Ecology and fisheries interactions of hammerhead sharks with implications for management in the western north Atlantic

The widespread human-induced removal of large marine predators as a result of global overfishing requires management strategies that effectively reduce the fisheries-related mortality to prevent species extinctions and the trophic downgrading of marine ecosystems. For my thesis, I primarily focused on advancing our understanding of the ecology of great and scalloped hammerhead sharks, Sphyrna mokarran and Sphyrna lewini, to improve their management in the western North Atlantic, since high at-vessel and post-release mortality of these two overfished species in a variety of fisheries may render management measures that solely limit their harvest insufficient. I first developed a new drill attachment for a faster and safer deployment of fin-mounted satellite-linked transmitters that can be used to generate the movement data imperatively needed to design effective management strategies. The new drill attachment allowed the deployment of four-bolt fin-mounted satellite-linked transmitters consistently in under 90 seconds, which may reduce the stress sharks experience and subsequently minimises the risk for detrimental effects during tagging studies. While opportunistically documenting the wound closure capacity of fin tissue in great hammerheads and the fate of fin-mounted satellite-linked transmitters after deployment, I found that these transmitters show a lateral posterior displacement over time until they are completely shed from the fin. The documented high wound closure capacity may support this process and reduces the risk for long-term trauma through fin-mounted transmitters. Using the newly developed drill attachment, I deployed fin-mounted satellite-linked transmitters on great and scalloped hammerheads and analysed observer data from the U.S. commercial longline fisheries to detail the sharks’ regional movements and their overlap with fisheries in the U.S. Atlantic and the Gulf of Mexico, aiming to suggest potential time area closures that may reduce their fisheries-related mortality. Multiple long-distance movements, a high degree of individual variation in movement behaviours and regional space use patterns highlighted the challenges in finding adequate management strategies for great and scalloped hammerheads. But the importance of waters off Florida and North Carolina for the great and scalloped hammerheads and their high overlap with commercial longline fisheries within, make these regions candidates for the design and implementation of year-round and/or time-area closures for the protection of these two species in the western North Atlantic. Anthropogenic activities altering movements could further complicate the management of hammerheads. I thus used passive acoustic telemetry to explore how long-term tourism-related feeding impacts the local space use of great hammerheads during seasonal residency periods along their movement tracks by changing the spatiotemporal availability of resources. I found that feeding progressively altered the space use of great hammerheads across short- and long-term time scales with persisting effects on fed individuals in Bimini, The Bahamas. How the effects of the continued declines of great and scalloped hammerheads may alter the functionality of marine ecosystems depends in part on how these changes are mediated by mesopredators. Thus, in the last part of my thesis I explored the trophic ecology and the body condition of an abundant marine mesopredator known to be an important prey species to great hammerheads, the southern stingray, Hypanus americanus. I identified southern stingrays in Bimini to be an important mesopredator of reef habitats with a preference for annelids and molluscs and provided baseline data to improve our capability to predict the consequences of possible trophic cascades in marine food webs. With one third of all sharks and rays being fished towards extinction, adequate and data-driven management strategies need to be implemented. My thesis, while focusing on great and scalloped hammerheads, provides methods and insights contributing to this effort for large-bodied and highly mobile sharks.