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      Residency Patterns and Migration Dynamics of Adult Bull Sharks ( Carcharhinus leucas) on the East Coast of Southern Africa

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          Abstract

          Bull sharks ( Carcharhinus leucas) are globally distributed top predators that play an important ecological role within coastal marine communities. However, little is known about the spatial and temporal scales of their habitat use and associated ecological role. In this study, we employed passive acoustic telemetry to investigate the residency patterns and migration dynamics of 18 adult bull sharks (195–283 cm total length) tagged in southern Mozambique for a period of between 10 and 22 months. The majority of sharks (n = 16) exhibited temporally and spatially variable residency patterns interspersed with migration events. Ten individuals undertook coastal migrations that ranged between 433 and 709 km (mean  = 533 km) with eight of these sharks returning to the study site. During migration, individuals exhibited rates of movement between 2 and 59 km.d −1 (mean  = 17.58 km.d −1) and were recorded travelling annual distances of between 450 and 3760 km (mean  = 1163 km). Migration towards lower latitudes primarily took place in austral spring and winter and there was a significant negative correlation between residency and mean monthly sea temperature at the study site. This suggested that seasonal change is the primary driver behind migration events but further investigation is required to assess how foraging and reproductive activity may influence residency patterns and migration. Results from this study highlight the need for further understanding of bull shark migration dynamics and suggest that effective conservation strategies for this vulnerable species necessitate the incorporation of congruent trans-boundary policies over large spatial scales.

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          Cascading effects of the loss of apex predatory sharks from a coastal ocean.

          Impacts of chronic overfishing are evident in population depletions worldwide, yet indirect ecosystem effects induced by predator removal from oceanic food webs remain unpredictable. As abundances of all 11 great sharks that consume other elasmobranchs (rays, skates, and small sharks) fell over the past 35 years, 12 of 14 of these prey species increased in coastal northwest Atlantic ecosystems. Effects of this community restructuring have cascaded downward from the cownose ray, whose enhanced predation on its bay scallop prey was sufficient to terminate a century-long scallop fishery. Analogous top-down effects may be a predictable consequence of eliminating entire functional groups of predators.
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            Patterns and ecosystem consequences of shark declines in the ocean.

            Whereas many land predators disappeared before their ecological roles were studied, the decline of marine apex predators is still unfolding. Large sharks in particular have experienced rapid declines over the last decades. In this study, we review the documented changes in exploited elasmobranch communities in coastal, demersal, and pelagic habitats, and synthesize the effects of sharks on their prey and wider communities. We show that the high natural diversity and abundance of sharks is vulnerable to even light fishing pressure. The decline of large predatory sharks reduces natural mortality in a range of prey, contributing to changes in abundance, distribution, and behaviour of small elasmobranchs, marine mammals, and sea turtles that have few other predators. Through direct predation and behavioural modifications, top-down effects of sharks have led to cascading changes in some coastal ecosystems. In demersal and pelagic communities, there is increasing evidence of mesopredator release, but cascading effects are more hypothetical. Here, fishing pressure on mesopredators may mask or even reverse some ecosystem effects. In conclusion, large sharks can exert strong top-down forces with the potential to shape marine communities over large spatial and temporal scales. Yet more empirical evidence is needed to test the generality of these effects throughout the ocean.
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              Transoceanic migration, spatial dynamics, and population linkages of white sharks.

              The large-scale spatial dynamics and population structure of marine top predators are poorly known. We present electronic tag and photographic identification data showing a complex suite of behavioral patterns in white sharks. These include coastal return migrations and the fastest known transoceanic return migration among swimming fauna, which provide direct evidence of a link between widely separated populations in South Africa and Australia. Transoceanic return migration involved a return to the original capture location, dives to depths of 980 meters, and the tolerance of water temperatures as low as 3.4 degrees C. These findings contradict previous ideas that female white sharks do not make transoceanic migrations, and they suggest natal homing behavior.
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                Author and article information

                Contributors
                Role: Editor
                Journal
                PLoS One
                PLoS ONE
                plos
                plosone
                PLoS ONE
                Public Library of Science (San Francisco, USA )
                1932-6203
                2014
                8 October 2014
                : 9
                : 10
                : e109357
                Affiliations
                [1 ]Department of Zoology & Entomology, Rhodes University, Grahamstown, South Africa
                [2 ]Port Elizabeth Museum at Bayworld, Humewood, Port Elizabeth, South Africa
                [3 ]South African Institute for Aquatic Biodiversity (SAIAB), Grahamstown, South Africa
                [4 ]Department of Zoology, Nelson Mandela Metropolitan University, Port Elizabeth, South Africa
                Hawaii Pacific University, United States of America
                Author notes

                Competing Interests: RD and MJS have an affiliation with the institute (Port Elizabeth Museum at Bayworld) that received funding from the National Research Foundation of South Africa for this study. This does not alter the authors' adherence to PLOS ONE policies on sharing data and materials.

                Conceived and designed the experiments: RD MJS PDC. Performed the experiments: RD. Analyzed the data: RD. Contributed reagents/materials/analysis tools: MJS PDC PWF. Wrote the paper: RD MJS PDC PWF. Obtained permission to conduct the study: RD.

                Article
                PONE-D-14-18926
                10.1371/journal.pone.0109357
                4190266
                25295972
                634e3f63-727b-4874-97c0-6771a55b245c
                Copyright @ 2014

                This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

                History
                : 28 April 2014
                : 3 September 2014
                Page count
                Pages: 11
                Funding
                Funding and support were provided by the Professional Development Programme of the South African National Research Foundation ( www.nrf.ac.za), Port Elizabeth Museum at Bayworld ( www.bayworld.co.za), Rhodes University ( www.ru.ac.za), and Bayworld Centre for Research and Education who administered the grant, 3 Fathoms Research, the Acoustic Tracking Array Platform maintained by the South African Institute for Aquatic Biodiversity ( www.saiab.co.za) with support from the Ocean Tracking Network ( www.oceantrackingnetwork.org) funded in part by the Save Our Seas Foundation ( www.saveourseas.com). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. One of the contributing funders of this study (the National Research Foundation of South Africa) only provided funding for one of the authors' salaries (RD). This funding was awarded to the Port Elizabeth Museum at Bayworld and administered by the Bayworld Centre for Research and Education. The Port Elizabeth Museum at Bayworld and Bayworld Centre for Research and Education are not commercial businesses. The Port Elizabeth Museum at Bayworld had no role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript.
                Categories
                Research Article
                Biology and Life Sciences
                Ecology
                Ecosystems
                Coastal Ecosystems
                Marine Ecosystems
                Tropical Ecosystems
                Marine Ecology
                Population Ecology
                Marine Biology
                Marine Conservation
                Earth Sciences
                Marine and Aquatic Sciences
                Aquatic Environments
                Marine Environments
                Ecology and Environmental Sciences
                Habitats
                Custom metadata
                The authors confirm that all data underlying the findings are fully available without restriction. Acoustic telemetry data files are available from the Ocean Tracking Network database ( http://members.oceantrack.org/data/discovery/POZS.htm).

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                Uncategorized

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