Introduction

Throughout the present century, several species of otter have suffered a significant decline in their numbers, frequently due to direct persecution, hunting and the destruction of their habitat (Foster–Turley et al, 1990). However, where populations have declined in the absence of these causes, contamination is thought to be involved and has been extensively demonstrated in Lutra lutra (Mason & Macdonald, 1986; Mason, 1989; Macdonald & Mason, 1994).

Those compounds that are accumulated in the organism through the food chain, reaching high levels in predators, have been specially studied. As well as affecting the mustelid directly, such components can also have an impact on prey, leading to a scarcity of food (Mason, 1989).

Amongst the compounds that are biomagnified, special attention has been paid to organochlorines (polychlorinated biphenyls (PCB), tetradichlordietan DDT, cyclodienes), being those on which most studies have been carried out (Mason, 1989; Olsson & Sandegren, 1991a,b; Smit et al, 1994). Amongst the most notable effects, besides death when the compounds are present in high concentrations, are the faults found in reproduction and the immune system, alterations in the nervous system, with changes in behaviour, and malformations (Mason, 1989; McBee & Bickham, 1990; Kihlstrom et al, 1992). Several of these effects have been found in Lutra lutra (Keymer et al, 1988; Mason & O'Sullivan, 1992).