The decline of the otter Lutra lutra L. in Britain: an analysis of hunting records and discussion of causes

The records of one pack of Otter Hounds hunting in southwest England are examined for the period 1907 to 1971 as well as the records of all packs active in Britain between 1950 and 1976. The hunting success per unit effort varies from year to year depending on changes in hunting conditions but longer term changes can also be identified. The hunting success of the Culmstock Otter Hounds (hunting in parts of south-west England) increased steadily from 1907 to 1956 but in most of England and south Wales the success rate of the hunts declined rapidly after 1957. There was also a decline in success in northern England and southern Scotland but to a lesser extent, while in north Wales and Eire, there is no evidence for a decline.
These changes are considered to reflect changes in otter populations but the extent of the decline in hunting success (to between 37% and 55% of previous levels in the southern hunts) is probably less than the actual decline in otter numbers. There are no signs of a recovery in the population but indications of a continuing decline up to 1976.
The reason for the increasing population in the first half of the century in south-west England is probably the decrease in persecution since the nineteenth century. A variety of causes for the crash in the late 1950s are considered and the factor most likely to be responsible is the introduction of the dieldrin group of insecticides in 1956. Use of these compounds has been increasingly restricted since 1963 and the possible reasons for the failufe of the otter population to recover are listed but no firm conclusions can be drawn as yet.