Ecologie de la loutre dans le Marais Poitevin III. Variations du régime et tactique alimentaire.

During the spring 1988, spraint samples were collected on the banks of 3 watercourses (one river and two canals) crossing the Marais Poitevin. Their general characteristics (flow rate, conductivity, hydrographic regime) are quite different. The fish community of the two canals was sampled by electrofishing and we estimated the relative abundance of the different fish species. After a check of the reliability of our spraint analysis technique (feeding trials with captive otters), we made an estimate of the relative abundance of the prey items in the otters diet and an assessment of the length and of the weight of each individual fish preyed upon. They are indeed very strong length-weight correlations in fish and also close relationships between the length of some skull bones and the total length od a fish. The diet is mostly made up by the eel. It comprises also a great variety of other fish species, nearly all that were observend in the habitat. Much frogs, some snakes, birds, mammals, insects and crustaceans were also discovered. They are only slight diet differences between the main watercourses which probably are related to the characteristics of their fish fauna (e.g. much more sticklebacks when waters are becoming brackish; big amount of Atherina in salt marshes). No marked variations were found during a normal summer but fish is obviously less eaten during a severe drought. Comparing the frequency-distributions of fish in the diet and in the habitat, we found no selective predation in respect with fish (eel, cyprinids) size. From that point of view, the otter display a generalistic-opportunistic pattern of foraging. As far as the relative abundance of the various species is considered, otters take much less roaches and much more eels and tenches than present in the habitat. It remains to be seen if that is the consequence of a 'deliberate choice' or of a peculiar way of underwater huntig. A fact is that the otter principally eats bottom living fishes.