Movement, Behavior, and Habitat Use of a Marine Apex Predator, the Scalloped Hammerhead

R J David Wells,Jennifer A Mckinney,William B Driggers,Matthew J Ajemian,J Marcus Drymon,Thomas C Tinhan,Michael A Dance,John A Mohan,Gregory W Stunz,Brett Falterman,Eric R Hoffmayer

doi:10.3389/fmars.2018.00321

Abstract

Conservation and management efforts of marine apex predators are more reliable when information on movement and habitat use patterns are known. The scalloped hammerhead (Sphyrna lewini) was the first shark species to be protected under the U.S. Endangered Species Act and has life history characteristics that make this species particularly at risk for local depletion. Consequently, the goal of this study was to better understand the movement dynamics of this species in the Gulf of Mexico (GOM) where discards through the longline fishery can be substantial. A total of 33 scalloped hammerheads were tagged with fin mounted satellite tags and tracked for an average of 146 days (ranging from 5 to 479 days) to examine horizontal movements and quantify space use. Scalloped hammerheads showed a wide range of movements throughout the GOM continental shelf with limited long-distance dispersal and females displayed a shelf-edge association relative to more mid-shelf use by males. A generalized additive model was developed to identify habitat suitability for scalloped hammerheads in the GOM, while state-space modeling was used to examine movement behaviors. Model results highlighted the use of continental shelf waters with high occurrence at close proximities to both artificial and hard-bottom habitat combined with low chlorophyll a concentrations (~0–4 mg m-3) and moderate salinities (33–35.5). Habitat suitability for scalloped hammerheads was predicted to be high on the mid to outer continental shelf inside the 200 m isobaths and state-space models support area-restricted behavior was most common relative to transient behavior. Findings from this study provide important information on movement of this species in the GOM and highlight their restricted use of continental shelf habitat and resident behavior that will need to be incorporated in future stock assessments and extinction risk analyses.

Highlights

Information on movement and habitat use of marine apex predators is needed to better understand population structure and to implement spatially explicit management strategies
In our study, scalloped hammerhead distribution was primarily restricted to the continental shelf, though track data revealed some cross-shelf movements
Duncan et al (2006) determined population genetic structure of scalloped hammerheads was substantial for a large marine predator, and that restricted dispersal over deep open ocean habitat may be the primary driver, but found high connectivity and dispersal across continuous coastlines along continental shelves

Summary

Introduction

Information on movement and habitat use of marine apex predators is needed to better understand population structure and to implement spatially explicit management strategies. Identifying important habitat and the factors responsible for movement are inherently difficult due to the mobility of large marine predators as they often traverse multiple jurisdictional boundaries, ecosystems, and habitats (Chin et al, 2017). Tagging methods (e.g., acoustic telemetry and archival tagging) have been used to discern habitat use and associated data provide fisheries managers with a more complete picture of the environmental and oceanographic conditions that influence movement and can be used in ecosystem-based fisheries management. Due to cost and logistical constraints, basic data on habitat use and movement are limited for many shark species, including many important stocks in the North Atlantic Ocean and associated basins, such as the Gulf of Mexico (GOM)

Objectives

Methods

Conclusion