Interactions between harbour seals (Phoca vitulina) and salmonids (Salmo spp.) in estuarine environments
There is great interest in the perceived conflict between salmon fisheries and seals, but little information to inform managers. This study therefore explored the interactions between harbour seals and salmonids within an estuarine system, the Cromarty Firth, NE Scotland (57°37’N, 4°21’W). The number of seals using this area in the 2000 pupping season was estimated to be 188 (138-286) using a novel Bayesian framework that corrected counts of seals on land for those remaining in the water. The presence of seals in the mouth of the River Conon, at the head of the Firth, and the occurrence of salmonid otoliths in seal scats, were related to changes in the abundance of adult salmonids. Overall, salmonid otoliths were found in 8% of scat samples and 21% of those collected during July. This represents the highest reported incidence of salmonid otoliths from scat samples collected in the UK; it is not clear if this is due to temporal or geographic differences with previous work. A novel diet estimation technique was constructed based on the assumption that seals may be employing one of a range of possible foraging strategies. This estimator was compared to two established methods and it was found that model choice introduced considerable introduced considerable bias (up to 3x) in the estimated importance of salmonids in the diet. It is unclear which of the models is appropriate and their outputs were combined to represent uncertainty in our estimates of diet more fully. It was not possible to partition consumption of salmonids between salmon and sea trout as otoliths were too badly digested to allow identification of species. A simple food web was used to suggest that removal of seal predation may improve fishery catch by 17% (5-52%). This figure must be treated with caution and potential biases, and caveats, are discussed.