Gulls are highly opportunistic seabirds, and the exploitation of fishery discards has led to many population increases worldwide. We investigated the importance of fish in the diet of yellow-legged Larus michahellis and Audouin’s gulls L. audouinii and assessed the influence of fishery discards on their feeding ecology. We collected pellets from 4 islands along the western Iberian coast during pre-breeding, breeding, and post-breeding seasons from 2014 to 2018. Stable isotopes (from adult blood, and chick and adult feathers) were used to investigate spatial, seasonal, and inter-annual differences in their feeding ecology. We used pellet, stable isotope (δ15N, δ13C, and δ34S), and biochemical (triglycerides, uric acid, total protein, and carotenoids in adult plasma) analyses to investigate their relationships with fish landings across the annual cycle. Results revealed that the fish species consumed by gulls matched those landed by local fisheries on all study islands, and there was a positive association of pelagic and demersal fish diets with fish landing quantities for 2 islands. δ34S values suggest different self-feeding and chick-provisioning strategies in relation to fisheries. δ15N values exhibited strong negative correlations with fish landings, and triglycerides were positively correlated with pelagic but not with demersal fish landing quantities, suggesting that gulls fed more on lower trophic level and higher energetic content pelagic fish than on demersal fish. Overall, our results based on several techniques suggest that gull feeding ecology was linked to fishery discards, which in view of the new EU landing obligation may have major implications for both gull populations across Europe.
The availability of anthropogenic food subsidies has promoted an increase in generalist opportunistic gull species, which currently breed and forage on predictable anthropogenic resources (e.g. landfills). Here we investigated whether marine resources are still important to urban-dwelling gulls. We studied 4 natural and 2 urban yellow-legged gull Larus michahellis colonies and compared (1) diet composition (through pellet analysis) and (2) isotopic niches of adults and chicks, (3) diet delivered to chicks of different ages, and (4) fatty acid (FA) composition of fledglings, in order to assess diet composition, diversity and quality, and the relevance of marine prey for natural and urban gull populations. Adult urban gulls consumed considerably lower proportions of marine prey when compared to gulls from natural colonies; however, they fed their younger chicks (<20 d old) mostly with fish, representing 61-80% of their chick food deliveries. Refuse items were mostly delivered to chicks older than 20 d. Overall, urban isotopic niches were not completely distinct from those of natural colonies, in some cases sharing ca. 50% of their niche space. Fledglings from the most urbanized colony presented overall higher FA concentrations and diversity, but they were lacking some omega-3 FAs relevant to their physiology. Our results highlight the importance of marine resources in the diet of urban gulls, particularly during early chick rearing, the relevance of food sources in the area around the breeding colonies and the fact that urban gulls benefit from year-round reliable anthropogenic food resources
Calado, J.G., Paiva, V.H., Ramos, J.A., Velando, A. & Munilla, I., 2020. Anthropogenic food resources, sardine decline and environmental conditions have triggered a dietary shift of an opportunistic seabird over the last 30 years on the northwest coast of Spain. Regional Environmental Change, 20(1), p.10.
Human activities and environmental conditions are the main drivers of ecosystem change. One major alteration near the western Iberian coast has been the collapse of the Atlanto-Iberian sardine Sardina pilchardus stock, with important cascading effects on marine top predators. We investigated the effect of long-term changes in fishery landings, sardine availability and environmental conditions on the diet of the yellow-legged gull Larus michahellis in the northwest coast of Spain, over the last 30 years (1987–2017). Dietary trends of gulls were investigated through the analysis of 5010 pellets that revealed a sharp decline of fish and refuse and a shift to a crustacean-based diet. General additive mixed models showed that both total fish and sardine occurrences in gull pellets were negatively associated with total fishery landings and positively associated with sardine landings, suggesting fish depletion and higher fishing efficiency (i.e. reduced discards) during the study period. The winter North Atlantic Oscillation (NAO) index was also positively related with sardine occurrence in gull pellets, possibly due to low sardine abundance and rough conditions in years with very low NAO values. The refuse decline was most probably caused by the closure of open-air landfills, implemented under the European Union Landfill Directive. Our results suggest that changes in fishing practices and waste disposal were the main factors responsible for the sharp decline of fish and refuse in yellow-legged gull diet.
Velando, A., Munilla, I., López-Alonso, M., Freire, J. & Pérez, C. EROD activity and stable isotopes in seabirds to disentangle marine food web contamination after the Prestige oil spill. Environmental Pollution 158, 1275–1280 (2010).
