The feeding ecology of the European otter (lutra lutra l.) in a marine environment

Abstract

The bulk of the research took place on the coast of Fetlar, Shetland, in1976-78.Otters were studied by direct observation of individuals recognised by variations in natural markings on the lips and throat, and in addition their diet was studied by means of spraint (faecal) analysis. Most foraging occurred in the sea within 150 m of the shore, a wide variety of fish species being eaten, the majority demersal, as well as some crabs. The pronounced seasonal) variation in dietary composition is considered in the light of available information on the behaviour of the prey species. Detailed observations of an adult female and her cub revealed a significant difference in the composition of their diets, the cub taking a lower proportion of the faster prey species as well as a significant number of crabs which were not predated at all by the adult. Most feeding was localised in bouts in relatively restricted areas. There was no clear correlation between otter activity and the diel or tidal cycles. The adult female and her cub had a home range consisting of 2.5km of coastline with a maximum foraging area of 42.6ha, and their pattern of use of this home range is described. Evidence suggests that for the effective exploitation of foraging areas, breeding females require holt sites (resting places) close by. In 1977 the minimum population density along 11km of coast was 1 otter: 1,2km of coast; and in 1978 along 16km of coast was 1 otter: 0.9km of coast. A positive correlation between dive duration and water depth was noted and it is hypothesised that this is a causal relationship determined by the necessity for minimising the up thrust produced by the air in the lungs during diving. The implications of this for otter foraging behaviour and population distribution are discussed.