NATURE AND THE ENVIRONMENT
In the footsteps of the otter
The otter, one of the Netherlands’ most appealing predators, is an indigenous species that died out but was reintroduced in 2002. Since then, Wageningen University & Research has been monitoring the otter population and investigating what the Netherlands can do to encourage its recovery.
The beast shoots through the water in search of its prey. Eel, perch and pike are on the menu, as are crayfish, snails, frogs and worms. The otter is one of the most appealing predators found in the Netherlands. It is an excellent swimmer, highly streamlined and extremely quick in the water. But it also moves well on land, sometimes travelling dozens of kilometres in a single night.
Yet such journeys are not without risk. Each year, a quarter of the Dutch otter population is killed on the roads. All kinds of provisions are being made in an effort to reduce this high mortality from traffic. For example, narrow planks are being installed under bridges in areas where otters live that they can then walk across. “That works really well,” says Loek Kuiters, an ecologist working at Wageningen Environmental Research, part of Wageningen University & Research (WUR). “Once one otter has walked over the plank, others can smell that and so they follow the track.”
The otter is indigenous to the Netherlands. It is a species subject to strict protection under the European Habitats Directive. This directive requires EU countries to protect habitats and certain species of plants and animals that are of particular significance for Europe as a whole. That is why the Netherlands is legally obliged to monitor how well the species is doing and make further improvements to its habitat. After the last otter was run over in the late 1980s, a programme to reintroduce the otter was initiated in 2002.
The otter is a strictly protected species. It is now found in the Netherlands after being reintroduced in 2002. Photo: Hugh Jansman
Wageningen University & Research has been tasked by the Dutch government with monitoring the recovery of the otter population. Where are the otters, where do they travel to, what age do they live to, are they reproducing and how much genetic variation is there in the population? “Unfortunately, I have never seen otters in the wild,” says Kuiters, who is in charge of the Wageningen otter project. “But our most recent count, for the winter of 2018-2019, shows that the otter population is still increasing and is spreading further across the country. There are now around 360 to 390 otters. But the numbers killed on the roads are also increasing substantially. Now over 100 otters are being run over every year, a quarter of the population. Inbreeding is also an issue for the Dutch otter population. As the otters are descendants of the small group that was initially released in the wild, many are related to one another.”
How do you count a species that is mainly active at night, notoriously shy and rarely seen? Kuiters: “We use DNA techniques. Otters leave droppings, known as spraints, scattered around their habitat. Small quantities of intestinal cells can be found on the outside of fresh droppings. You can then use DNA analysis to identify individual animals. We have now recorded all the Dutch otters known to us in a database. We know roughly how many there are in total and where they are. This is a very elegant method.” Many other European countries count otters by looking for their tracks. But you can’t tell very well from the tracks whether you are looking at different animals or the same one again and again.
Otters are a heavily protected species in the Netherlands, meaning there is a nation-wide obligation to improve the animal’s habitat. WUR researcher Hugh Jansman is monitoring otter population recovery: where can they be found, where do they travel, how old do they grow and are they reproducing?
Most spraints are collected during the winter, when temperatures are lower and the fresh droppings stay suitable for DNA analysis for longer. Another reason is that otters leave their spraints in clearly visible spots in their habitat between October and March as a way of marking their territory. Each year, WUR collects large numbers of spraints in the core area of De Wieden-Weerribben in Overijssel province. Spraints are also collected in all other habitats that are known to have otters.
‘Our latest counts show that otters are spreading further across the country’
Much of the material is collected by Freek Niewold of Niewold Wildlife Infocentre and by volunteers, often members of the CaLutra workgroup of the Dutch Mammal Society (Zoogdiervereniging). The spraints are placed in alcohol in jars and sent to the laboratory in Wageningen. In addition to the spraints, the corpses of dead otters are also collected with the help of large numbers of nature management staff and others. DNA samples are taken from the corpses and they are autopsied to determine the cause of death. The females are also examined to see whether they were pregnant or lactating.
The genetic analyses show evidence of inbreeding in the otter population but that has not got worse in recent years. About 30 otters from all over Central and Eastern Europe were released originally in the Netherlands. Kuiters: “A number were killed early on by vehicles or they disappeared from the radar. Only about 12 to 15 otters went on to reproduce and there were no new additions to the gene pool from elsewhere in those first few years. That has started happening in recent years, though – we are increasingly seeing otters that come from Germany.”
Otters live in a kind of harem system. The male covers a large area known as the home range, which incorporates the territories of several females. In the early years, only a single dominant male mated with all the available females in the marshes of Wieden-Weerribben in Overijssel. His son took over that role later on. Breeding between closely related animals happened a great deal in the early years, which had an adverse effect on the genetic variation.
Fauna tunnels are designed to help protect the otter population. Photo: Loek Kuiters
Researchers use DNA analyses of spraints to count the number of otters. Photo: Niall Benvie / Hollandse Hoogte
Kuiters: “The scientific literature tells us that inbreeding in otters can reduce resistance and make them more susceptible to diseases, have a negative impact on reproduction and lead to deformities such the absence of teeth or an abnormal number of digits. But we don’t see such abnormalities in practice. Reproduction levels are high and the animals look healthy.”
How do you count a species that is active at night, notoriously shy and rarely seen? By analysing its droppings
Otters have their own mechanism for preventing inbreeding. Young males are chased out of the territory and then roam around until they find a new habitat. But it is precisely that mobility that leads to the large numbers of otters killed on roads. Incidentally, the Netherlands is far from ‘full’ as far as otters are concerned. “As numbers increase, the individual territories are becoming smaller. But it turns out otters can live at much closer quarters than we had anticipated, as long as there is enough food and shelter. There are still many suitable habitats that do not yet have any otters at all, for example in the province of Noord-Holland,” says Kuiters.
The monitoring shows how rapidly progress can be made when an animal is reintroduced and how quickly a species like the otter can become re-established, where females have one litter a year with an average of two to three young. Otters are no longer hunted for their fur, there is plenty of food and they are benefiting from improvements in water quality. They do however need sufficient cover along the river banks where they can rest and raise their young. There are still a lot of otters that drown in (usually illegal) fish traps. You can modify the traps to make it impossible for an otter to get into them. Dogs that are not on a lead are a threat for young otters in particular.
Kuiters: “Our monitoring pinpoints the hotspots where the most road deaths are. We report this regularly to the road authorities. They can then prioritize measures on regular migration routes where a lot of otters are killed crossing busy roads. The A6 motorway at the point where it crosses Tjeuke Lake was notorious for the number of otter fatalities. This section of motorway has now been fenced off and fauna tunnels have been installed. Now we only get reports of road deaths when a fence is damaged or a tunnel flooded. These measures make a real difference.”
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