Examination of some factors affecting selection of fish prey by otters (Lutra lutra) living by eutrophic fish ponds

In this study the predation and fish prey selection of otters Lutra lutra L . living by eutrophic fish ponds on agricultural land and in a protected ar ea of temperate climate in Hungary were investigated. The correlations betw een fish in the diet of the otters (by spraint analysis involved 1942 and 1 280 samples for the two habitats) and the fish stock available (by harvest and sample fishing) were generally close. Prey selection was significantly related to selected species of a particular size range (p < 0.01). The pref erence calculations were performed with Ivlev's index of preference (E-i, m inimum - 1. maximum +1). Regardless of species, the otters avoided (E-i= -0 .51) fish heavier than 1000 g, with a preference for individuals weighing b etween 500 and 1000 g (E-i = 0.79). No substantial or clear preference was observed in the weight range below 500 g (E-i = - 0.02-0.38). The preferenc e for fish in accordance with their characteristic sites of occurrence with in the body of water was also significant (P < 0.0 1). They avoided fish li ving primarily in open water (E-i= -0.64) and to a lesser degree those occu rring near the pond bed (E-i= -0.22). They favoured fish inhabiting the are a with a covering of aquatic plants (E-i = 0.46), and showed a preference t o a lesser degree for fish living in the shallow littoral regions (E-i = 0. 14). With the cessation of fish farming and the effect of the drastic chan ges which occurred in the vegetation, the otters fed substantially on the s tock of alternative sources of prey such as amphibians and water insects as well as terrestrial animals, and at such times, depending on season, fish became a secondary source of prey.