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.

Trophic peculiarities of the Great Cormorant, Grey Heron and Long-tailed Duck on the Baltic Sea Lithuanian coast: a stable isotope approach

“…Therefore, Great Cormorants are present for half a year on the Baltic Sea coast and Curonian Lagoon and during this pe- Figure. Mean isotopic values of tip and middle parts of bird feather riod consume the largest biomass of fish. Žydelis, Kontautas (2008) proved that the direct competition between piscivorous birds and fishermen for fish recourses is low. The estimated consumption by common piscivorous bird species is equal to about 9% of the total fish resources in the lagoon and two-thirds of the amount of fish landed by commercial fishermen.…”

“…The estimated consumption by common piscivorous bird species is equal to about 9% of the total fish resources in the lagoon and two-thirds of the amount of fish landed by commercial fishermen. In addition, there is a size difference between fish consumed by cormorants and commercially valuable fish size (Žydelis, Kontautas, 2008).…”

“…The Curonian Lagoon was the main feeding area for Great Cormorants. They mainly fed on the most common and abundant species in the lagoon (Repečka, 2003;Žydelis, Kontautas, 2008) and gregarious fish species (Suter, 1997). Marine fish contributed only a low portion to the overall diet (6.5% of biomass; Pūtys, Zarankaitė, 2010).…”

Do flocks of great cormorants and goosanders avoid spatial overlap in foraging habitat during the non-breeding season?

“…During migration and wintering several of the species of larger piscivorous birds present in Central Europe may be involved in interspecific interactions (including direct competition or avoidance) for foraging places (Ž ydelis and Kontautas 2008). The piscivorous guild includes raptors, herons, gulls, grebes, divers, mergansers, and cormorants.…”

Population dynamics, diet and ecotoxicology of the Great Cormorant Phalacrocorax carbo (Linnaeus, 1758) in the Carska bara marshland

“…Takođe je i mali broj istraživanja koja ukazuju na potencijalne efekte kormorana na riblje stokove. Procenjuje se da kormorani tokom zime pojedu od 22 kg/ha do čak 79 kg/ha ribe na reci Vltavi u Češkoj (Čech & Vejřík, 2011), 77 kg/ha do 84,3 kg/ha na reci Meza u Belgiji (Evrard et al, 2005), oko 4 tone godišnje (8,2% ukupne biomase) u Nacionalnom parku Wigierski u Poljskoj (Wziaqtek et al, 2007) ili čak oko 345 tona ribe godišnje u litvanskom delu Kurskog zaliva (Žydelis & Kontautas, 2008) S druge strane, gubici koji se javljaju u proizvodnji ribe, bilo kroz direktnu predaciju ili povredjivanje riba mogu se smatrati realnim štetama. I pored toga, malo je kvantitativnih procena koje govore o razmerama ovog problema.…”