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La Tour du Valat

Institut de recherche pour la conservation des zones humides méditerranéennes

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Titre :	Deterring cormorants (Phalacrocorax carbo sinensis) using overhead lines at common carp (Cyprinus carpio) wintering ponds in the Bavarian Oberpfalz 1995-1996
Auteurs :	Juergen P. Schmidt, Auteur
Type de document :	Article
Dans :	Ornithologischer Anzeiger (37(1) Jan., 1998)
Article en page(s) :	1-18
Résumé :	1. The continental subspecies of Cormorant (Phalacrocorax carbo sinensis) wintering in Bavaria can cause significant losses of common carp (Cyprinus carpio) from wintering ponds at fish-farms. Various types of covering over fish-ponds have been used to deter fish-eating birds by forming both a physical and optical barrier. The barrier can be built as a net cover being an effective protection for tanks, small ponds and raceways. In Bavaria trout farm ponds have been effectively protected by entirely enclosing with narrow-spaced parallel overhead lines and netting walls. The system is suspended by poles allowing good access for harvesting and feeding (table 4, fig. 2). To protect larger ponds, the use of wide-spaced (1-20 m) overhead lines of rope or wire arranged as a grid pattern is said to be more practicable (fig. 3). The idea of the overhead lines is to interfere with landing and take-off of visiting birds and thereby reducing the attractiveness of the fish ponds. 2. Previous attempts to deter cormorants with overhead lines have reported conflicting results. This study evaluated the effect of line spacing on deterring cormorants from nine carp wintering ponds at two fish-farms in the Bavarian Oberpfalz in autumn 1995. At farm I, eight ponds ranging in size from 0.19 to 0.7 ha (total 3.1 ha) were fitted with overhead wires (2.5-2.8 mm diameter) arranged as grids. Three spacing distances were evaluated: 5, 7.5 and 10 m (figure 1). At farm II a single pond of 0.85 ha was fitted with wires spaced at 7.5 m. The technical specifications, construction methods, material costs and labour requirements (per ha) are described. The ponds were stocked with two-year old (C2) carp (Cyprinus carpio) in polyculture with tench (Tinca tinca), pikeperch (Stizostedion lucioperca), grass carp (Ctenopharyngodon idella) and (only farm II) pike (Esox lucius). 3. Visits by foraging cormorants to the study ponds were monitored from mid October to the end of November (1995), the period reported by the fish farmer to have greatest cormorant predation. After November ponds froze over and observations were discontinued. At farm I, where most monitoring was undertaken, a permanently installed caravan was used as a hide from which observers could watch visiting cormorants undisturbed. At all three spacing distances evaluated the overhead wires effectively deterred visiting cormorants. Although a total of 113 cormorants were recorded at farm I during 27 observation days, no cormorants were observed landing at any of the study ponds during the watches. The fish firmer reported no fishing by cormorants until April of the following year. Nine watches are analysed to describe the cormorants reaction to the overhead wires. Because all observed visiting cormorants of these nine watches were successfully deterred by the wires, insufficient data were available to determine the optimal wire spacing at farm I. At farm II the good results of the spring harvest seem to indicate an effectiveness of the 7.5 m spacing. This is supported by another study in Bavaria during summer 1996. Evening counts at nearby roost sites showed that large numbers of potential foragers (ca. 100 in October and ca. 170 in November) were present in the study area throughout the study period (table 1). 4. The change in yield of harvestable fish from the study-ponds provided an important additional criterion to evaluate the effectiveness of the bird deterrents. In spring 1995 at farm I, prior to the provision of overhead wires, the losses had been estimated at 88%. In spring 1996, following the winter with the overhead wire deterrents, the total losses of carp at both fishfarms were about 10% (table 2 and 3). This level was considered satisfactory by the fish farmers and is in accordance with published data. No fish injured by cormorants were found during spring 1996 harvest. 5. In the Bavarian Oberpfalz Cormorant Project, foraging cormorants were successfully kept away from the wintering ponds by a system of overhead wire. Fish farmers and other studies have reported a lessening in the effectiveness of wide-spaced overhead wire deterrents with time because cormorants become adept at manoeuvring between the wires. This did not happen at the study ponds of the Oberpfalz Project even in the second winter (1996/97) after installing the wires. Other questions of interest concerning the protection of fish farms are discussed.

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