This problem of fish-eating birds was widespread at Scottish fish farms. Grey heron (<i>Ardea cinerea</i>), cormorant (<i>Phalacrocorax carbo</i>), shag (<i>Phalacrocorax aristotelis</i>), goosander (<i>Mergus merganser</i>) and red-breasted merganser (<i>Mergus serrator</i>), the principal species implicated, were studied in an area on the west coast of Scotland from September 1985 to August 1987. Data were also collected from farms in south Argyll, Highland and Tayside Regions. Herons visited both pond farms, taking fish in the shallows, and cages, taking fish through top nets. Fish were either eaten or dropped, the resulting wounds often making them unmarketable. Birds tended to arrive at cages during the hours of darkness, selecting those cages containing fish weighing less than 300g and taking fish swimming close to the surface. These fish were generally smaller than the majority held in the cage. Adults were aggressive towards first-years who had a lower feeding success. However, both age classes increased their food intake rate by feeding at fish farms. In some cases this increased food supply appeared to influence breeding, allowing birds to lay earlier, produce larger clutches and larger broods surviving to fledge. Such increased production was offset by widespread persecution. Between 1069 and 1936 birds were estimated to be killed annually, of which c. 14% were adult, but it was considered unlikely that this reduced the overall Scottish population. Cormorants visited marine and freshwater cage farms, particularly during the winter. In freshwaters they may have been attracted by large numbers of escaped rainbow trout living in the waters adjacent to cages. Underwater attacks could cause considerable damage to fish in cages, but birds did not remove fish from them. Anti-predator nets reduced this damage, but were not toally effective. Between 980 and 3047 birds were estimated to be killed annually, and as 25% were adult, the Scottish population could have been affected. Shags may have been attracted to marine cages by the large concentrations of wild fish living in adjacent waters. Juveniles, but not adults, had an increased food intake rate fishing close to farm cages when compared with elsewhere. There was almost no evidence of damage to stock and persecution was therefore unwarranted. Between 1930 and 4255 birds were estimated to be killed annually, but almost all were juveniles and it was unlikely that such pesecution reduced the Scottish population. Goosanders were a problem at a few sites during the spring. Gulls and crows scavenged dead fish and fish food opportunistically, while other species which visited farms caused no problems. Losses to fish-eating birds were small in comparison to other forms of fish mortality and represented only a small fraction of a farm's running costs.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:642641 |
| Date | January 1988 |
| Creators | Carss, David N. |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1842/13326 |
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