Recruitment to a Population of Otters (Lutra lutra) in Shetland, in Relation to Fish Abundance

(1) Numbers of cubs and litters of otters (Lutra lutra L.) were analysed in relation to numbers of adults and of prey present, over a 5-year period in a study area in Shetland. Data are presented on cub mortality and its causes. (2) An estimated nine adult females in the study area produced a mean total of 56 litters year 1, as observed when cubs left the natal holts at the age of about 2 months. Mean litter size in the study area was 1 64, elsewhere in Shetland 1 86. (3) The estimated number of juvenile females per 100 adult females attaining the age of first reproduction (2 years old) was 33 7, similar to the estimated proportion of adult females dying annually (31 %). (4) The annual number of cubs from nine females varied between four and sixteen. There was a high correlation between prey density (eelpout Zoarces viviparus and rocklings Ciliata mustela) in July-August and numbers of cubs observed over the following months. (5) It is argued that annual variation in numbers of cubs observed was caused by early mortality, and at least part of this was due to females abandoning cubs.