Prey selection by coastal otters (Lutra lutra L.)
The thesis investigated the proximate factors affecting selection among different prey types by coastal otters. From a broad description of coastal otter diet, and its relationship to prey availability, the work progressed to a detailed study of the predatory behaviour of individuals and age classes, and thence to the formulation and testing of specific hypotheses regarding prey choice. Spraint analysis showed that the diet of otters in Loch Spelve consisted principally of small, demersal inshore fishes and shore crabs. It was demonstrated that while the frequency of occurrence of most fish species in spraints seemed to reflect availability, the occurrence of shore crabs did not. Direct observations of foraging otters showed that there were large differences in prey selection and foraging behaviour between age classes of otter. In particular shore crabs were consumed almost exclusively by juvenile otters and rarely appeared in the diets of adults. Foraging efficiency was shown to improve gradually with age and experience, and as it did so the proportion of shore crab in the diet decreased. It was surmised that shore crabs were not a preferred prey, but were relatively easy for juvenile otters to locate and capture. It was hypothesised that the apparent preferences of otters for certain prey types, such as for fish over shore crabs, were based on the relative energetic profitabilities of those prey types. A detailed examination of the costs and benefits, in terms of time and energy respectively, to otters of feeding on the main prey species supported this hypothesis. Shore crabs were markedly unprofitable, requiring considerable time expenditure for relatively little energy return. An apparent preference for rocklings and eelpout over butterfish was also consistent with the relative profitabilities of these species.