Otters Lutra lutra in Shetland feed almost exclusively in the sea. Their diet was assessed from direct observations of 13, 313 dives, of which 27% were successful, and 2028 prey could be identified. The results are compared with published data on the availability offish for different seasons, time of day, state of tide, and different types of coast. The most frequently taken prey was eelpout Zoarces viviparus (34%), but they were relatively small, and rocklings Ciliata mustela dominated the diet in terms of biomass (18%, against eelpout 15%). Small eel-shaped, bottom-living species dominated in the diet at all times, but of those, otters took the larger fishes in the populations (median weight of prey 28 g). The size and species composition of prey of males was the same as that of females with cubs, but females without cubs took smaller fish.
In spring, prey was smaller than at other times, and otters were least successful when diving. Diving success and mean prey weight was highest in winter, although prey availability was highest in summer.
Eelpout were caught mostly along sheltered coasts, rocklings in more exposed areas. It is argued that for optimal exploitation of the seasonal availability of different species of prey, otters need to use different types of coast, and the size of their ranges would be related to the spacing of coastal types.
Most fishes were caught in daytime, and around high tide otters fed less than at other times; hunting therefore took place during periods of inactivity of prey. For the main prey species, Ciliata mustela, it was estimated that otters took a substantial part of the total population.