Fine-Scale Movements of the Broadnose Sevengill Shark and Its Main Prey, the Gummy Shark

Adam Barnett,Barry D Bruce,Kátya G Abrantes,John D Stevens,Jayson M Semmens,Richard K F Unsworth

doi:10.1371/journal.pone.0015464

Abstract

Information on the fine-scale movement of predators and their prey is important to interpret foraging behaviours and activity patterns. An understanding of these behaviours will help determine predator-prey relationships and their effects on community dynamics. For instance understanding a predator's movement behaviour may alter pre determined expectations of prey behaviour, as almost any aspect of the prey's decisions from foraging to mating can be influenced by the risk of predation. Acoustic telemetry was used to study the fine-scale movement patterns of the Broadnose Sevengill shark Notorynchus cepedianus and its main prey, the Gummy shark Mustelus antarcticus, in a coastal bay of southeast Tasmania. Notorynchus cepedianus displayed distinct diel differences in activity patterns. During the day they stayed close to the substrate (sea floor) and were frequently inactive. At night, however, their swimming behaviour continually oscillated through the water column from the substrate to near surface. In contrast, M. antarcticus remained close to the substrate for the entire diel cycle, and showed similar movement patterns for day and night. For both species, the possibility that movement is related to foraging behaviour is discussed. For M. antarcticus, movement may possibly be linked to a diet of predominantly slow benthic prey. On several occasions, N. cepedianus carried out a sequence of burst speed events (increased rates of movement) that could be related to chasing prey. All burst speed events during the day were across the substrate, while at night these occurred in the water column. Overall, diel differences in water column use, along with the presence of oscillatory behaviour and burst speed events suggest that N. cepedianus are nocturnal foragers, but may opportunistically attack prey they happen to encounter during the day.

Highlights

Information on the foraging behaviour of large mobile predators provides additional information to methods such as dietary analysis to better understand predator-prey relationships and their effects on community dynamics [1,2]
Study area A VEMCO radio-acoustic positioning system (VRAP) was deployed in Norfolk Bay in southeast Tasmania, Australia (Fig. 1) to track 18 N. cepedianus and 10 M. antarcticus that were fitted with coded acoustic transmitters
Frequency of occurrence N. cepedianus were detected in the VRAP on 52% of days and

Summary

Introduction

Information on the foraging behaviour of large mobile predators provides additional information to methods such as dietary analysis to better understand predator-prey relationships and their effects on community dynamics [1,2]. The foraging behaviour of a predator can determine anti-predatory behaviour of its prey such as increased vigilance, or it can influence habitat selection [3,4,5]. Determining fine-scale movement patterns of predators and their prey is an important component of studying predator-prey interactions and evaluating the likely consequences of these interactions for the predator, prey and overall community [1]. Active acoustic tracking, radio-acoustic positioning and animal-borne video, audio and data collection systems have been used to study fine-scale movements of a relatively small number of large shark species [9,10,11,12]

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Discussion

Conclusion