ERRORS ASSOCIATED WITH OTTER LUTRA-LUTRA FECAL ANALYSIS .2. ESTIMATING PREY SIZE DISTRIBUTION FROM BONES RECOVERED IN SPRAINTS

New methods of estimating the size of salmonids Salmo spp. and eels An guilla anguilla consumed by otters Lutra hutra are presented, based on feeding trials involving captive, tame otters. These methods involve modelling the size-related differential recovery of key fish bones rec overed in faeces (spraints) and avoid two sources of errors that may h ave occurred using previous techniques which relied solely on a series of correlations between fish length and the length of individual vert ebrae. Sources of errors avoided are: (a) that vertebrae recovered in spraints could not always be assigned to the correct correlation equat ion for mean, maximum, or minimum-sized bones; and (b) that the number of fish represented in a spraint was assumed to be one, in the case o f similarly-sized vertebrae, and two in the case of bones varying cons iderably in size. We tested the use of salmonid atlas vertebrae to det ermine the largest minimal numbers estimate, the length frequency dist ribution of salmonids consumed, and to estimate the proportions of tro ut and salmon in the diet. Eels do not contain an atlas which is resis tant to digestion and so equivalent estimates of the minimum numbers o f fish represented in spraints are not possible. A model was therefore developed to estimate the distribution oflengths of-fish consumed fro m the lengths of thoracic vertebrae recovered in the spraints using a series of equations. For each type of prey (salmonids or eels), the pr oportions of identifiable bones recorded in spraints were related to t he original size of ingested fish. Such models greatly enhance the val ue of otter spraint analyses, particularly for size-selection studies.