C om bining GPS tracking and stable isotope analysis to predict
changes in guii dynamics
Sotillo A le ja n d ro 1, Eric Stienen2, Magda V in cx3 and Luc Lens'
1 T errestrial Ecology Unit, Ghent U niversity
K.L. Ledeganckstraat 35, B-9000 Ghent, Belgium
E-mail: aleiandro.sotilloqonzales@ uqent.be
2 In stitu u t voor Natuur- en Bosonderzoek
Kliniekstraat 25, B-1070 Brussels, Belgium
3 Marine Biology Section, Ghent U niversity
Krijgslaan 281, Campus Sterre - S8, B-9000 Ghent, Belgium
During the second half o f the 20th century, populations o f Herring Guii (Larus argentatus) and
Lesser Black-backed Guii (Larus fuscus) have grown su bstantially along the coasts o f the North Sea.
T he ir protected status, and a dd itional food supply in the fo rm o f urban waste and fish e ry discards,
are considered responsible fo r the p ro life ratio n o f these species. Belgian populations o f Herring and
Lesser Black-backed Guii breed m ainly on the docks o f the o ute r Port o f Zeebrugge and the roofs o f
the city o f Ostend. They are hig hly dependent on anth ro p ic sources o f food, nam ely fishery
discards, which m ig ht be crucial during the chick rearing stage o f the breeding season. The
prospect o f a strong drop in the production o f discards in the near fu tu re brings up the question on
which responses can be expected fro m large gulls in term s o f habitat use and feeding patterns, in a
co nte xt o f global decline o f th e ir populations.
To answer th is question, I study the habitat use, fo od dependencies and m ovem ent patterns o f
breeding L fuscus and L. a rg en ta tus using a com bination o f GPS tracking and stable isotope
analysis techniques. Data from the firs t breeding season show the im portance o f specific, individual
and seasonal variation in habitat use. Specific variation is driven by the d iffe re n t fo ra gin g niche o f
the tw o studied species, which overlaps m arginally. Individual variation suggests the developm ent
o f particular fo ra gin g strategies th a t may be influenced by ontogeny. Seasonal variation reflects
changes in n u tritio n a l and energetic dem ands o f the breeding population, determ ined by the stage
o f the breeding season: egg-laying, hatching, chick-rearing, fle dg in g and post-fledging.
GPS tracking data o f fu tu re breeding seasons w ill inform on annual variations in habitat use. Stable
isotope ratio analyses w ill inform on the real dependence o f the breeding population on each o f its
main food sources at the d iffe re n t stages o f the breeding season.
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