LINKED PAPER
Evaluating the dependence of opportunistic Yellow-legged Gulls (Larus michahellis) on marine habitat and fishing discards. Zorrozua, N., Granado, I., Fernandes-Salvador, J. A., Louzao, M., Basterretxea, M., Arizaga, J. 2024. IBIS. DOI: 10.1111/ibi.13227. VIEW
There are numerous examples throughout nature of wildlife making use of anthropogenic food sources, in both marine environments (e.g. fishing discards) and terrestrial ones (e.g. landfills), with impacts on aspects such as movement ecology. In some cases, bird populations have become dependent on anthropogenic food sources leading to improved survival and reproduction, along with promoted population growth (Duhem et al. 2008, Weiser & Powell 2010). The introduction of legislation reducing such food sources, such as Europe’s discard ban through the Landing Obligation (LO), can potentially have dramatic negative effects on the populations dependent on them.
In a recent study in Ibis, Nere Zorrozua and colleagues studied Yellow-legged Gulls (Larus michahellis) from the south-eastern Bay of Biscay to quantify the population’s potential use of fishing discards, evaluate their dependence on this resource, and determine the possible impacts of introduced legislation.
Yellow-legged Gulls in the Bay of Biscay
Previous studies of Yellow-legged Gull populations on the Iberian coast and in the Mediterranean region have shown that reduced availability of fishing discards can lead to breeding success declines of up to 46% (Oro et al. 1995) and strong population declines (Payo-Payo et al. 2015). For the resident population within the south-eastern Bay of Biscay, although marine prey comprise a large proportion of the diet composition (Zorrozua et al. 2020), it remains unknown whether they obtain fish prey from fishing vessels within their foraging range, or from activity at harbours such as fish remains and vessel cleaning.
The researchers used data from GPS-tracked adult Yellow-legged Gulls in the south-eastern Bay of Biscay and Vessel Monitoring System (VMS) data to describe habitat use by two of the main colonies and assess the overlap between marine trips by gulls and the presence of fishing vessels. By compiling information on the foraging ecology of the gull population, the fishing gear types, and fishing discards, they aimed to determine whether the gull’s potential overlap with fishing vessels could be related to the trophic ecology of the population.
Figure 1. Foraging patterns of Yellow-legged Gulls breeding in the SE Bay of Biscay. The 95% (light grey) and 50% (dark grey) kernel density estimates (without considering positions in the colony) estimated from the GPS tracking data are shown. The black dots represent the location of the two breeding colonies, Getaria and Ulia. The main fishing harbours and landfills of the area are also indicated (white dots and white stars, respectively).
Marine habitat use variation
The results showed sex and seasonal variation in habitat use, with more use of the marine environment by males in the breeding period. Female gulls had more overlap in their positions with fishing vessels than males, and gulls mainly followed purse seiners and trawlers. Overall, the gulls showed relatively low use (average 19%) of the marine habitat, compared to terrestrial habitats such as landfills and urban areas. This differs from other Yellow-legged Gull populations and is interesting given previous findings that marine prey makes up 40% of adults’ diet (Zorrozua et al. 2020).
The results indicated a low spatial overlap between gull positions and fishing vessels, in contrast to other gull populations. While this may reflect methodological differences, the results suggest that this population obtains their fish prey either by active fishing at sea or by exploiting discards at harbours, rather than by scavenging for discards at sea. Future studies could investigate the energetic and nutritional quality of fishing discards at sea and in harbours to evaluate the relative importance and trade-offs of alternative feeding resources.
There was seasonal variation in habitat use, with gulls spending more time in the colony when they were breeding and increasing their presence in landfills and urban areas post-breeding. This may reflect the exploitation of feeding sources over a wider geographical range following the end of breeding duties (Ackerman et al. 2018). The higher use of the marine habitat and higher overlap with fishing vessels during the breeding period may be due to the need to feed both themselves and their chicks, but this requires further investigation. Furthermore, as previous studies have found that seabirds consume more fishing discards offshore in winter (Depestele et al. 2016), it could be beneficial to investigate the winter period for this population in future.
Figure 2. Habitat use by GPS-tracked adult Yellow-legged Gulls in three different study periods: breeding (up to the end of June), transition (July) and post-breeding (August). Positions in the colony have been removed.
This study tested the spatial overlap between fishing activity and the foraging patterns of a resident, breeding seabird species within the Bay of Biscay for the first time. Overall, the researchers found little spatial overlap, which may suggest a relatively low dependence on offshore fishing discards. They suggest that European fishery policies would probably have a negligible effect on the Yellow-legged Gull population of the southeastern Bay of Biscay during the breeding season, although some questions do still remain around foraging efficiency by gulls at vessels and around the overlap with fishing vessels in the autumn/winter period. It is likely that this population would be more affected by management actions focused on other anthropogenic food sources, such as landfills, which could be taken into consideration during future policy planning.
References
Ackerman, J.T., Peterson, S.H., Tsao, D.C. & Takekawa, J.Y. (2018). California Gull (Larus californicus) space use and timing of movements in relation to landfills and breeding colonies. Waterbirds 41: 384-400. VIEW
Depestele, J., Rochet, M.-J., Dorémus, G., Laffargue, P. & Stienen, E.W.M. (2016). Favorites and leftovers on the menu of scavenging seabirds: Modelling spatiotemporal variation in discard consumption. Canadian Journal of Fisheries and Aquatic Sciences 73: 1446–1459. VIEW
Duhem, C., Roche, P., Vidal, E. & Tatoni, T. (2008). Effects of anthropogenic food resources on Yellow-legged Gull colony size on Mediterranean islands. Population Ecology 50: 91–100. VIEW
Oro, D., Bosch, M. & Ruiz, X. (1995). Effects of a trawling moratorium on the breeding success of the Yellow-legged Gull Larus cachinnans. Ibis 137: 547–549. VIEW
Payo-Payo, A., Oro, D., Igual, J.M., Jover, L., Sanpera, C. & Tavecchia, G. (2015). Population control of an overabundant species achieved through consecutive anthropogenic perturbations. Ecological Applications 25: 2228–2239. VIEW
Weiser, E.L. & Powell, A.N. (2010). Does garbage in the diet improve reproductive output of Glaucous Gulls? Condor 112: 530–538. VIEW.
Zorrozua, N., Aldalur, A., Herrero, A., Diaz, B., Delgado, S., Sanpera, C., Jover, L. & Arizaga, J. (2020). Breeding Yellow-legged Gulls increase consumption of terrestrial prey after landfill closures. Ibis 162:50-62. VIEW
Image credits
Top right: Larus michahellis in Farnese Gardens, Rome, Italy | Krzysztof Golik | CC BY-SA 4.0 Wikimedia Commons
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