Key landscape factors for Eurasian otter Lutra lutra visiting rates and fish loss in estuarine fish farms

Conflicts between the conservation of large vertebrates and the use of biological resources are common and affect many species and activities. The vast majority of the Portuguese fish farms are located in estuarine protected areas and production of marine fish species is done mostly in semi-intensive regimes and with the imposition of some restrictive rules. This exposed form of production is prone to predation by wild animals. The Eurasian otter Lutra lutra is widely distributed in Portugal and this combination of factors has led to a conflict scenario between fish farming and the conservation of otters. This study aimed to determine the influence of landscape factors in otter damages in fish farms of Sado estuary (SW Portugal) and the usefulness of visiting rates and otter diet as surrogates for damage assessment. The impact of otter predation (damage) in each fish farm was assessed by combining the percentage of consumption of reared species with otter daily food consumption, daily visitation rates, and number of otters visiting each fish farm. Landscape descriptors were recorded in the field and several landscape metrics were calculated using a geographic information system. Multiple linear regressions were used to identify key landscape factors for damage and for damage surrogates. Distance to streams and refuge cover areas were identified as key landscape features to damages. This information can be used as a management tool in landscape planning, specifically in estuaries with fish-farming activity, as high-risk farms can be identified and selectively protected.