Volume 4 Pages 1 - 45 (March 1989)

Citation: Martens,  V. (1989) Otter Survey in the Northern Part of the Netherlands  IUCN Otter Spec. Group Bull. 4: 27 - 28

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Otter Survey in the Northern Part of the Netherlands

Vincent Martens

Gedempte Molenwijk 72, 8442 BH Heerenveen, The Netherlands

INTRODUCTION

In the Netherlands, the number of otters in the early 1960s was estimated at about 300. This estimate was based on indirect investigations by questionnaires and research of journals on hunting and fishing for reports of otters that were killed (van Wijngaarden & van de Peppel, 1970). Although the otter population in the Netherlands was never investigated by a field survey it was clear that the otter had declined dramatically since the early 1960s. Since 1980, signs of otters were only found regularly in the northern part of the Netherlands In the provinces of Gröningen, Friesland and Overijssel. It was generally accepted that a viable otter population was still present in this part of the country. However, the only records of otters found dead after 1980 have come from the province of Friesland (van Moll, 1980).

To Investigate the actual status of the otter in the northern part of the Netherlands a field survey was made in 1987 and 1988.

STUDY AREA

In the early 1960s two otter populations were present in the northern part of the Netherlands: one small population (estimated at about 30 animals) in the lake area of Gröningen and northern Drente, another population (estimated at about 120 animals) in the lakes and marshes of Friesland and the northwestern part of Overijssel. These are the two areas that were investigated during the field survey. In the province of Overijssel, the river Vecht (which could be regarded as a possible connection with otter populations in Western Germany) was also surveyed.

METHODS

The survey was based on a 5 x 5 km square grid. In total , 69 squares were surveyed: 6 in the lake area of Gröningen and Drente, 48 in Friesland, 9 in the northwestern part of Overijssel and 6 in the river Vecht area. In each square three sites were searched for otter spraints and footprints over 600 m. In Friesland, eight of the squares were surveyed much more intensively (over more than 600 in as well in different. seasons) in order to check the accuracy of the survey method and to find out whether or not otters were present at a very low density.

RESULTS

In the lake area of Groningen and Drents no signs of otters were found during the survey. The last signs of otters in this area were found in 1986.

In 1987, signs of otters were found within 15 squares in Friesland and only one square in northwestern Overijssel. In the same year an otter was killed by road traffic. In 1988, no new squares containing signs of otters could be added.  Moreover, of the 15 squares in Friesland only seven adjoining squares were found to contain otter signs in 1988 although they were all visited again.

In September 1988 another otter was killed by road traffic and since then no further signs of otters were found in the seven adjoining squares nor in any other square.

DISCUSSION

In 1987, five or six otters must have been present within the study area. Only three permanent otter home ranges could be located overlapping nine squares in total. In 1988, otter activity was restricted to two areas which were situated 8 km from each other, each of them containing most probably just one otter. One of these two otters must have been the one which was killed by road traffic in September 1988, the other one just disappeared at the same time.

In the northern part of the Netherlands the otter can be considered extinct now. From other parts of the country no reliable otter signs were reported in 1987 and 1988. Whether there are otters still present in any other part of the Netherlands is very doubtful.

In 1987, water and sediment within three otter habitats in Eriesland were found to be polluted with PCBs and heavy metals. Analysis of the few otters found dead after 1980 showed alarmingly high levels of PCBs in liver and fat tissue (Broekhuizen & de Ruiter-Dijkman, 1980).

Considering the widespread water pollution, the highly fragmented otter habitat and the still growing traffic intensity, reintroduction of otters in the near future will not be responsible. Much work will have to be done on suppressing water pollution, improving the quality of otter habitats and restoring the ecological infrastructure until a well considered reintroduction of otters in the Netherlands will be possible.

REFERENCES

Broekhuizen, S. and de Ruiter-Dijkman, E. (1988) Otters met PCBs: de zeehondjes van het zoete water.  Lutra, 31: 63 - 79
Martens, V. (1988) Verspreiding van de otter in noord Nederland.  50 pp., Stichting Otterstation Nederland, Gröningen
van Moll, G. (1988): Causes of Otter Mortality in the Netherlands between 1965 and 1897. IUCN Otter Specialist Group Bulletin3: 9
van Wijngaarden, A. and van de Peppel, J. (1970) De otter in Nederland. Lutra 12: 1-72