There are more otters in the Netherlands following efforts to repopulate the country’s lakes and rivers but inbreeding is becoming an issue, RTL news says on Tuesday.

Researchers at Wageningen University say the Dutch otter population had reached around 140 in 2013, but that the otters are showing fewer genetic variations.

This means new blood is needed to freshen the genetic balance, the researchers say.

While the impact of inbreeding in otters remains unclear, in other animal populations it has led to a loss of fertility and resistance to disease.

Otters had died out in the Netherlands. They were reintroduced from Lithuania and other eastern European countries between 2002 and 2008 in the north of the country and have since been spotted in the central belt.

In November, the appeal court in The Hague told the transport ministry to tackle 21 traffic blackspots and make them safer for otters. Despite the growing population, 31 were killed by cars this year alone, news agency ANP reported.