Introduction

The otter, Lutra lutra, was once a top piscivorous predator in coastal and riparian habitats throughout much of Britain, continental Europe, the former USSR, Asia and Japan (Mason & Macdonald, 1986). However, otter populations have declined severely over much of this range during the last century, particularly in freshwater habitats, and the species is now rare or absent in many countries (Mason & Macdonald, 1986; Foster–Turley et al., 1990). The species is listed on the EU Habitats Directive, the Bern Convention and the Convention on International Trade in Endangered Species (CITES), and is classified by the International Union for Conservation of Nature and Natural Resources (IUCN) as ‘vulnerable’ as a result of the declining or endangered status of many of its populations (for full legislative details, see JNCC, 1996). Several countries, including the UK (see JNCC, 1996), have conservation strategies for the species that include such things as population monitoring, enhancement of populations through habitat management, the promotion of natural recolonization, and improved knowledge through appropriate research.

Several of these factors involve knowledge of the relationships between otters and their food supply. Two approaches have been adopted to investigate the use of food resources by animals.