat diversity and continuity, water quality, abundance, diversity and quality of preys, etc. Th is level consists of the protection and management of habi-tats to provide good conditions for reproduction in a limited area (local level of management).

Th e second level is the river basin management. It consists of building or restoring continuities, river networks and in permitting migration of otters from one river to another (for example: management of “Otter ducts” on dams).

Th e last level of otter protection and management is the European level. To restore the western European otter populations and to facilitate the migration of otter popula-tions, Reuther et al. ( 2004 ) have defi ned various corridors, for example from the north of Germany and the centre of France in the direction of the Benelux countries ( fi gs. 3 and 4 ).

In the present paper, we will compare and discuss results of riverbank assessments from 1994 and 2007 in the north of Luxembourg. Th is comparison mostly concerns habitat assessment and management priorities to be defi ned for the otter. Other aspects like bioaccumulation of pollutants, pollutions, risks of accidents due to the presence of roads (“black spots”) are certainly also important for otter habitat assessment but are not discussed in this contribution.

Th e availability of food (fi sh biomass mostly) constitutes a limitation for the living of otters in good conditions. Th e threshold of about 120 kg fi sh/km of river for the otter (Libois et al., 1982 ; Maizeret et al., 1981 ) is always guaranteed in the principal rivers of our study area.

Materials and methods

Methodology used in 1994

Th e methodology used for the 1994 survey was based on Weber ( 1990 ) and Mason and Macdonald ( 1987 ). Th e survey was made for all habitats (refugia) on both riverbanks in winter. Each river was divided in sectors of 1 km with subsectors of 100 m. Each 100 m sector was then assessed for its habitat quality according to fi ve scores:

1. No cover (see glossary) on the banks 2. Not enough cover on the banks. Not enough cover means that the presence of

vegetation and other structures is not suffi cient to allow an otter to hide during a full day.


3. A. One daily resting-site (see glossary) with frequent disturbance. Frequent distur-bance means that the otter cannot rest during a whole day in this area.

B. One daily resting-site during the summer season. 4. One daily resting-site during the whole year with poor disturbance. Poor distur-

bance means that the otter can rest one day in this area. 5. One daily isolated resting-site during the whole year without disturbance.

For each sector of 1 km, the average score of the 10 sectors were then calculated and given an evaluation on a scale of three values:

Figure 3. Priority areas (corridors) for Benelux otter migration from the north of Germany and the Netherlands (Reuther et al., 2004).


• average below 3: the habitat is considered as unfavorable. • average between 3 and 4: the habitat is considered as potentially favorable. • average equal or up to 4: the habitat is considered as favorable.

Assessment of habitat quality is based on this scale with three values: excellent or favorable habitat, medium or potential habitat, no suitable habitat.

Methodology used in 2007

Riverbank assessment . Th e methodology used in 2007 was based on the one of Libois ( 2000 ) adapted by us, which consists of surveying both sides of the riverbanks and in noting on a map all the habitats present, e.g. permanent structures (see glossary) like burrows, temporary structures (see glossary) like wood heaps, and potential structures like bank forests ( fi g. 5 ).

Figure 4. Priority areas (corridors) for Benelux otter migration from the south of France (Reuther et al., 2004).


Figure 5. Illustration of diff erent types of otter habitats. A: assessment of habitats on both riverbanks on the River Wiltz. B: burrow on the Upper Sûre River. C: wood heap on the Upper Sûre River. D: dense willow clumps on the River Wiltz. E: natural alder forest on the River Wiltz. F: cavity under roots on the River Our.

A value is established for the diff erent structures which is depending on proximity to the river (5 or 10 meters), disturbance (e.g. a road or a camping site), remarkable features of the structure (see glossary), which allow otters to live there in quiet conditions during a day at least (examples: burrow, dense willow clump, big wood heap, etc.). Disturbances were taken into account by a devaluation of a remarkable structure into a potential structure (see glossary). All these structures are noted on an adapted sheet and on a map (at a scale of 1/5000).

