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Auteur (up) Thiebault, A.; Mullers, R.H.E.; Pistorius, P.A.; Tremblay, Y.
Titre Local enhancement in a seabird: reaction distances and foraging consequence of predator aggregations Type Article scientifique
Année 2014 Publication Revue Abrégée Behavioral Ecology
Volume 25 Numéro 6 Pages 1302-1310
Mots-Clés camera; foraging cues; gannet; Gps; group hunting; network foraging; prey detection; social information.
Résumé Seabirds foraging on pelagic fish develop behavioral strategies specifically adapted to locate inconspicuous prey that are aggregated in spatially dynamic patches. In the marine environment, they may use various mechanisms to detect cues of prey availability. The aggregation of predators at a patch of food is a particularly obvious cue to locate prey, a mechanism known as local enhancement. Pioneering studies described the formation of foraging groups at sea, showing that seabirds are attracted to feeding conspecifics. Improved foraging success due to local enhancement has been suggested from modeling studies, but no direct validation of these results exists. We deployed video cameras concomitantly with GPS loggers on Cape gannets to study the behavioral responses of equipped birds to the aggregation of predators at food patches. We showed that the reaction distances of equipped birds increased with the size of an aggregation, demonstrating that predator aggregations enhance food detectability for foragers. For small aggregations (<50 gannets), reaction distances were mostly less than 10 km, and they increased up to almost 40 km for larger aggregations (100–150 gannets). In addition, we showed that the number and frequency of dives increased with the number of conspecifics aggregated, up to a threshold. The predator aggregations on a patch of food could, therefore, not only inform about the presence of prey but also entail information about foraging conditions. From direct observations on the various components involved, our study provides justification of the use and advantages of local enhancement in foraging seabirds.
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ISSN 1045-2249, 1465-7279 ISBN Médium
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Numéro d'Appel MARBEC @ isabelle.vidal-ayouba @ collection 1207
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Auteur (up) Thiebault, A.; Semeria, M.; Lett, C.; Tremblay, Y.
Titre How to capture fish in a school? Effect of successive predator attacks on seabird feeding success Type Article scientifique
Année 2016 Publication Revue Abrégée J Anim Ecol
Volume 85 Numéro 1 Pages 157-167
Mots-Clés boid; Competition; dolphins; facilitation; foraging; gannets; group hunting; Modelling; sardine run; video
Résumé * Prey aggregations, such as fish schools, attract numerous predators. This typically leads to the formation of multispecific groups of predators. These aggregations can be seen both as a place of increased competition and as a place of possible facilitation between predators. Consequently, the functional role of such predator–prey aggregation is uncertain, and its effect on individual feeding success is virtually unknown. * Using underwater film footage of different predators feeding on fish schools during the sardine run in South Africa, we directly measured the in situ feeding success of individual Cape gannets Morus capensis in different foraging situations. * We determined the types of Cape gannet attacks (direct plunge dive or plunge dive followed by underwater pursuit) and we measured the occurrences and timing of attacks from the different species (mostly Cape gannets and long-beaked common dolphins Delphinus capensis). We also estimated the size of the targeted fish schools. These observations were complemented with a simulation model to evaluate the cumulative effect of successive predator attacks on the prey aggregation structure. * The probability to capture a fish in one feeding attempt by Cape gannets averaged 0·28. It was lower when gannets engaged in underwater prey pursuit after the plunge compared to direct plunge (0·13 vs. 0·36). We found no effect of the number of prey on gannets’ feeding success. However, the timing and frequency of attacks influenced strongly and positively the feeding success of individuals. The probability to capture a fish was the lowest (0·16) when no attack occurred in the few seconds (1–15 s) prior to a dive and the highest (˜0·4, i.e. more than twice) when one or two attacks occurred during this time window. The simulation model showed that a prey aggregation disorganized just after an attack and that the maximum of disturbance was obtained a few seconds after the initiation of the successive attacks. * Our study suggests that, in multispecies predator assemblages, the cumulative effect (through disorganization of school cohesiveness) of the multiple species attacking a prey aggregation may increase the feeding success of each individual. Therefore, facilitation between predators is likely to overcome competition in these multispecific assemblages.
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ISSN 1365-2656 ISBN Médium
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Numéro d'Appel MARBEC @ isabelle.vidal-ayouba @ collection 1463
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