Module 3: Diet and Damage Assessment

Abstract

Data on the diet of an aquatic top predator are essential to assess the impact of predation on the fisheries resources. Quantitative data are sparse however. Field data is usually collected by analysis of undigested parts in the food remains of scats, pellets, or in stomach and intestine of dead animals. Advanced techniques as fatty acid or stable-isotope analysis makes it possible to assess diet of components not leaving identifiable remains and to look at diet over larger time-scales. A general observation is that aquatic predators tend to be opportunistic in their choice of prey. In the absence of field data the assumption can be made that diet reflects local availability of prey of appropriate size for the predator. To analyse real or perceived resource competition calculation of the amount consumed by the predator should be based on current and methodologically sound data. A simple comparison of the mass or number of a resource species taken by the predator and the fishery respectively can be highly misleading. An analysis should always be made in an ecosystem context, taking size distribution and density dependence into account.