The reintroduction of the otter

The otter, a freshwater-dwelling predator, was reintroduced into the Netherlands in 2002. Thirty-one otters from other countries were released in the Wieden-Weerribben natural reserve in the province of Overijssel. The Dutch Ministry of Agriculture, Nature and Food Quality commissioned Alterra to monitor the new otter population. Alterra has successfully utilised genetic methods for the monitoring process.


The otter, a solitary freshwater-dwelling predator, could be found in large numbers in the Netherlands until the middle of the previous century. The population decreased as a result of water pollution from PCBs, increasing traffic, fragmentation of their habitat, and expansion of the fishing industry. The last otter in the Netherlands died in 1989, and the animal became extinct in this country. The Dutch Ministry of Agriculture, Nature and Food Quality (Ministerie van Landbouw, Natuur en Voedselkwaliteit, LNV) decided to reintroduce the otter, and between 2002 and 2008, 31 otters from Eastern Europe were released in the Wieden-Weerribben nature reserve in the province of Overijssel. Alterra kept track of these otters between 2002 and 2009.


In order to monitor the primarily nocturnal otters, it is necessary to be able to observe them. Therefore the otters were fitted with transmitters at the time of their release, so that their initial distribution could be recorded. Because the transmitter batteries only work for approximately one year, a blood sample was also taken of each animal released. A genetic “fingerprint” was taken of the DNA obtained from each otter. This data makes it possible to chart the otters’ behaviour. On the basis of the DNA obtained from the spraint (otter dung) collected, it is possible to determine the activities of the different otters. The fact that new DNA profiles have been found suggests that the otters are reproducing, and it is thereby also possible to determine who the parents are.

This indirect observation of the otters by means of non-invasive genetic monitoring, a method perfected by Alterra specifically for use with the otters, is working well. With the DNA database, it is possible to make maps of the otters’ annual distribution. If dead otters are reported, often as a result of traffic accidents, it is also possible to determine their origin.


The DNA monitoring has demonstrated that male otters require large territories, while daughters use a portion of their mother’s territory. At the time of reintroduction, two dominant males chased the other males from the release area. This increased the chance of inbreeding, necessitating the introduction of fresh bloodlines. This advice was presented to the Ministry during reintroduction. It was also stated that new releases of otters could best consist of females in surrounding areas, because the males will naturally follow them.

The locations of otters which had been run over resulted in suggestions being presented to managers of infrastructure, such as fencing off roads, reducing speed limits, or building tunnels.


Hans Peter Koelewijn, molecular ecology researcher and geneticist for the otter project, is positive about the project. “The genetic techniques have enabled us to gain a great deal of insight into the otter population at the time of reintroduction. However, it is unfortunate that because of the limited budget, it is nearly impossible to continue the intensive monitoring at this time. Now, after several years, this is the time when it is most interesting to see whether the population can sustain itself. We are currently limiting the monitoring to dead animals which are brought in.”