Individual study: The efficacy of land versus raft-based live cage-traps in the capture of feral coypu Myocastor coypus in the Yare River Valley, Norfolk, England
Baker S.J. & Clarke C.N. (1988) Cage trapping coypus (Myocastor coypus) on baited rafts. Journal of Applied Ecology, 25, 41-48
Feral populations of coypu Myocastor coypus (a large rodent native to South America) exist in many countries and a number of different control methods are employed. In Britain (where there were formerly feral populations of coypus established in wetlands in eastern England through individuals escaping from fur farms), it is illegal to use spring traps or poison against coypus, and trappers use cage traps to catch and hold coypus until they can be humanely killed. The experiments reported here compared the efficacy of cage trapping where cages were set on a baited raft ('raft trap') as opposed to a more traditional position on the land ('land trap').
Study area: Trapping was carried out over the winter of 198 1-82 by three experienced trappers in a 1,100 ha area of the Yare river valley in Norfolk, eastern England. The land is a flat patchwork of pasture and arable fields intersected by drainage ditches, and at this time supported a feral population of coypus.
Cage traps: The cage traps used were made of welded wire mesh and measured 850 x 250 x 250 mm. A drop door was released when an animal entered and depressed a treadle. The rafts used weighed 20 kg each and comprised a 2 m x 1 m plywood base, with blocks of expanded polystyrene fixed underneath to provide additional buoyancy. Three traps were mounted in a row at one end facing a bait tray.
Experimental design: The study area was divided into four blocks, and each subdivided into two. Six land traps were allocated to one half and two rafts to the other. These were placed in what was judged to be the optimum positions. Traps were baited with carrot on the first Monday and checked daily until Friday. Land traps were then closed and moved out of position to leave the runs free; rafts were left baited but traps were locked open. All traps were reset the following Monday and this procedure was undertaken for 5 weeks.
All traps were then removed for 5 weeks, after which trapping recommenced for a further 5 weeks. The areas that had been previously allocated rafts received land traps and vice versa. Because of heavy snow, 92 trap-nights were lost during the last week of the first period.
Trapping success: During the comparison of land and raft traps, 150 coypus were caught during 1,656 trap-nights with 90 caught in raft-based and 60 in land traps. The total number of coypus caught on areas initially trapped with raft traps was similar to that from areas initially trapped by land traps (n =73, 77). Although proportionally more adult males than adult females were caught using raft traps there was no significant difference in the overall proportions of the four sectors of the population caught by the two techniques.
Non-target species: Several bird species e.g. moorhen Gallinula chloropus (741) and mallard Anas platyrhynchos (313) (the commonest captures) and mammal species e.g. water vole Arvicola terrestris (61) were caught during this and subsequent trials between January 1982 to April 1986 (33,067 trap-nights with land traps and 8,803 trap-nights on rafts). Almost all such individuals were released, apparently unharmed. Casualties included eight were moorhens, four unidentified ducklings (probably mallard), one an adult mallard and two water voles.
Conclusions: Overall, raft traps were 50% more effective than land traps on the study area, where the habitat was predominantly drained marsh. No attempt was made to 'cost' the relative effort needed to deploy rafts and land traps, as much depends on factors such as the terrain, distance equipment has to be carried, whether land traps are to be used singly or in groups, etc. However, as one raft is equivalent to more than four land traps, the authors conclude that rafts are cost effective.
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