All data were encoded in a Geodata base with point feature classes (see glossary) ( fi g. 6 ). Th is database is connected to a Geographic Information System (GIS) to elaborate maps and statistical models. All habitat structures are represented by a point on the map.


Th e assessment of habitat quality (refugium) was based on the number of remarkable structures present in a square of 500 m 2 . When three diff erent remarkable structures were present, the square was rated as having an excellent potentiality of living for the otter. When two diff erent remarkable structures were present, the potentiality was considered as good. When one remarkable structure was present, the potentiality was considered as medium. When potential structures were only noted, the potentiality was considered as poor and when no interesting structure was observed, the site was rated as having no potentiality for the establishment of the otter ( table 1 ). Th e potentiality was based on the possibility of a habitat to accept one otter during a day in a quiet place.

Defi nition of priorities for habitat restoration . On the basis of the number of structures per 500 m 2 , priorities for habitat restoration by river sector (at least three successive squares) ( table 2 ) was defi ned as follows.

Restoration priority was considered as: maximal: when three squares of no potentiality or two squares of no potentiality and –one square with poor potentiality were contiguous; high: when one square of no potentiality and two squares of poor potentiality or –three squares with poor potentiality were contiguous; medium: when one square of no potentiality was next to one square of poor poten- –tiality and one square of medium potentiality, or one square with medium potentiality and two squares of poor potentiality or one square with poor potentiality and two squares with medium potentiality;

Figure 6. Database for the encoding of the diff erent structures.

Table 1. Assessment of the riverbank quality per square.


low: when one square of poor potentiality was next to one square of medium poten- –tiality and one square of good potentiality or one square with medium potentiality was next to one square of good potentiality or three squares of medium potentiality; no: when one square of excellent potentiality was present. –

Results

Riverbank assessment

Results for 1994 . Th e results of the habitat assessment conducted in 1994 ( fi g. 7 ) indicate that the Upper Sûre River and the upper part of the River Our have very good conditions for otter survival and that the River Wiltz, the Mid Sûre River and the lower part of the River Our are not favorable for otter survival. Th us, for example, on the River Wiltz, twelve excellent sectors, four medium sectors and ten sectors of no poten-tiality were recorded. On the Upper Sûre River, twenty-four excellent sectors, two medium sectors and eleven sectors of no potentiality were noted, whereas on the Mid Sûre River only two medium sectors were recorded, all the other thirty-seven sectors having no potentialities. On the River Our, fourty-two excellent sectors, thirty-two medium sectors and sixteen sectors of no potentialities were noted; on a part of the River Wark, four excellent sectors, eight medium sectors and sixteen sectors of no potentialities were observed.

Results for 2007 . Th e results of the habitat assessments done in 2007 indicate that on the River Wiltz ( fi g. 8 ), four excellent or good sectors, six medium sectors and sixteen sectors of no or poor potentialities are present. On the Upper Sûre River ( fi g. 9 ), twenty-four excellent and good sectors, eight medium sectors and fi ve sectors of no potentiality were observed, whereas on the Mid Sûre River ( fi g. 10 ), nineteen excellent or good sectors, eight medium sectors and twelve sectors of no or poor potentialities were noted. On the River Our, thirty-two excellent or good sectors, thirty medium sectors and twenty-eight sectors of no or poor potentialities were noted. On the River Wark fi ve excellent or good sectors, eleven medium sectors and twelve sectors of no or poor potentialities were recorded.

Table 2. Defi nition of restoration priorities.


Figure 7. Map of the assessment for otter habitats in the north of Luxembourg in 1994 (Groupe Loutre Luxembourg, 1997 ).

Results of priorities for habitat restoration

Figure 11 shows the priority river stretches for the implementation of habitat restora-tion measures in the north of Luxembourg in 2007. Th e results per sector indicate that on the River Wiltz three sectors of medium priority were obtained. On the Upper Sure River, only three sectors of low priority were found, whereas on the Mid Sûre River, one sector of maximal priority and one sector of medium priority were recorded. On the Tretterbaach Streamlet, a tributary of the River Clerve, two sectors of restoration were obtained: one of high priority and the second of medium priority. On the River Our, nine sectors of restoration were present: one sector of maximal priority, seven


Figure 8. Map of the habitat’s assessment of the River Wiltz.


