Number of otters hardly grown

Published on
September 1, 2017

The number of otters in the Netherlands has hardly grown in the past year and is stabilising at an estimated 200 animals. The genetic variation within the population still benefits from the introduction of genetically non-related animals. The average genetic variation within an individual was again a bit lower than in the previous year. This indicates a slowly progressive inbreeding. Research by Loek Kuiters and Arjen de Groot shows this, as published on early September 2017.

Wageningen Environmental Research (Alterra) is commissioned by the Ministry of Economic Affairs to annually monitor the genetics of the Dutch otter population. Information from this research sheds light on the genetic variation present in the population. That is important, because in small, isolated populations there is always a danger of inbreeding with possible effects on the vitality of the animals and their reproductive success. Measurement of genetic variation is an early warning system, that points out in an early stage whether the population may be going downhill. The results of the survey of the 2016/2017 have been disclosed recently.

Since the onset of the monitoring in 2002 when the first group of otters was released  in National Parc Wieden-Weerribben, droppings, and otters that were found dead, have been collected for DNA research.  Also of all individuals, released extra, the DNA profile was recorded. On the basis of DNA profiles a picture appears of the genetic variation that is present within the population. This way of monitoring renders also information about the total number of animals in the population. During the past winter of 2016/2017 over 1100 droppings were collected in the distribution area, that includes over time now habitats in Friesland, Overijssel, Drenthe, Groningen, Flevoland, Gelderland and Zuid-Holland.

Genetic variation

The results show that  the genetic variation in the Dutch otter population fluctuates slightly from year to year, whereby restocking with genetically non-related animals and the sometimes premature disappearance of these,  does still have a large effect on the genetic variation in the population. It is important that this genetic material is actually mixing. Only then, the new genes are being incorporated sustainably. The past year showed a positive development for the core population. The total genetic variation, that lagged behind compared to the total population earlier, has increased clearly in the past year. This implies a better mix of the genotypes present. The core population still constitutes an important source for colonising new habitats. The population at large and especially the cohort new offspring showed a slight decrease again in the average genetic variation within the individual (heterozygosity), an indication for inbreeding to slowly increase.

Population size

Based on the DNA profiles found, the total size of the population can also be estimated. The population seemed hardly growing for the first time in years. The size of the population is now estimated to be about 200 animals. The core population in The Wieden–Weerribben comprises circa 80 animals. The current distribution area is mainly contracting and hardly extending. A finding that strikes most, is a strong growth of the number of individuals in Drenthe. It is also striking that the eldest females in both the Weerribben and The Wieden, in the meanwhile over 10 years old, appeared still to be alive.


The number of reported and verified fatal casualties was high again in 2016 with 50 animals, which constitutes approximately a quarter of the population. This relatively high mortality is the most important reason for further growth to be stagnating. Road authorities work hard to make the roads safer on places where otters might cross, but it is for sure that not all bottlenecks are solved yet. This problem demands continuous attention. The usage of stop grids, and making this obligatory in areas where otters occur, is another point of special interest. Mortality, caused by traffic and unwanted by-catch in catching utensils, has to be curbed, if ever the population is to grow to a size that can be regarded as sustainable.