The survey relied heavily on volunteer input across Wales, and was supported by grant funding from Natural Resource Wales’ legacy bodies.
Overall, 89% of the sites surveyed showed the presence of otters, compared with 20% in 1977-78, 38% in 1984-85, 53% in 1991 and 74% in 2002.
Such an increase in otter distribution underlines the increase in otter numbers in Wales from the very low levels of the 1970s.
Although otter numbers are undoubtedly increasing, significant numbers are killed each year on the road network in Wales, and threats such as entrapment in fishing gear may be an issue.
The results of the 2009-10 surveys underlines the improvements in water quality in Welsh rivers and lakes in recent decades.
The report contains a number of recommendations to maintain the recovery of this species in Wales.