Piscivorous birds as top predators and fishery competitors in the lagoon ecosystem

Abstract

Piscivorous birds have received much attention with respect to competition with fisheries for resources. The majority of studies have been focused on cormorants Phalacrocoracidae, while predation by other piscivorous bird species has often been overlooked. This study was designed to supplement sociological research (Bell, 2004), which revealed that the fishermen community at the Lithuanian section of the Curonian Lagoon considers great cormorants Phalacrocorax carbo, but not other fish-eating birds, to be significant competitors of fish resources. In this paper, we estimate fish consumption by cormorants and other abundant piscivorous birds, and attempt to interpret this level of predation in relation to fish resources and commercial fishery landings. We estimate that four piscivorous bird species consumed nearly 700 tonnes of fish during the breeding season of 2001 and winter 2001/2002, which corresponds to ~9% of the total fish resources in our study area. Bird consumption equalled two-thirds of the amount of fish landed by commercial fishermen. However, we argue that direct competition between birds and humans for fish resources is low, because there is a size segregation of exploited fish stock segments and abundant fish species that dominate the diet of birds. Fish monitoring and commercial fish landings indicated no apparent changes in fish stock size and composition, which could be attributed to a recent increase in piscivorous birds. Great cormorants consumed the largest biomass of fish compared to other piscivorous bird species. However, total fish intake by grey herons Ardea cinerea, great-crested grebes Podiceps cristatus, and goosanders Mergus merganser combined, equalled that of cormorants. Our results do not support the common public perception that cormorant predation greatly exceeds that of other piscivorous birds, and is detrimental to commercial fisheries.