In this study, we measured via surgical sampling hepatic EROD activity in yellow-legged gulls from oiled and unoiled colonies, 17 months after the Prestige oil spill. We also analyzed stable isotope composition in feathers of the biopsied gulls, in an attempt to monitor oil incorporation into marine food web. We found that yellow-legged gulls in oiled colonies were being exposed to remnant oil as shown by hepatic EROD activity levels. EROD activity was related to feeding habits of individual gulls with apparent consequences on delayed lethality. Capture–recapture analysis of biopsied gulls suggests that the surgery technique did not affect gull survival, giving support to this technique as a monitoring tool for oil exposure assessment. Our study highlights the combination of different veterinary, toxicological and ecological methodologies as a useful approach for the monitoring of exposure to remnant oil after a large oil spill.
Velando, A., Munilla, I. & Leyenda, P. M. Short-term indirect effects of the Prestige’ oil spill on European shags: changes in availability of prey. Marine Ecology Progress Series 302, 263–274 (2005).
In 2003, immediately following the ‘Prestige’ oil spill in Galicia, Spain, we studied the reproductive performance of European shags Phalacrocorax aristotelis at Illas Cíes, the main breeding ground for the species in Spain. Over the 10 yr preceding the spill, we monitored the breeding and the dynamics of the Illas Cíes population. We performed 1000 simulations of the population dynamics using the population parameters and their variability for the pre-spill period. The number of breeding
pairs counted in 2003 was lower than any of the values predicted by the simulation models, suggesting that population parameters changed after the ‘Prestige’ wreck. Environmental conditions cannot explain the observed effects because weather conditions were far from severe in 2003. The analysis of shag casualities showed that despite the extensive oiling of Illas Cíes and nearby waters it is unlikely that shags were killed in large numbers. Nevertheless, the shag feeding grounds around Illas Cíes, which are mainly sandy bottoms, were continuously exposed to oil pollution throughout the shag breeding season as revealed by the high levels of pollutants in sediments, plankton, and other organisms. In the pre-spill years, shags showed low seasonal dietary variation, feeding mainly on sandeels. Nevertheless, in 2003, there was a dietary shift with a lower occurrence of sandeel that, together with sandeel fishery data, indicate lower sandeel availability at foraging areas. In addition, reproductive performance in 2003 was significantly lower and chick condition was poorer compared to the pre-spill years. When all this information is taken into account, the picture that emerges strongly suggests that the European Shag population in Illas Cíes is suffering a negative impact of an indirect nature mediated through a reduction on the availability of a highly preferred forage-fish.
Munilla, I. Henslow’s swimming crab (Polybius henslowii) as an important food for yellow-legged gulls (Larus cachinnans) in NW Spain. ICES Journal of Marine Science: Journal du Conseil 54, 631–634 (1997).
An analysis of the contents of 2562 pellets sampled from 1987 to 1993 at breeding colonies and roosting sites showed that Henslow’s swimming crabs (Polybius henslowii) are by far the most important marine prey for yellow-legged gulls (Larus cachinnans) on the coasts of Galicia (north-western Spain), occurring in 36.4% of pellets. The results also suggest that yellow-legged gulls in Galicia are to a great extent marine foragers. Galicia has one of the largest yellow-legged gull populations in western Europe, largely dominating the seabird community. Polybius henslowii is the most abundant decapod crab over the continental shelf of Galicia. It enters coastal waters in large shoals and frequently stays close to the sea surface. Compared with the diets of other yellow-legged gull populations or any of the other closely related gull species, such as the herring (L. argentatus) and the lesser black-backed gull (L. fuscus), Polybius henslowii appears as a characteristic and even exclusive prey of yellow-legged gull populations in the Iberian Atlantic. There is also some evidence that the regular irruption of large Polybius henslowii shoals is a phenomenon peculiar to Iberian Atlantic waters.
Moreno, R., Jover, L., Munilla, I., Velando, A. & Sanpera, C. A three-isotope approach to disentangling the diet of a generalist consumer: the yellow-legged gull in northwest Spain. Marine Biology 157, 545–553 (2010).
The widespread omnivory of consumers and the trophic complexity of marine ecosystems make it difficult to infer the feeding ecology of species. The use of stable isotopic analysis plays a crucial role in elucidating trophic interactions. Here we analysed d15N, d13C and d34S in chick feathers, and we used a Bayesian triple-isotope mixing model to reconstruct the diet of a generalist predator, the yellow-legged gull (Larus michahellis) that breeds in the coastal upwelling area off northwest mainland Spain. The mixing model indicated that although chicks from all colonies were fed with a high percentage of fish, there are geographical differences in their diets. While chicks from northern colonies consume higher percentages of earthworms, refuse constitutes a more important source in the diet of chicks from western colonies. The three-isotope mixing model revealed a heterogeneity in foraging habitats that would not have been apparent if only two stable isotopes had been analysed. Moreover, our work highlights the potential of adding d34S for distinguishing not only between terrestrial and marine prey, but also between different marine species such as fish, crabs and mussels.