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The Scottish populations of salmonids are important both ecologically and economically. Game fisheries for Atlantic salmon, sea trout and brown trout are all highly acclaimed and generate substantial levels of income for Scotland, but many populations are in decline and efforts are being made to ensure the future sustainability of these species. These declines have led to a focus on the impact of piscivorous bird predation on fish populations. The purpose of this review was to assess the evidence for population-level impacts on salmonid populations, and/or economic impacts on Scottish game fisheries of predation by the four primary UK freshwater piscivorous bird species; cormorant Phalacrocorax carbo, goosander Mergus merganser, red-breasted merganser Mergus serrator and grey heron Ardea cinerea. There is evidence that these birds can, in some situations, remove large numbers of fish from stocked and natural fisheries. However, a lack of information on fish population levels, the numbers and species composition of feeding birds, and robust calculations of fish consumption has hampered the conversion of the results of the existing studies into useful quantitative measures of impact. As a consequence, few studies have demonstrated any reductions in numbers of breeding fish or fish productivity due to predation by piscivorous birds, or direct economic loss to fisheries in Scotland. We support a previous recommendation for a reiterative procedure of modelling, experimentation and remodelling to assess the impacts of piscivorous birds on fisheries. Wide-scale studies of the movements of piscivorous birds, their feeding locations in relation to river characteristics, and the factors that make fish particularly vulnerable to predation are seen as important areas for future research.

The influence of predators on the distribution, density and dynamics of their prey species has long been of interest to ecologists and wildlife managers. Where the prey population is also utilised by humans, conflicts may arise through competition for a limited resource. Because gamebird shooting in the UK provides employment, recreation and income, the impact of birds of prey on gamebird populations has been the subject of intense debate for many years. A variety of approaches has been used to assess the impacts that raptors have on gamebird populations. Here we review the applicability and limitations of the methods used and assess the scientific evidence for population-level and economic impacts of raptors on gamebird populations in the UK. Raptors may, in some situations, take large numbers of gamebirds and may be an important proximate cause of mortality, although few studies have assressed the impacts of raptors on either breeding or pre-shooting densities. Two exceptions are studies of Hen Harrier and Peregrine predation on Red Grouse on moorland in Scotland and Sparrowhawk predation on Grey Partridge on farmland in England. Both these studies suggested that raptors could have population-level impacts when their gamebird prey was already at low density. Studies on predation of captively bred gamebirds suggest that numbers taken by raptors at release pens vary considerably and in a few cases raptors have been documented killing relatively large numbers. On the whole, however, it appears that raptors account for a relatively small proportion of mortality among released birds and the impact on subsequent shooting bags is unknown. We summarise important gaps in current knowledge and recommend specific areas for future research.