AQUATIC CONSERVATION: MARINE AND FRESHWATER ECOSYSTEMS, VOL. 5, 87-96 (1995) Home range and diet of re-introduced European otters Lutra lutra (L.) in Hertfordshire rivers KEVIN ROCHE’, REBECCA HARRIS2, STUART WARRINGTON and GORDON H. COPP Division of Environmental Sciences, University of Hertfordshire, College Lane. Hatfield, Herts. ALIO 9AB, UK ABSTRACT 1. The distribution and diet of two groups of captive-bred, re-introduced otters Lutra lutra (L.) was assessed through the collection and analysis of spraints (faeces), collected between July 1992 and March 1994 from the Rivers Stort, Lee and Rib in Hertfordshire (UK). 2. Almost immediately after release in 1991, both groups of otters moved upstream of their release sites. In February 1992, one male was found dead on a road near the River Stort, and both groups appeared to have moved downstream, their total range eventually extending to about 40 km, from 8km downstream of the confluence of the Rivers Stort and Lee to about 16km up the Stort and 16km up the Lee. 3. Fish, principally cyprinids, formed the major dietary element, with spatial differences in prey availability influencing the range of secondary items taken. Eels were the only prey category to show significant seasonal variation in spraints, occurring more often than expected in spring and summer spraints. Mammals/amphibia and birds were taken as prey only occasionally. Where environmental quality and/or prey species diversity were low, small-bodied fish species, such as three-spined stickleback Gasterosteus aculeatus, bullhead Cottus gobio and stone loach Barbatula barbatula, occurred more frequently in the diet. 4. Only two of the seven IUCN criteria for re-introductions appear to have been met before the otters’ release, and subsequent assessments suggest that most other criteria were not. As there has been only one possible, but unconfirmed, sighting of a female with cubs, it remains unclear whether the released otters have reproduced and thus the success of the re-introduction is doubtful. INTRODUCTION The decline of the European otter Lutra lutra (L.) in large areas of western Europe is now well documented (Chanin, 1985). In Britain, populations started dropping around the 1950s, with the use and subsequent bioaccumulation of pesticides and polychlorinated biphenyls (PCBs). Dieldrin and PCBs have been put forward as the chief culprits (Mason and Macdonald, 1993a), though there may well be further factors involved (Mason and Macdonald, 1983). Between 1977 and 1979, and again between 1984 and 1986, a national survey of otters in Britain indicated that the remaining populations were limited to south-west England, Wales and Scotland, with small isolated groups in Northumberland, East Anglia and the South of England (Strachen et al., 1990). ‘Current address: Institute of Landscape Ecology, KvEtna 8, 603 65 Brno, Czech Republic. 2Current address: 57 Lowther Road, Wokingham, Berks, RGl 1 1 JB, UK CCC 1052-7613/95/020087-10 01995 by John Wiley & Sons, Ltd Received 4 July 1994 Accepted 4 February 1995