Mortality of Otters (Lutra lutra) in Shetland

(1) A sample of 113 dead otters Lutra lutra L., collected in 1984-88 in Shetland, was analysed for cause of death, body condition, age, and the presence of some organochlorines, PCBs and heavy metals. (2) Although 54% of the sample had died of non-natural causes (42% road-mortality), this figure was an artefact of the sampling method, and natural, non-violent mortality was most important. (3) Proximate causes of non-violent mortality were various; most deaths occurred during times of low food availability (spring). (4) Mortality increased linearly with age from year 1 onwards, and the mean adult life expectancy was 3 1 years. (5) Concentrations of DDE, dieldrin (HEOD), lindane (BHC), Cd and Pb in livers were considered insufficient to have contributed significantly to mortality. (6) Mercury occurs naturally in Shetland inshore fishes on which otters feed; its concentration in the liver of otters increased with age. Almost 40% of otters of 5 years old and over had accumulated enough mercury for potential sublethal or lethal effects. There was no correlation between mercury and selenium, and it was unlikely that selenium alleviated the effects of mercury. (7) Concentrations of PCBs were high enough to have had possible sublethal effects in some otters. (8) The results suggest that food shortage was most probably the ultimate cause of death in most otters in Shetland, with the effects of mercury and perhaps PCBs as contributing factors.