RESEARCH OBJECTIVES

Movement patterns of great hammerhead sharks (Sphyrna mokarran) 

 

Animal movement influences ecosystem structure and function (Nathan et al., 2022; Riotte-Lambert & Matthiopoulos, 2020) by shaping the distribution and abundance of organisms and driving ecological processes at population, community and ecosystem level. (Bestley et al., 2013). Understanding movement patterns can reveal habitat preferences (Mercker et al., 2021), inter- and intra-specific interactions, foraging ecology (Sims et al., 2008) and potential responses to disturbance (Doherty et al., 2021). Therefore, effective conservation and management strategies should include a comprehensive understanding of movement patterns (Lascelles et al., 2014; Heupel et al., 2015). However, determining where, when and why a species moves remains a challenge (Allen and Singh, 2016). For large marine vertebrates the complexity of this challenge is further amplified, as they live in a concealing environment, are low in abundance, move far distances and are difficult to capture (McClenachan et al., 2012; Dulvy et al., 2014). 

​

The great hammerhead shark, Sphyrna mokarran, is a charismatic, ecologically important species. They are of global conservation concern, but their movement and behaviour is poorly documented. Great hammerheads have experienced substantial population declines due to being targeted or caught as bycatch in various fisheries across its range (Denham et al., 2007; Miller et al., 2014). These declines have led to the species being classified as Critically Endangered by the International Union for Conservation of Nature (IUCN) Red List (Rigby et al., 2019) and listed in Appendix II of the Convention on the Conservation of Migratory Species of Wild Animals (CMS, 2019).  Overfishing is a major driver of population declines in threatened shark species globally (Dulvy et al., 2021; Dulvy et al., 2024) whether targeted or as bycatch. S. mokarran is particularly vulnerable to catch-related death due to its valuable fins and high sensitivity to capture-related stress, which results in elevated rates of post-release mortality (Gallagher et al., 2014; Gulak et al., 2015; Morgan and Burgess, 2007).

​

The S. mokarran occurs throughout the Western North Atlantic, including in the USA from the Florida Keys to Virginia, with migration distances up to 1200 km recorded (Kohler et al., 1998; Miller et al., 2014; Kohler and Turner, 2001; Hammerschlag et al., 2011). The species is highly mobile, and distributed widely in coastal and pelagic tropical waters, inhabiting a range of ecosystems from shallow lagoons to deep oceanic waters (Compagno, 1984; Cliff, 1995). As an upper-trophic level predator, they can play a pivotal role in maintaining ecosystem structure, function, and stability by serving as a mobile link species between habitats through its extensive movements (Mourier et al., 2013; Lundberg and Moberg, 2003; Heupel et al., 2015; Ferretti et al., 2010; Roff et al., 2016). 

​

Despite increased information about the habitat use and movements of S. mokarran  over the last decades, crucial gaps in our understanding of their movement ecology persist and need to be addressed to inform effective conservation strategies.  Previously conducted research indicates that great hammerheads exhibit predictable horizontal movements, with core habitat areas in coastal regions and seasonal migrations to temperate oceanic waters. However, these studies highlight the urgent need to identify spatial hotspots and migratory corridors to mitigate risks posed by anthropogenic threats. Given the conservation concern of the species and its elevated sensitivity to capture in commercial fisheries, additional  tagging efforts are necessary to understand the movement patterns of great hammerhead sharks to develop and implement the most effective conservation strategies. An integral component of this is to determine movement patterns across age classes, and if this changes over time. 

​

The goal of this study is to use acoustic telemetry data of juvenile to adult great hammerhead sharks to identify movement patterns in the Florida Keys and the internal and external factors that might influence these movement patterns. 

 

Main research questions:
 

1. Based on previous findings of predictable horizontal movements and seasonal migrations, can we identify distinct clusters of seasonal movement patterns for S. mokarran?

2. Which internal factors, such as size and sex, as well as external factors, including salinity, water temperature, and others, influence the movement patterns of S. mokarran?

3. Can we identify potential hotspots for future receiver array deployments or to support proposed management strategies?