Browsing by Author "Visser, Fleur"
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- Context-driven communication during deep-sea foraging in a social toothed whalePublication . Hessing, Sanne; Risser, Nolwenn Morgane; Pichot, Loanne; Oudejans, Machiel G; Guilpin, Marie; Barcelos, Luis M. D.; Curé, Charlotte; Visser, FleurSocial deep-diving odontocetes face the challenge of balancing near-surface proximity to oxygen and group members with foraging in the deep sea. Individuals rely on conspecifics for critical life functions, such as predator defence, but disperse during foraging to feed individually. To understand the role of social acoustic mediation during foraging in deep-diving toothed whales, we investigated the context of social burst-pulse call production in Risso’s dolphin (Grampus griseus) using biologgers. Dolphins produced context-specific burst pulses predominantly during daytime foraging, preceding or following foraging dives and in the early descent of daytime deep dives. Individuals applied differential short and long burst-pulse calls intended for either near-surface receivers (horizontal transmission) or deep-foraging receivers (vertical transmission). Our results show that deep-diving toothed whales are reliant on acoustic communication during certain foraging contexts, to relay information including foraging conditions or an individual’s location. Moreover, they accentuate the importance of maintaining acoustic contact with conspecifics, specifically when dispersed during deeper foraging. It also signifies that our oceanic top predators may be specifically vulnerable to the current strong increase in anthropogenic noise. Potential masking of the signals from group members communicating at a distance could undermine their social cohesion, and hence their capacity to maintain vital life functions.
- Temporal residency patterns of Risso's dolphins, Grampus griseus, off Pico Island, AzoresPublication . Hartman, Karin L.; Geelhoed, Steve C. V.; Visser, Fleur; Azevedo, José M. N.Temporal residency patterns of Risso's dolphins, Grampus griseus, occurring off Pico Island, Azores archipelago, were studied by conducting photoidentification surveys during 25 sampling months (May 2004-January 2007). A total of 1102 dolphins was identified, of which a restricted population (n= 996) was further analyzed. Temporal residency patterns were classified using year, season and monthly presence, and defined in three groups: non resident, partial resident and resident. Risso's dolphins were present in the research area in all seasons and all years. The seasonally parameter (4 seasons sighted) resulted in the highest percentage of residents (17%, n = 173). Age-classes and sexes showed different temporal residency patterns. Residents consisted largely of adult males and sub-adults, and were present in the research area throughout the sampling period. Partial and resident females were less frequently sighted, but showed a resident pattern when calving and nursing. The presence and number of old resident females strongly decreased. A calving peak was found in summer. Together with the existence of a resident population, this indicates that Pico is functioning as a critical habitat for Risso's dolphins occurring off Pico Island, was set up in 2000 by the Nova Atlantis Foundation. The aim of the present study was to obtain information on the residency pattern of Risso’s dolphins around Pico Island.