Figure 9. Map of the habitat’s assessment of the Upper Sûre River.


Figure 10. Map of the habitat’s assessment of the Mid Sûre River.


Figure 11. Priorities for the implementation of habitat restoration measures in the north of Luxembourg.

sectors of high priority and two sectors of medium priority. On the River Wark, three sectors of restoration were found: two sectors of high priority and one sector of medium priority.

Discussion

Th e two methodologies used in 1994 and in 2007 are similar but not identical. To compare the results of 1994 and 2007, we have done some adaptations (geographic scale adaptation: 1 km of river versus a square of 500 m of side, habitat potentiality scale adaptation: 5 versus 3 levels).

For the River Wiltz, the number of excellent and good potentialities of habitats was three times higher in 1994 than in 2007. Th e River Our data seem to have the same


Table 3. Table comparing data from 1994 and 2007 in the assessment of habitats for the rivers Wiltz, Upper and Mid Sûre, Our and Wark (Groupe Loutre Luxembourg, 1997).

River/potentialities

Wiltz Upper Sûre Mid Sûre Our Wark

1994 2007 1994 2007 1994 2007 1994 2007 1994 2007

Excellent and good

12 4 24 24 0 19 42 32 4 5

Medium 4 6 2 8 2 8 32 30 8 11Poor or no 10 16 11 5 37 12 16 28 16 12

tendency as the River Wiltz data, with a decrease of the good and excellent potentialities and an increase of the poor or no potentialities. For the Upper Sûre River, the results are quite similar but seem to be a little bit better in 2007 than in 1994. On the Mid Sûre River, the data are really the opposite, worse in 1994 and excellent in 2007 (table 3).

Besides possible changes in the riverbanks, these very diff erent assessments are possi-bly linked to the diff erent methodological approaches of Weber ( 1990 ) and Libois ( 2000 ). Weber’s methodology (1990) is now 18 years old and was based according to the author on a simple method due to the lack of knowledge on the relationship between the otter and its environment. Th us Weber indicates: “We are missing a lot of information about otter ecology and behavior. Our methodology supposes that the otter habitat will provide a large possibility of independent resources with limit values”. Th e diffi culty is to defi ne those limits. For example, if we look at the relationship between habitats and disturbance, many authors consider that the presence of otters is correlated with the abundance of bank vegetation and that the presence of these structures is more important when human disturbance occurs (Macdonald and Mason, 1985 ; Elliot, 1983 ; Prauser, 1985 ). Assessing the impact of disturbance on the habitat of the otter is one of the diffi culties of this methodology. Weber ( 1990 ) has tried to quantify the presence of habitats and their relationship with disturbance (values from 1 to 5).

To avoid an over assessment, the methodology was always used in the winter in the 1994 study by the Groupe Loutre luxembourgeois. Furthermore, all the observers were trained at the same time with the same teacher. But this approach does not pre-vent any subjectivity of the observer in the interpretation of the values related to the experience of the observer, the complexity of the landscape, the number of factors implicated in the assessment of the habitat, etc. Weber ( 1990 ) recognized the subjec-tivity of his methodology and the diffi culty for the observer to imagine an otter hiding !

Libois’ methodology (2000) is about 7 years old and has profi ted of all the knowl-edge gained from the recent ecological studies on the otter and was adapted for our particular project, which needs very practical conclusions to defi ne priorities of man-agement. Th e interpretation of the habitat quality is based on three diff erent remark-able structures per square of 500 m of side, which is an assessment with a very high requirement.


Th e diff erences between the two assessments may also be related to the subjectivity of the observer, but also to the complexity and the lack of clarity of the defi nitions of the fi ve scores of Weber’s methodology to assess the habitats’ quality for the otter.

