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Journal of Fish Biology - 2022 - Guttridge - Vertical space use and thermal range of the great hammerhead Sphyrna mokarran

Authors and Affiliation

  • Tristan L. Guttridge, Lukas Müller, Bryan A. Keller, Mark E. Bond, R Dean Grubbs, William Winram, Lucy A. Howey, Bryan S. Frazier, Samuel H. Gruber

  • Affiliations:

    • Saving the Blue, Cooper City, Florida, USA

    • Bimini Biological Field Station Foundation, South Bimini, Bahamas

    • Marine Animal Ecology Group, Wageningen University, Wageningen, The Netherlands

    • The Watermen Project, Geneva, Switzerland

    • Coastal and Marine Laboratory, Florida State University, St. Teresa, Florida, USA

    • Department of Biological Sciences, Institute of Environment, Florida International University, North Miami, Florida, USA

    • South Carolina Department of Natural Resources, Marine Resources Research Institute, Charleston, South Carolina, USA

    • Biosciences, College of Life and Environmental Sciences, University of Exeter, Exeter, UK

Abstract

  • Great Hammerhead (Sphyrna mokarran): A large, predatory shark found in coastal and semi-oceanic waters. Highly mobile and key to marine ecosystems but critically endangered due to fishing pressures.

  • Research Focus: Examined vertical space use and thermal range in the western North Atlantic to contribute to conservation strategies.

  • Key Findings: Predominantly shallow habitat use (<30 m), with temperature preference between 23-28°C. Individual differences noted, indicating potential vulnerability to fisheries.

  • Conservation Implications: Findings can enhance fisheries management and conservation efforts to reduce by-catch mortality.

Introduction

Movement Patterns in Marine Species

  • Fish exhibit complex movements driven by various biotic and abiotic factors.

  • Movements significantly impacted by species' ecological roles, life history, and environmental conditions.

  • Overfishing affecting species like sharks and rays increases the need for species-specific ecological data for effective management.

Importance of Great Hammerhead Sharks

  • S. mokarran: A top predator influencing marine ecosystems, slow growth rates, low fecundity, and critical importance for fisheries balance.

  • Habitat use includes both shallow and deeper pelagic waters, with documented long-distance migratory behaviors.

  • Critically endangered due to over-exploitation; need for conservation strategies is paramount.

Materials and Methods

Tagging and Field Research

  • Research conducted under various regional permits in Bimini, Bahamas, Florida, and South Carolina.

  • Shark tagging in known aggregation sites, using pop-off satellite archival tags (PSAT).

  • Data collected on depth, temperature, and reproductive status.

Data Analysis Techniques

  • Analyzed depth and temperature records, used statistical tests to examine diurnal differences and seasonal habitat use.

  • Investigated potential periodicities in depth distributions through FFT analysis and time-at-depth distributions.

Results

Tagging Overview

  • Total of 18 sharks (15 females, 3 males) equipped with PSAT, providing depth and temperature records over various durations.

  • Movements documented across significant distances, showcasing extensive use and regional connectivity between habitats (e.g. U.S. to Bahamas).

Vertical Space Use and Thermal Range

  • Mean maximum depth reached by individuals was 133 m, with 75% of time spent <30 m.

  • Sharks demonstrated preference for warmer waters between 23-28°C, beneficial for growth and reproduction.

  • Individual sharks displayed diverse depth and thermal patterns, emphasizing ecological variability.

Diel Variation in Depth Use

  • Significant differences in depth usage between day and night, with some individuals favoring deeper waters at specific times.

  • Frequency analysis indicated a 24-hour cycle in the depth usage of several sharks, correlating to nocturnal predation strategies.

Discussion

Conservation and Impacts of By-Catch

  • The species relies on shallow depths, raising concern about their vulnerability to fishing gear, particularly in commercial operations.

  • Effective management requires integration of vertical habitat use data to develop better conservation strategies and reduce by-catch.

  • Proposed actions include gear modifications and temporal closures to minimize interactions based on identified depth and diel patterns.

Future Research Directions

  • Emphasis on filling gaps in understanding S. mokarran movements across life stages and environments is critical.

  • Ongoing studies should consider the ecological dynamics of predators and prey interactions to refine management practices.