Abstract
Environmental cycles often influence the presence of animals, creating patterns at different temporal scales, which may mean that their effects overlap and/or interact. Interactions between diel and seasonal cycles have been reported to influence fish behaviour but little is known about such interactions in marine top predators. Here, we studied the combined effect of seasonal, tidal and diel cycles on the occurrence of bottlenose dolphins (Tursiops truncatus) within a Marine Protected Area in Scotland. Our analyses were based on echolocation detections from passive acoustic devices (CPODs) deployed at three coastal sites between 2010 and 2016. We described patterns of dolphins’ occurrence using circular statistics and then used generalised additive mixed models to explore the relative importance of each cycle and any interactions between them. We found site-specific cyclical patterns of presence that remained constant across years. There was a highly significant interaction between seasonal and diel cycles at two sites around deep channels, where occurrence was diurnal in summer but became nocturnal in autumn. The study demonstrates the highly plastic behaviour of bottlenose dolphins and shows a previously unreported behaviour that has management implications for this and other marine protected areas.
Highlights
Environmental cycles create rhythmic patterns that modify the abiotic conditions of ecosystems, and biological rhythms that match these cycles are widespread[1]
Cetaceans are considered less likely to be influenced by tides in open coastal areas than in narrow channels[35,36]
Occurrence at Chanonry was www.nature.com/scientificreports strongly influenced by tide in all months of the year (Fig. 3). This site was within a narrow channel and subject to strong tidal flows which are known to influence fine-scale spatial movements of dolphins in this area[37]
Summary
Environmental cycles create rhythmic patterns that modify the abiotic conditions of ecosystems, and biological rhythms that match these cycles are widespread[1]. The main driver of tidal migrations in coastal areas is the avoidance of unsuitable conditions, but tidal currents are used for transport, feeding, predator avoidance and reproduction[7] Since these environmental cycles can create patterns at different temporal scales, their effects may interact. For animals with a flexible diel behaviour, one strategy they can follow to adapt to local conditions is to modify their diel activity patterns. Examples of such seasonal changes in diel activity patterns can be found across many taxa: see Hut et al for a review[8]. The management of potential stressors relies on accurate information about the distribution of focal species; the inclusion of both spatial and temporal variables becomes necessary to implement efficient protection measures for highly mobile species
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