Influence of fisheries on the spatio-temporal feeding ecology of gulls along the western Iberian coast

Abstract

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.

Fatty acids composition in yellow-legged (Larus michahellis) and lesser black-backed (Larus fuscus) gulls from natural and urban habitats in relation to the ingestion of anthropogenic materials

“…In fact, EPA and DHA, both ω-3 FAs, were the most important FAs in segregating wildlife rescue centres, all showing higher percentages in individuals from RIAS, the least urbanized area, and LxCRAS. The higher percentages of these ω-3 FAs in individuals from LxCRAS and RIAS is consistent with a diet based on marine resources (Calado et al, 2018(Calado et al, , 2021Dalsgaard et al, 2003). On the contrary, the deficiency in ω-3 FAs and the lower diversity of FAs (i.e.…”

“…We acknowledge that some individuals in each habitat may not be strict urban or natural dwellers as we stated, because the movement ecology of each individual before capture is unknown. However, our previous research indicates that gulls from Porto are largely urban dwellers year-round (Pais de Faria et al, 2021aFaria et al, , 2021b, and those from Ria Formosa forage mostly in interaction with fishing activities also year-round (Calado et al, 2021).…”

“…Both YLG and LBBG, adults and immatures, males and females, are known to benefit from reliable and predictable food sources, either by interacting with fishing boats and feeding on marine species with higher nutritional value (i.e. fishery discards; Calado et al, 2018Calado et al, , 2021Mendes et al, 2018), mainly in natural habitats, or by feeding on human meal leftovers collected from trash containers or in nearby landfills, in urban habitats (Lopes et al, 2021;Pais de Faria et al, 2021a;Spelt et al, 2019). Also, in this study, we compared FA composition between adults and immature gulls (1-3 years), and we did not consider fledglings.…”

Evaluating the dependence of opportunistic Yellow‐legged Gulls (Larus michahellis) on marine habitat and fishing discards

“…2017, Calado et al . 2020b, 2021a). Thus, breeding success declines of up to 46% have been observed when these resources dramatically decrease or disappear (Oro et al .…”

“…Studies of the Yellow-legged Gull Larus michahellis on the Iberian coast and the Mediterranean region show that fish prey can constitute one of the main feeding sources for some populations, and high availability of fishing discards allows a very fast population growth (Arizaga et al 2009, 2011, Real et al 2017, Calado et al 2020b, 2021a. Thus, breeding success declines of up to 46% have been observed when these resources dramatically decrease or disappear (Oro et al 1995), as well as strong population declines (Payo-Payo et al 2015).…”