| Summary: | 1. Many organisms undertake dramatic seasonal migrations to overcome local fluctuations in the availability of resources. The marine environment is characterised by a heterogenous distribution of resources at a variety of spatial scales (Boyd, Staniland & Martin 2002), yet the way that predators respond to this variability remains poorly understood. Basking sharks {Cetorhinus maximus) are model organisms in which to study foraging strategies as they have been shown to track the distribution of their zooplankton prey at both fine and broad spatial scales (Sims & Quayle 1998, Sims et al 2003b), and their movements are not restricted by the need to surface to breathe. 2. The patterns of vertical movement in pelagic predators can provide important insights into their foraging strategies, but can also be highly complex. Signal processing techniques were used to idenfify periodicity in vertical movements of 5 basking sharks using archival tag data. The main periodicities identified in the vertical movements of these sharks were found to be associated with foraging behaviour. 3. Diel vertical migration (DVM), a well-documented behaviour in zooplankton (Hays 2003), represented the main periodic dive behaviour in basking sharks, occurring for up to 97% of the time in any given month. Sharks showed two distinct modes of DVM, including one where sharks appeared to track the vertical migrations of its prey directly. 4. A fidal pattern of vertical movement was also idenfified. It is proposed that this represented a previously undocumented foraging behaviour where a shark fed on tidal aggregations of prey at depth. 5. The only juvenile tagged showed a markedly different pattern of vertical behaviour, which may be a result of foraging inexperience. 6. Large-scale patterns in the occurrence of vertical behaviours revealed that the sharks responded to changes in zooplankton abundance and behaviour at a seasonal scale. Basking sharks were also found to make dramatic behavioural transitions at an hourly timescale associated with changes in thermal habitat. This indicated that basking sharks possess highly flexible foraging strategies and can respond rapidly to changes in zooplankton availability associated with individual water masses. Keywords: behaviour, pelagic predator, planktivore, archival telemetry. Running headline: Foraging strategies in basking sharks Faculty of Science, The Marine Biological Association of the United Kingdom
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