Dietary trends of yellow-legged gulls in Galicia, north-west Spain, over the last 30 years
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1
Centre of Molecular and Environmental Biology (CBMA), Department of Biology, University of Minho, Portugal
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2
Animal Ecology Group (GEA), Department of Ecology and Animal Biology, University of Vigo, Spain
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3
Marine and Environmental Sciences Centre (MARE), Department of Life Sciences, University of Coimbra, Portugal
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4
Munilla Biodiversidade, Santiago de Compostela, Spain
Human activities can affect the structure and dynamics of wild populations, communities, and overall ecosystems. Commercial fisheries and climate change are responsible for modifying the stability of marine food webs, shaping the ecology and behaviour of several marine species. Anthropogenic activities can also generate huge amounts of waste that can be opportunistically used as food resource by scavenger species. Gulls are generalist seabirds that can feed on human food subsidies at both terrestrial (e.g. landfills) and marine (e.g. in association with fisheries) areas. As top predators, gulls are particularly vulnerable to environmental changes owing to their specific life-history characteristics, such as long life span and delayed maturity. In this study we examine temporal trends in the diet of the yellow-legged gull (Larus michahellis) in Galicia (north-west Spain) over the last 30 years (1987-1994 and 2004-2017). Diet composition was analysed by identifying hard parts of prey present in 5227 gull pellets, and the percentage of occurrence of prey items was calculated for each year. We also assessed the effects of commercial fisheries and environmental variables on gull diets. We found a major dietary change from refuse to a marine invertebrate (i.e. crustaceans) based-diet. The occurrence of sardine (Sardina pilchardus) was 12.4% in 1987 but became practically non-existent in gull pellets since the 2000's. Overall fish species also decreased from 36.9% of occurrence in 1987 to 4.9% in 2017. Our results suggests that the extended winter North Atlantic Oscillation (NAO) index has an effect on the consumption of fish, especially of pelagic fish species. In 2010 the NAO index was one of the lowest values recorded (-4.64), and such conditions can lead to lower marine prey abundance. This climate event could have caused the increase in refuse occurrence (62.5%), decrease in marine crustaceans (12.5%), and no occurrence of fish species that year on gull pellets. On the contrary, we did not found a relationship between fishery landing data and the diet of gulls. Fish landings increased over the study period, which could be related to increased fishing effort and not fish availability. Overall, changes in fishing practices and waste disposal may be responsible by the decreased consumption of fish species and refuse by yellow-legged gulls. Indeed, European policies promoting the reduction (and future ban) of fishery discards and the closure of open air refuse dumps to properly managed landfills were implemented during the 2000’s. Furthermore, the breeding population of yellow-legged gulls at Galicia has decreased ca. 60% since its peak in the 2000’s, suggesting that these environmental policies had a significant effect on the gulls’ demography. Continued decrease of human food subsidies availability and climate change can have important implications for gull species, affecting directly their diet composition, and lead to long-term population declines.
Acknowledgements
We acknowledge the support provided by ‘Fundação para a Ciência e a Tecnologia’ (FCT), through the strategic project UID/MAR/04292/2013 granted to MARE and the fellowships PD/BD/127991/2016 and SFRH/BPD/85024/2012 granted to JGC and VHP, respectively.
Keywords:
Larus michahellis,
pellet analysis,
fishery landings,
Refuse,
NAO index
Conference:
IMMR'18 | International Meeting on Marine Research 2018, Peniche, Portugal, 5 Jul - 6 Jul, 2018.
Presentation Type:
Oral Presentation
Topic:
Biodiversity, Conservation and Coastal Management
Citation:
Calado
JG,
Munilla
I,
Ramos
JA and
Paiva
VH
(2019). Dietary trends of yellow-legged gulls in Galicia, north-west Spain, over the last 30 years.
Front. Mar. Sci.
Conference Abstract:
IMMR'18 | International Meeting on Marine Research 2018.
doi: 10.3389/conf.FMARS.2018.06.00095
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Received:
03 May 2018;
Published Online:
07 Jan 2019.
*
Correspondence:
Ms. Joana G Calado, Centre of Molecular and Environmental Biology (CBMA), Department of Biology, University of Minho, Braga, Portugal, joana.gomes.calado@gmail.com