Impact of Cormorant Phalacrocorax carbo sinensis, Black-headed Gull Larus ridibundus and Grey Heron Ardea cinerea in a fish farm in Navarra (northern Spain) during the winter season.

Published:

Volume 45(2), December 1998. Pages 171-182.

Language:

Spanish

Original title:

Impacto del Cormorán Grande Phalacrocorax carbo sinensis, la Gaviota Reidora Larus ridibundus y la Garza Real Ardea cinerea en una piscifactoría de Navarra durante la época invernal.

Keywords:

Ardea cinerea, fish farm, ichthyophagous birds, impact, Larus ridibundus, Oncorhynchus mykiss, Phalacrocorax carbo sinensis.

Abstract:

In the winter 1994-95 and during five consecutive months (October-February), the impact of three ichthyophagous birds (Cormorant Phalacrocrocax carbo sinensis, Grey Heron Ardea cinerea and Black-headed Gull Larus ridibundus) was estimated in a rainbow trout Oncorhynchus mykiss fish farm located in southwestern Navarre (northern Spain). Some foraging parameters (number of foraging bouts, number of feeding attempts, percentage of success, feeding time, number of trouts caught by each bird and biomass taken; Table 1) were studied by direct field observations. Both adult Herons and Cormorants caught more trout biomass per foraging bout than juveniles. Black-headed Gulls and Grey Herons were the main avian predators due to their relative numbers in the first case and to the fish biomass taken in the second. Seasonal variation in the numbers of Cormorants, Herons and Gulls at the fish farm was found during the study period (Fig. 2). Adult Herons and Cormorants dominated the age structure of their wintering populations at the fish farm (Fig. 3). Large differences in the average number of foraging birds were found at the fish farm according to day time: Cormorants fed in the morning and in the afternoon, and both Herons and Gulls fed mainly in the afternoon (Fig. 4). The location of beak-prints in the body of injured trouts was studied (Table 2). Significant differences were found in the length of trouts caught by Cormorants and Herons during the wintering season (Fig. 5). Small trouts (50-90 g) were caught more frequently by Black-headed Gull, medium-sized trouts (130-180 g) by Cormorants and large trouts (250-360 g) by Herons. The impact of Gulls, Herons and Cormorants was estimated as less than 0.1% of the annual yield of the fish farm, but this supposed a considerable economic loss in absolute terms (5000000 ptas or $36000) due to the high productivity of the fish farm.