44 SEPTEMBER 2013 • WORLD AQUACULTURE • WWW.WAS.ORG decreased competition and better food availability. The calculation of the compensation is based on hypothetical growth without the impact of the otter. The following example illustrates this approach: A pond was stocked with 600 carp, each approximately 0.4 kg. At the end of the season 200 carp each approximately 2 kg were harvested. Assuming a natural loss of 5 percent, 570 carp should have been harvested. Therefore, by excluding other causes, 170 carp must have been captured by the otter. Without the reduction of the fish stock by the otter, the fish would have grown to 1.6 kg, rather than 2 kg. On that basis, 1.6 × 170 = 272 kg were lost by otter and the owner will be compensated for that loss. The calculation of financial loss is based on the price of fish sold as stocking material, independent of the actual marketing strategy of the producer. For instance, fish sold directly to the consumer fetch a higher price than fish sold wholesale or as stocking material. For the loss of 272 kg carp, assuming a price of US$ 4.22/kg, US$ 1,147.84 will be compensated. Since 2010, actual financial compensation also depends on pond size and is limited to US$ 960/ha. Prevention Strategies Compensation helps to mitigate the financial damage to the fish producer but cannot stop the otter from feeding on pond fish. Therefore, effective strategies to protect fish and reduce damage caused by otter were and are still being developed. Fences (especially electric fences) are the most commonly used countermeasure against otters. A carefully serviced electric fence effectively prevents otters from reaching the pond. Maintenance, however, is costly and labor-intensive throughout the year, such as keeping fences free from snow and grass. In general, fencing a pond of 1,000 m2 costs approximately US$ 1,640, excluding installation and service. Between 1998 and 2007, 156 ponds smaller than 1,000 m2 were registered as suffering losses from otters. Fencing all these ponds would have reduced the damage by 5 percent (US$ 83,200). Approximately US$ 256,000 would have been necessary to fence all 156 ponds. This is more than three times the compensation of US$74,920 paid in 19982005 to the owners of these ponds. In addition, ponds smaller than 1,000 m2 only account for 8 percent of the total amount of granted compensations. This demonstrates that fencing has only a limited potential to prevent losses caused by otter because overall the most damage is done in ponds too large for fencing. There are also some disadvantages of electric fences. Not only otters are kept away from ponds but also other animals, including roe deer. Moreover, electric fences are deadly traps for amphibians, hedgehogs and other animals. Unfortunately some otters may have learned to breach fences by climbing or tolerating electric shock. There are alternative prevention strategies, some of which are feasible for large ponds. Alternative Prevention Strategies One preventative strategy is to reduce the amount of prey available for the otter. This can be achieved by emptying the pond for the winter and transferring fish to a smaller, well-protected pond. A second method is to fish the pond out, but leave it full of water during the winter. Both strategies force the otter to look for food elsewhere. Another strategy is to provide otters with a different source of food. Otters do not feed on fish exclusively, but also eat amphibians and aquatic insects. To supply otters with these other food sources, ponds need vegetation-rich and semi-natural littoral zones. Especially for large ponds and during the winter when the food supply is limited, it would be reasonable to install so-called distractive ponds. These small ponds are situated close to the large pond and have a high stocking density of non-commercial fish. Hunting in these small ponds is more efficient for the otter and it keeps them away from valuable fish in the large pond. A temporary strategy is to scare off otters using acoustic, visual or olfactory signals. The effect is temporary however and the otter gets attuned to these signals and ultimately ignores them. Ultrasound has no impact on otters. Conclusion Ponds provide a suitable wildlife habitat not only for otters but a broad variety of birds, amphibians, insects and plants. However, fish ponds are artificial structures constructed for fish production and are often privately owned. The existence of the ponds depends on their regular use. Once abandoned, ponds tend to silt up and become quickly overgrown. Therefore, recognizing the economic problems resulting from otter predation has a knock-on effect on pond conservation. The great number of otters is caused by the non-natural high availability of resources unintentionally provided by fish ponds. The resulting economic damage is but one side of the coin. There is also increased predation pressure on natural fish populations in stream ecosystems, amphibians and aquatic birds. This pressure tends to rise where otters are denied access to fish ponds and must hunt for food in other aquatic systems, possibly threatening other endangered species, such as crayfish Astacus astacus or freshwater pearl mussel Margaritifera margaritifera. Although prevention strategies help to mitigate the conflict between fish producers and otters, the perfect strategy for untroubled coexistence of human and otter has not yet been found (Myšiak et al. 2004). Notes Christian Bauer, Federal Agency for Water Management, Institute for Ecology and Aquaculture Gebharts 33, 3934 Schrems, Austria, oeko@baw.at, Fax +43 2853 78463 References Kranz, A. and L. Polednik. 2009. Zur aktuellen Verbreitung und jüngsten Ausbreitung des Fischotters in Niederösterreich. Bericht im Auftrag der Abteilung Naturschutz des Amtes der Niederösterreichischen Landesregierung – in german. Myšiak, J., K. Schwerdtner and I. Ring. 2004. Comparative analysis of the conflicts between carp pond farming and the protection of otters (Lutra lutra) in Upper Lusatia and South Bohemia. UFZ Centre for Environmental Research LeipzigHalle GmbH, UFZ-Discussion Papers, 1-27.
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