Foraging behaviour of cormorants Phalacrocorax carbo in pound nets in Denmark: the use of barrel nets to reduce predation

Yields in pound nets are in particular damaged or reduced by cormorant Phalacrocorax carbo predation. In this study we examined the effect on predation in pound nets by mounting barrel nets vertically under the water in the pot in order to obstruct cormorants during their hunt. To keep control of the content of fish we used an experimental pot with a closed throat. The pot was stocked repeatedly with rainbow trout Oncorhyncus mykiss and its content of fish was thereby maintained at between 10 and 100 individuals during all periods of data collection. Video recordings, both above and under water, were carried out during May ‐ August 1995 under various experimental conditions. We tested five different barrel net arrangements including a control situation without barrel net. Two bird categories were defined a posteriori: those that caught one or more fish during their visit to the pot (successful birds) and those that failed to catch a fish (unsuccessful birds). Successful and unsuccessful birds differed in a number of ways. Successful cormorants stayed longer in the pot, performed more dives, and spent more time diving and shorter time on the surface. When barrel nets were present, fewer cormorants of both categories visited the pot. Furthermore, the barrel nets affected successful birds by increasing the number of dives required to catch their first fish, reduced the time that both successful and unsuccessful birds remained in the pot, and reduced the number of dives performed by unsuccessful birds. Therefore, it seems that barrel nets may have a potential as a predation reducing device in pound net fishery. Introducing the use of barrel nets in pound net fisheries may force cormorants to hunt in free waters, possibly with reduced foraging success. This may eventually lead to a reduction in the cormorant population which will make it better fit the natural carrying capacity of the environment.