Diet of the Eurasian otter (Lutra lutra) in relation to freshwater habitats and alien fish species in southern Italy

Abstract

The diet composition of otters in the upper catchment of the River Agri (southern Italy) was investigated by spraint analysis (N = 555) between May 2001 and January 2003. Once a season, spraints were collected along four riverine stretches, two with swift-waters and two with slow-waters. Overall, fish represented the dominant food (percent mean volume, Vm% = 57.30), followed by amphibians (Vm% = 18.90) and crayfish (Vm% = 15.60). The food composition of otters feeding along swift-water sections differed significantly from that recorded along slow-water sections. The trophic niche was larger for slow-water sections, because of the presence of more fish species. In both sampled habitats (swift- and slow-waters) the importance of fish decreased in winter when flooding increasing water speeds and turbidity and thus reduced the hunting efficiency of the otters. Hibernating amphibians represented a primary alternative resource. As expected, insects and reptiles were eaten more frequently in summer, while birds predominated during their breeding season. Seasonal variation in fish consumption could have been related to changes in fish activity and to environmental conditions, which influenced the availability of prey to the otters. For both habitats, trophic niche breadth showed a minimum value in autumn, when fish were dominant in the otters' diet, whilst a peak was reached in summer for slowwater sections and in spring for swift-water sections. Comparing three sampling periods (1989–1990, 1996–1997, 2001–2002), a time–lag of about 13 years seemed to have occurred between the introduction of centrarchids, mainly the largemouth bass (Micropterus salmoides), and their predation by otters. This relatively long lag phase could have been related to the pattern of alien fish invasion rather than to the inability of the otters to readily prey on newly available resources. Our results confirm the opportunistic feeding behaviour of the otter in Mediterranean habitats, the species diet tending to match the seasonal and spatial variation in the relative abundance of different prey items.

Keywords:

• Lutra lutra
• diet
• alien fish
• swift- and slow-waters
• southern Italy