Otters are believed to be back and living along the River Calder in Calderdale.
The Yorkshire Wildlife Trust wants any sightings reported to it as it works to help re-establish populations.
Otters, or signs of otters, have been found all the way up to Todmorden.
It is thought, however, that there may be a single male with a large territory which travels up and down.
Trust programme manager Brian Lavelle said: “I have over the years tracked the slow return of otters up the Calder until there is some presence along the whole of the Calder but in low numbers.
“If you are lucky enough to see an otter please let Yorkshire Wildlife Trust know so that we can continue to track the presence of these fantastic creatures.”
Otters were once common in Britain but virtually disappeared from rivers in the 1950s.
Their demise was blamed on the increased use of pesticide which was running off into rivers and polluting water sources.
Otters are at the top of the food chain and the pesticides accumulated in their bodies.
They are currently protected by the Wildlife and Countryside (1981) Act preventing intentional destruction, injury, disturbance or their sale.
The Trust has been enhancing the otter’s habitat on Yorkshire’s riverbanks.
That work has included creating further wetland sites, planting shrubs to provide otters with areas to hide safely for the day, constructing otter platforms and installing artificial holts along river banks.
The Trust can be contacted on 01904 659570 or www.ywt.org.uk