title: Temperature-mediated shifts in the foraging behaviour of the Eurasian otter, Lutra lutra L
Creators: name: McCluskie, Alan Edward
Publisher: name: University of Glasgow
Year: 1998
Subjects: subject: Ecology, physiology.

description: Environmental variables will influence the behaviour of an animal by changing its state, and a relationship exists between such variables and the animal's behaviour. Aquatic animals are particularly under the influence of the medium in which they forage, and changes in this may have profound effects on the behaviour Recent studies have demonstrated that the metabolic costs of foraging in the Eurasian otter (Lutra lutra) are increased substantially by depressed water temperature. Otters are common around the west coast of Scotland, and yet they are frequently exposed to low water temperatures. This project examined the foraging behaviour of otters at the Taynish peninsula, Mid-Argyll, Scotland, and the effect of low water temperatures on otters and their prey, by observing the behaviour of otters at a range of water temperatures around. A regime of stationary fish trapping was initiated over 15 months to examine seasonal and spatial fluctuations in the abundance of the prey species. While the limitations of this method of assessing prey populations are discussed, it was clear that there were profound differences in the prey composition of the three habitats studied and that these changed seasonally. An examination of the accuracy of faecal analysis as a means of quantifying the diet of otters, was carried out. From this, potentially more accurate methods of assessing diet were suggested. The diet of the otters around the study site, as determined by observations and analysis of the faeces, demonstrated that the otters were selective of their prey at certain times of year, corresponding with those times of high prey abundance, but at times of poor prey abundance such selectivity was reduced. Foraging site use by the otters was examined in the context of temperature mediated fluctuations in prey biomass, as determined from the trapping. While the overall use of the habitats matched that with the highest biomass, temporal variations in this did not correspond with variations in biomass. Potential explanations for this are discussed. Changes in the activity levels and escape responses of some of the otter prey species were examined experimentally, and were found to have a significant positive relationship with water temperature. However, the actual capture times of these species, as determined from direct observations of the otters feeding, did not change with water temperature. This may have been caused by longer search times in colder water, or be due to the fact that otters forage for their prey when it is in the inactive component of its activity cycle. From this it was hypothesised that the otters would change the timing of their foraging in cold water, as it would no longer be dependent on the behaviour of their prey. This was tested by direct observation, and while no relationship was observed at any time of year with the tide, the observed relationship between foraging activity and time of day was altered in the winter. The relationship between the parameters of dive behaviour and water temperature was investigated. There was no strong relationship with any of them, however following recent studies in the literature, it was hypothesised that the metabolic costs of foraging would largely be met after the foraging bout was completed. A mathematical model was constructed to describe the relative amount of on land recovery time needed after a foraging bout at different water temperatures. This predicted that more time would be spent on land in lower water temperatures, and these predictions were upheld by observations from the wild. It was also apparent that the otters made greater use of deeper water during warmer water temperatures. This phenomenon was investigated by cost benefit analysis. Fish trapping revealed that there were better prey in deeper water and the success rate of dives was also higher. Conversely, dive times were longer in deeper water, and the prey, though of better quality were associated with longer handling times. Furthermore the rate of heat loss from the otter pelt was determined experimentally to be greater in deeper water. These data were combined in the form of an optimality model, which confirmed the results of observations that the otter only foraged in deeper water when the temperature of the water was relatively high. In conclusion it was found that the foraging behaviour of the otter was influenced by water temperature in the following ways: 1. The temporal pattern of foraging changes in the winter 2. Post-foraging recovery times on land are increased in colder water temperatures 3. Otters are restricted in their use of depth in colder water These results have demonstrated that the foraging behaviour of the otter cannot simply be viewed in terms of handling costs and prey energetic value, rather the complex influences of environmental variables on the otters physiological state, and the complex relationship this has with its behaviour, must be considered.