DETERRING CORMORANTS Phalacrocorax carbo sinensis WITH WIDE
SPACED OVERHEAD WIRE GRIDS AT COMMON CARP Cyprinus carpio
WINTERING PONDS IN THE BAVARIAN OBERPFALI, 1995-1996

Jürgen P. Schmidt

Ganzenmüllerstr. 11, D-85354 Freising, Germany

The continental subspecies of the Great Cormorant Phalacrocorax carbo sinensis, wintering in Bavaria, can cause significant losses of Common Carp Cyprinus carpio from wintering ponds at fish-farms (Keller & Vordermeier 1994). Various types of covering of fish-ponds have been used to deter fish-eating birds by forming both a physical and optical barrier (Keller 1996, Veldkamp 1996, Mott & Boyd 1995). The barrier can be a net which is an effective protection for tanks, small ponds and raceways. Trout farm ponds in Bavaria have been effectively protected by complete enclosure with narrow-spaced parallel overhead lines and netting walls (Bohl 1997). This system is suspended by poles allowing good access for harvesting and feeding. To protect larger ponds, partial enclosures using wide-spaced (1-20 m) overhead lines of rope or wire arranged as a grid pattern were found to be more practicable (FAO 1989; Fig. 1). The idea of the overhead lines is to interfere with landing and take-off of visiting birds and thereby reduce the attractiveness of the fishponds.

Figure 1. CAD graph of a wire-covered pond, 10 m line spacing.

Previous attempts to deter cormorants with overhead lines have reported conflicting results (Keller & Vordermeier 1994). This study evaluated the effect of line spacing on deterring cormorants at nine Carp wintering-ponds at two fish-farms in eastern Bavaria in autumn 1995.

Study area, ponds and deterrent material

The two farms are situated in the Bavarian Oberpfalz in the county of Schwandorf, a traditional pond farming area. According to the fish farmers there had been cormorant visits in 1994. The production from one- to two-year old Carps (C1, to C2) was stopped (pers. comm.). 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 square grids. Threc spacing distances were evaluated: 5, 7.5 and 10 m (Fig. 2). At farm II a single pond of 0.85 ha was fitted with wires spaced at 7.5 m. The wires are supported in the ponds by metal poles (galvanised steel pipes, 3!4", 26.7 mm outer diameter) of 2 or 3 m length. The poles were installed by boat at each crossing point of the wires for the 10 m grid and at every second point (staggered between rows) at the other two spacings. To anchor the ends of the wires, metal L-profiles of 0.50 m length were driven into the banks of the ponds. In order to not interfere with the mowing of the bank vegetation, the anchor points together with the first part of the wire were set below surface level into small ditches. For steeper banks it was found to be more solid to use 2 m-poles instead of the L-profiles. At the end of the project experience has shown that the installation of L-profiles was too labour-intensive. Their use cannot be recommended. The wires were usually installed by two workers and tensioned by inserted turnbuckles. Material costs were about 2,700 DM/ha (5 m spacing) and 1,400 DM/ha (10 m spacing). For the 7.5 m-type 1,200 - 1,600 DM/ha could be worked out. At farm I labour requirements were about 171 man hours per ha, at farm II (7.5 m spacing) the installation took 81 man hours per ha.

Figure 2. Survey of fish farm I (T1-T8, pond numbers); pond 1-8 with overhead lines 1995. Pond B, C and D were covered in summer 1996 by the farmer himself. Pond A. E and F are not managed by him (after aerial photograph 1:5000, StMLU 1995).

Stocking and harvesting data

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 at farm II additionally Pike Esox lucius. The lengths and weights of Carp samples from autumn 199S are listed in Table 1.

Table 1. Fish farm l; pond number, sample size and sizes of Carp Cyprinus carpio. The fish samples were taken in autumn 1995 during fish stock.

The change in yield of harvestable fish from the wire-grid protected ponds provided an important criterion to evaluate the effectiveness of the bird deterrents. In spring 1995 at farm 1, 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 fish-farms were about 10 % (Table 2, farm I). This level was considered satisfactory by the fish farmers. It is in accordance with published data, e.g. Bohl (1982) mentions a maximum of 10% loss during wintering of two-year old Carps (C2). The physical condition of the Carps was examined by a fish veterinary surgeon in autumn and spring. No fish injured by cormorants were found during the spring 1996 harvest (Fuchs & Hoffmann 1996).

Table 2. Fish farm I; pond sizes. fish density (carp) and numbers of Carp stocked in spring (pers. comm.) and autumn I995, numbers of carp harvested in spring 1996.

Cormorant observation

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 farmers to usually have greatest cormorant predation. After November ponds froze over and observations were discontinued. At farm I, where most monitoring was undertaken, a permanently sited caravan was used as a hide from which observers could watch visiting cormorants without interference. 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 ponds during the watches. The fish farmer reported no fishing by cormorants until harvesting in April 1996 (pers. comm.). Nine watches were analysed in greater detail to describe the cormorants reaction to the overhead wires. In two cases a bird was quite near the wire grid over the pond surface, but then suddenly interrupted landing and flew off. Others circled over the area and then left without any attempt to land. Because all observed visiting cormorants during these nine observation periods were successfully deterred by the wires, data are insufficient for determining 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 at the research station of Wielenbach/Oberbayern also operating with 7.5 x 7.5 m wire grid (Severin-Edmaier 1996).

Evening counts at nearby roost sites (distance to Farm I ~13 km, to farm II ~3 km as the crow flies) were also made during the study period. These counts showed that large numbers of potential foragers (c. 100 in October and c 170 in November) were present in the study area throughout the study period.

Summary

Within the Bavarian Oberpfalz Cormorant Project, foraging cormorants were successfully kept away from the wintering ponds by a system of overhead wires. Fish farmers (Goldschmitt 1996) 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 (Baird et al. 1993, Trolliet 1993). This did not happen at the study-ponds of Oberpfalz Project even in the second winter (1996-97) after installing the wires (Wimmer pers. comm.).

References

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Fuchs, I. & R. Hofi'mann 1996. Methodik und Ergebaisse der Fischuntersuchungen vom Herbst 1995 und Frühjahr 1996 an den Teichen des Betriebs 1. 5 p. (unpublished).

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Severin-Edmaier, G. 1996. Die Wirksamkeit der weitmaschigen überspannung (Raster: 7,5 m x 7,5 m) eines Karpfenteiches als Abwehrmaßnahme gegen den Kormoran während der Sommermonate. AbschIußbericht über das "Teichüberspannungsprojekt Wielenbach". Im Auftrag des Bayerischen Landesamts für Wasserwirtschafi, München, 39 p (unpublished).

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