Comparison of results from 1994 with 2007 illustrates the diffi culty to assess otter habitats due to a lot of factors integrated in those methodologies and the complexity of otter ecology. For example, diff erent studies have demonstrated that otters often prefer holts (underground resting-sites) to overground resting-sites like dense willow clumps, bramble bushes when the landscape is open and the human disturbance important (Rosoux, 1995 ). But otters can also adapt their behaviour and use overground resting-sites with low human disturbance and in a ‘bocage’ landscape (Reuther, 1985 ).

Th e 2007 assessment as the one carried out in 1994 indicates that the Upper Sûre River remains one of the best habitats for the otter and that some rivers like the River Wiltz, the Tretterbaach Streamlet, the River Clerve have to be restored to improve the quality of the otter habitat (Groupe Loutre Luxembourg, 1997 ).

At the European level, Reuther et al. ( 2004 ) have defi ned a methodology to assess the habitat quality for the otter taking into account the potential confl icts with anthropic activities (nine parameters including agriculture, roads, altitude, etc.) with a rating of fi ve classes (from low to very high) processed by a common GIS classifi cation. Th ese results have the advantage to integrate more parameters but are more theoretical and diffi cult to use at a local or a regional level for management actions like habitats and water quality improvement, etc.

Conclusions

Th e diff erent available methodologies for otter habitat assessment cause problems due to lack of standardization and subjectivity in the interpretation of the results. A European standardized methodology would be useful to compare the diff erent situations in diff erent countries and also to manage adapted actions. Th is European standard methodology must also provide clear defi nitions of terms like “cover” and “structure”.

It would be of great interest to adapt the GIS methodology of Reuther et al. ( 2004 ) at a local level in order to integrate more objectively potential confl icts in the habitat assessment for otter management plans. A multifactor analysis of the diff erent confl ict factors, based on the GIS, has to be considered. All the parameters like human distur-bances, pollutions, roads, etc. have to be incorporated in this GIS analysis of habitats to try to assess the complexity of the human impact on otter populations and the development of habitat quality in Luxembourg.

It is also important to know the subjectivity of this methodology due to the observer and his knowledge on practical otter ecology. Th us, the knowledge and the training of the observer are very important to have a reproducible methodology, which can guar-antee the success of the assessment and the management of restoration plans for the otter.


Glossary

Cover : structure of vegetation like willow clump, reed or bramble bush along the riverbank which can be used by the otter as resting-side during the day.

Disturbance : is related to human activity, presence of dogs, roads, railways, etc. Th e otter is particularly sensible to disturbance by dogs which can keep the otter away from such area.

Permanent structure : long-lived structure used by the otter as resting-site, like bur-row, cavity in rocks, etc.

Point feature class : structure encoded in the database by class and represented by a point in a Geographic Information System (GIS).

Potential structure : structure not optimal as otter resting-site which can be improved and than eventually reach the level of remarkable structure.

Remarkable structure : structure where the otter can reproduce or stay a long time without human disturbance, for example large burrows, holes in rocks, cavities in trees or under roots, big and dense clumps of willow, important bramble bushes, long, dense and large hedges, etc.

Resting-site : area where an otter can rest during a day in a quiet and hidden place. Temporary structure : structure, which will disappear with time like wood heap, bram-

ble bush, etc.

Acknowledgements

We would like to thank the European Commission, the Ministry of the Environment and the Ministry of Internal Aff airs of Luxembourg for the fi nancial support in the framework of the LIFE Nature project (LIFE05 NAT/B/000085). We also acknowledge Vinciane Schockert, Roland Libois from the University of Liège and Hans-Heinrich Krüger from the Otterzentrum for the scientifi c support on otter ecology and methodol-ogy and all the people involved in this LIFE Nature project: Hélène Ghyselinck, Bob Glesener, Christine Leclercq, Sven Plattes, Yves Krippel, Eva Rabold and Anne Scheer.

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Riverbank assessment and management for the Eurasian otter in the north of Luxembourg

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