Science - September 9, 2010

‘The otter mustn’t die out twice’

The reintroduction of the otter into the Dutch landscape eight years ago seems to have been a success, but inbreeding and high mortality rates are taking their toll.

The 31 otters that were released into the weerribben in 2002 seem to be doing well, says Hugh Jansman, animal ecologist at knowledge institute Alterra. The have produced 70 to 90 young and are also being spotted beyond the weerribben with increasing frequency. 'Otters are nomadic and they get everywhere, in spite of the countless barriers', says Jansman. 'That illustrates the resilience and the opportunism of the species.' In the report The return of the otter to the delta area, Jansman argues for the introduction of new otter populations besides the small existing source population in the Wieden-Weerribben national park. This would increase the chances of being able to put this mammal - the Netherlands' largest - back on the fauna map, after the species was officially declared extinct in 1988.
A fish-eating water rat
The European otter (Lutra lutra) is found in large tracts of Europe and Asia. It lives both in fresh water and along rocky sea coasts. With its weight of up to 13 kilos and its length of almost one and a half metres, the otter is slightly lighter yet longer than the badger. Its streamlined body, webbed toes, closeable ears and watertight skins make this fish-eater a real water rat.

As things stand, there is cause for concern, however. The otter population grew spectacularly in 2007-2008, only to fall back significantly a year later. This winter, numbers of the animals actually decreased, says Jansman. The harsh winter played a role in this, but Jansman believes that the consequences of inbreeding are also revealing themselves. 'A handful of extremely dominant males fertilized all the females. Those offspring then mate with each other, and as a result the genetic variation is halved.'
Over the next few years, the ecologist hopes to obtain more insight into the effects of inbreeding. The introduction of fresh genetic material from newly released animals is a must, says the researcher, who argues for the release of new otters on a small scale in places including the Gelderse Poort, where there is room for a fair-sized population, the Vechtplassen, and possibly in the longer term the river area of central Limburg.
Traffic victims
Another problem besides inbreeding is the high mortality rate. The heavy traffic on Dutch roads poses a serious threat to the otter's survival. The otter is no match for a tonne of steel that approaches almost soundlessly at a hundred kilometers an hour over 'quiet asphalt'. This makes traffic cause of death number one for the otter. 'About 80 percent of otter deaths can be directly put down to the traffic', estimates Jansman. Another 15 percent are due to otters meeting an untimely end by swimming into a hoop net and drowning.
Jansman sees solutions for these threats. He thinks the traffic danger could be reduced by relatively simple measures. 'We know the otters' main migration routes. Wherever a route crosses a busy road, you can take specific precautions', he claims. 'Lowering the speed limit from 80 to 60 kilometres an hour would be enough to reduce the death rate spectacularly. And then you could consider putting corrugations in the asphalt at certain places, to make the approaching cars more audible'. The death rate among otters could be further cut, says the researcher, by redesigning hoop nets so that otters cannot swim into them any more.
Hard to see but not to study
Otters are difficult to spot, but thanks to modern technology, Jansen and his colleagues have been able to learn a lot about Dutch otters. Their droppings contain a certain small percentage of intestinal wall cells which contain DNA that can, with the help of technology, be extracted and analysed. Because each animal has a unique DNA profile, researchers can not only identify individual animals, but also analyse their parentage and the genetic variation in a population. Like this, you can learn a lot about the otter population without ever clapping eyes on an otter. The population introduced into the Netherlands is therefore the best-researched otter population in Europe.
Model country
Alongside releasing more otters and taking steps to reduce the death rate, Jansman also argues for a systematic improvement to the otter's habitat. That need not cost a fortune, he thinks. 'The otter doesn't need much. What it takes is patches of shrubs or 'wilderness' along waterways, which the otter can use as resting places during the day. And every 5 to 10 kilometres there should be a larger natural area where the animals can reproduce.'
In spite of all the problems and sticking points, Jansman is optimistic about the otter's chances. 'I am convinced that in 20 years' time the otter will once again occupy most of its original habitats in the Netherlands', he asserts. 'But this is a critical phase. We have to watch out that the otter doesn't die out for the second time due to genetic impoverishment.'
What this would take, says Jansman, is financing for monitoring the population, for example. At present there are no funds for this. And that is a pity, says Jansman. 'A project such as this generates a lot of unique knowledge that can be applied elsewhere as well. The problems China is having with the conservation of the panda and the tiger are really no different to what we come up against in the conservation of the otter: the fragmentation of its habitat and inbreeding. With the otter research, the Netherlands can profile itself as a model country for the maintenance of sustainable populations in a strongly human-dominated context.'