THE EFFECTS OF OTTER (LUTRA-LUTRA) ACTIVITY ON SPRAINT PRODUCTION ANDCOMPOSITION - IMPLICATIONS FOR MODELS WHICH ESTIMATE PREY-SIZE DISTRIBUTION

This study is an investigation into the influence of activity on otter (Lutva lutra) spraint production and composition, based on feeding tr ials involving captive, tame otters. Trials were of two types: 'active ' where animals were encouraged to run, swim and dive for at least 33% of the time during the first six hours after each meal, and 'inactive ' where activity varied between zero and 18% of this period. Trial typ es were balanced for number of fish, meal volume and fish size. Activi ty was found to have a large effect on spraint production and composit ion. For active trials, median minimum gut transit time was 67 minutes and median number of spraints per trial was 23. Corresponding figures for inactive trials were 170 minutes and 14, respectively. Of ingeste d salmonid (Salmo spp.) atlas vertebrae, 60% were recovered in spraint s after active trials, while only 32% were recovered after inactive on es. Analyses of these effects showed that our previous models for esti mating prey size distribution from bones recovered in spraints gave a better fit for inactive than for active trials. As otters in the wild are likely to spraint mostly during periods of activity, especially fo raging, further feeding trials were undertaken to refine an earlier mo del better to mimic the field situation. Trials, involving active otte rs, were undertaken to: (i) produce in vivo bone to fish length equati ons for salmonid atlases and eel (Anguilla anguilla) thoracic vertebra e; and (ii) model the size-related differential recovery of these bone s. These refined equations are more soundly based to provide estimates of prey size distribution than the earlier ones and their use in futu re studies of size-selection is recommended. The present study also hi ghlights the potential limitations of extrapolating from the captive s ituation to the wild.