The English otter, a beloved mammal once thought to have all but disappeared from the nation’s waterways, is staging an impressive comeback. Otters can now be found in nearly 35 percent of England’s rivers and wetlands, a five-fold increase over numbers from 25 years ago, according to survey results released by the government yesterday. Otter populations dropped steeply in the 1960s in Britain and throughout Europe, due in large part to widespread use of pesticides. Improved water quality has helped the species rebound, as have higher fish stocks and changes in riverbank management. But otter advocates caution against complacency, pointing out that the animals are not returning quickly enough to some parts of the country and that cars still pose a significant threat to otters, which periodically wander onto roadways.