Grey seal Halichoerus grypus breeding sites contribute substantial carrion biomass to the Firth of Forth

Decomposing organic matter is central to the recycling of energy and nutrients in all ecosystems. Few studies have investigated the role of animal carrion biomass in ecosystem functioning, and quantitative data on carrion biomass are lacking. The role of carrion inputs in the marine environment specifically is poorly understood. The grey seal Halichoerus grypus breeding colony on the Isle of May in the Firth of Forth, Scotland, provides insight into the contribution of regular carrion pulses to the surrounding marine ecosystem. This study analysed 3 breeding locations with a range of topographies, elevations and tidal influences. Carcasses were mapped from aerial images and ground visual surveys in the 2008 and 2012 breeding seasons. Generalised linear mixed models were used to explore the degree to which breeding location and the position of a carcass influenced its availability to marine scavengers. Carcasses closer to shore were more likely to be completely displaced to the marine environment, and this effect varied with breeding location. An approximate 0.9 to 1.3 t of biomass per hectare of breeding site per year were released into the marine system. For carcasses that were below the high-water spring tide range but remained on shore, we quantified the typical duration of submersion to range from 5% to 44% of the time carcasses were ashore. Additionally, up to 808 kg of carrion was accessible to marine scavengers while washed by tides. Our results suggest breeding colonies of grey seals may contribute significantly to the carrion biomass available in local marine systems.