ISSN:
1365-2109
Source:
Blackwell Publishing Journal Backfiles 1879-2005
Topics:
Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
Notes:
Abstract. The aims of the present study were to determine the number and age structure of grey herons, Ardea cinerea L., visiting a large freshwater cage trout farm in a study area in Argyll, western Scotland and to quantify heron predation at the site. Heron abundance was assessed on sea loch shores, running and standing freshwater bodies and at the trout farm between September 1985 and August 1987. Herons visited the farm almost exclusively at night or during twilight periods and, as a consequence, were seen more often by farm staff in winter than in summer. Herons selected cages containing small trout (〈300g) and removed them from cages by standing on the top nets and fishing through the mesh. Stock losses were of two kinds; trout were either eaten directly or were dropped by herons during manipulation, when characteristic wounds increased susceptibility to disease or made the fish unmarketable. Adult herons were more successful at feeding than first-years and although younger birds spent more time feeding their intake rates remained lower than those of adults in terms of their total time at the farm. Within a cage, smaller fish aggregated closer to the surface than larger ones and so were more vulnerable to heron predation. In many cases, a high proportion of the fish attacked by herons were blind and/or in poor condition. By recording only wounded fish, farm staff considerably underestimated their losses to herons. Nevertheless, attacks sometimes appeared serious to farmers. However, such losses were small compared with other forms of fish mortality and loss.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1111/j.1365-2109.1993.tb00826.x
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