Seasonal Movements and Habitat Use of Juvenile Smooth Hammerhead Sharks in the Western North Atlantic Ocean and Significance for Management

Mark Sampson,Lara L. Sousa,Mahmood S. Shivji,Jeremy J. Vaudo,Ryan K. Logan,Bradley M. Wetherbee

doi:10.3389/fmars.2020.566364

Abstract

Upper trophic level predators dramatically impacted by fisheries include the large-bodied hammerhead sharks, which have become species of conservation concern worldwide. Implementing spatial management for conservation of hammerhead populations requires knowledge of temporal distribution patterns and habitat use, identification of essential habitat for protection, and quantification of interactions with human activities. There is little such information for the smooth hammerhead shark, Sphyrna zygaena. We used fin-mounted satellite tags to examine the movements and habitat use of juvenile smooth hammerheads, a demographic segment particularly threatened by exploitation. Six sharks were tagged off the US mid-Atlantic and tracked for 49–441 d (mean 187 d). Sharks consistently showed area-restricted movements within a summer core area in waters of the New York Bight and a winter core area off Cape Hatteras, North Carolina, with directed movements between them in autumn. There was high overlap of shark winter core area use and the Mid-Atlantic Shark Area (MASA) - a seven-month per year, bottom-longline fishery closure - indicating that this area closure offers seasonal reduction in fishing pressure for this species. Based on timing of shark movements and the MASA closure, protection for juvenile smooth hammerheads may be increased by beginning the closure period one month earlier than currently scheduled. Generalized additive mixed models revealed that area-restricted movements of sharks in their summer and winter core areas coincided with high primary productivity, strong sea surface temperature fronts and elevated sea surface temperature. Consistency in use of summer and winter core areas suggests that the coastal waters of the New York Bight and Cape Hatteras, North Carolina could be considered for Essential Fish Habitat designation for this species. This study reveals the first high resolution movements and habitat use for smooth hammerheads in the western North Atlantic to inform management planning for this population.

Highlights

The rapid expansion of elasmobranch fisheries and trade globally are principal drivers of population decline for many shark species (Dulvy et al, 2014)
The conservation of this species is an international priority, e.g., Vulnerable listing on the International Union for Conservation of Nature (IUCN) Red List (Casper et al, 2009); Appendix II listing on the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES); Appendix II listing on the Convention on Migratory Species of Wild Animals (CMS)
After testing the importance of various combinations of autocorrelation structures while holding other variables constant, we found that the Generalized additive mixed models (GAMMs) without an autocorrelation term was deemed more robust with better wAIC and AIC; no autocorrelation structure was used in the final model

Summary

Introduction

The rapid expansion of elasmobranch fisheries and trade globally are principal drivers of population decline for many shark species (Dulvy et al, 2014) Some species, such as the large-bodied hammerhead sharks (great – Sphyrna mokarran, scalloped – S. lewini, and smooth – S. zygaena hammerheads), are especially vulnerable to fishing pressure because of their slow rates of population growth (Cortés et al, 2010) and high at-vessel and post-release mortality due to elevated stress response to capture (Morgan and Carlson, 2010; Eddy et al, 2016; Gallagher and Klimley, 2018). Conservation-relevant data on many aspects of the biology of smooth hammerheads are extremely limited, including information relating to their movements, seasonal distributions and habitat use (Miller, 2016; Gallagher and Klimley, 2018)

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