The impact of cormorants (Phalacrocorax carbo carbo and Phalacrocorax carbo sinensis) on inland fisheries in the UK

Cormorants are piscivorous birds with a daily food intake (DFI) of approximately 500 g. They are a protected species under the Wildlife and Countryside Act 1981. In the UK, the number of over-wintering, inland cormorants increased steadily between 1970 and 1987, at a rate of between 5 and 10 % per annum. An increase of 74 % occurred between winter 1987/88 and 1990/91, and the population is still believed to be rising. The population growth was observed in all regions of the UK, on all habitat types. As cormorants exploited new habitats, ornithologists welcomed their increased presence. This contrasts with the views of angling bodies, who assert that the presence of cormorants, feeding daily on their fisheries, has a damaging impact on fish stocks with inevitable financial losses. Due to a lack of effective non-lethal control methods, the angling bodies wish to see the cormorants removed from the protected species list so their inland numbers can be controlled. Ornithologists insist that there is no scientific evidence proving cormorants are damaging to inland fisheries and so are opposed to any culling. A review of previous cormorant studies was undertaken to evaluate information on their ecology, feeding behaviour and predation impact. The general conclusion was no study had been able to prove cormorant predation damages fish populations, because few studies had moved beyond determining the mass of fish removed by the birds over the particular study period. No assessment had been made of the impact of that fish removal on the fish population dynamics and the angling performance of the fishery. This highlighted the requirement for research into the impact of cormorant predation on inland fisheries. This study was formulated to estimate cormorant predation impact on fisheries in a more realistic and robust manner than had previously been undertaken. The principal objective of the study was to integrate fish population and cormorant feeding dynamics data on specific fisheries (study sites) in such a way as to quantify, where possible, the full impacts of the cormorant predation. This required the following criteria at each study site: " evaluation of the historical status of fish and cormorant populations; " determination of the population and community dynamics of the fish stocks; " analysis of the angling effort and angling performance; " identification of the species, and estimation of the numbers and sizes of fish consumed and wounded by cormorants, and comparison with the numbers and sizes of the fish populations present; " determination of the occupancy on, and use by, cormorants at the selected sites. The work programme ran between September 1995 and July 1998, covering three winters of cormorant predation. At each study site, the cormorant feeding dynamics were assessed by detailed feeding observations and cormorant counts. This enabled data to be collected on the species, size and amount of fish being ingested during each foraging bout, and the diurnal and seasonal patterns of cormorant occupancy. As feeding observations were unable to completed at each site everyday, a modelling system was designed, using a Monte Carlo Simulation (MCS), to estimate the number and mass of