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Fishery, Conservation, Management & Ecological Role of Hammerhead Sharks 

IN THE BAHAMAS AND U.S. ATLANTIC 

Overview

Documented global declines of shark populations ask for adequate conservation management strategies (Dulvy et al. 2014). Targeted and indirect fishing is one of the major anthropogenic threats today’s shark populations are facing (Davidson et al. 2017, Queiroz et al. 2016). Traditional management methods such as limited catch quotas or prohibited landing can benefit some shark species (Ward-Paige et al. 2012) but might be ineffective as a strategy for the conservations of great, Sphyrna mokarran, and scalloped, Sphyrna lewini, hammerhead sharks.

These large, mobile and circumtropical species that inhabit coastal-pelagic and semi-oceanic habitats are both present in the U.S. Atlantic and the Gulf of Mexico (Guttridge et al. 2017, Wells et al. 2018). Scalloped and great hammerheads have been captured as target or bycatch species in a wide variety of fisheries. As a result, the two species have suffered from significant population declines throughout their range and are listed as Endangered by the IUCN Red List of Threatened Species. Compounding these declines are the two species’ high sensitivity to capture as 70-95% of the hammerheads are dead before they are landed (Miller et al. 2013, Miller et al. 2014). According to Jiao et al. (2011) scalloped hammerheads, which were periodically overfished from the early 1980s to 2005, showed a high risk of overfishing in recent years. Data about the status of the great hammerhead sharks in this region are scarce in quality and quantity. Most likely they became overfished in the mid-1980s and periodically from 1983 to 1997. However, currently the overfishing risk is low. Due to their high at-vessel and post-release mortality traditional management methods such as prohibited landings are expected to show limited success regarding their conservation and alternative management strategies are needed.

In many ecosystems, large shark species function as apex predators that exert a strong force in shaping marine communities, and there is increasing evidence that the loss of apex predators can lead to trophic cascaded and change dynamics in an ecosystem (Heithaus et al. 2008, Roff et al. 2016). The ecological consequences of the loss of such predators due to fishery related population declines are not fully understood yet.

Using satellite and acoustic telemetry in combination with fishery dependent and independent data my project aims to discuss the efficiency of potential time-area closures as a strategy for the conservation of great and scalloped hammerheads in the U.S. Atlantic and the Gulf of Mexico. Acoustic tracking, stable isotope analysis, body condition assessment and accelerometer-enabled data storage tags will help advancing our understanding of predator-prey dynamics and how the absence of an apex predator might change the behaviour and role of mesoconsumer populations.

Research Questions

  1. Identify the seasonal distribution and relative densities of scalloped, S. lewini, and great hammerhead, S. mokarran, sharks from fishery dependent and independent data sources.

  2. Quantify horizontal movement patterns of scalloped and great hammerhead sharks to assess the efficacy of hypothetical time-area closures.

  3. Characterize the potential predator-prey interactions between an endangered transient apex marine predator, S. mokarran, and two smaller mesoconsumers (southern stingray, Hypanus americanus, and Atlantic spotted eagle ray, Aetobatus narinari) and investigate underlying aspects of microhabitat choice.

  4. Understand potential implications from a potential loss of top-down regulation in a marine ecosystem by assessing the behaviour and physiology of H. americanus and A. narinari.

Conservation importance

In 2017, hammerhead sharks were defined as one of 7 priority groups of shark species that would benefit the most from better management (Dulvy et al. 2017). A possible approach to reduce the fishery and bycatch related mortality of sharks are Marine Protected Areas (MPAs) and time-area closures. However, the high mobility of some predatory sharks such as great and scalloped hammerhead sharks might reduce the efficiency of MPAs as they are restricted in size. A recent study (Davidson and Dulvy 2017) found that only 0.9 % of current MPAs are fit to prevent extinctions. The hammerhead’s high mobility together with their high sensitivity to capture might make time-area closures a valuable strategy to improve the conservation of these sharks. However, age-specific data on the spatial dynamics and habitat use needs to be included to discuss the efficiency of such closures as a conservation tool (Lea et al. 2016). Such data is often scarce as these sharks are typically hard to access, difficult to tag and low in abundance.

Sharks can serve as apex predators in marine ecosystems and their decline in numbers and/or removal is suggested to have significant consequences for ecosystem functioning (Roff et al. 2016, Heithaus et al. 2008). Potential effects on mesoconsumer populations might be particularly important to understand as those species often serve as important link between higher and lower trophic levels (Vaudo & Heithaus et al. 2013).

Principal Investigator: 

Vital Heim - Bimini Biological Field Station Foundation/University of Basel, Basel (Switzerland)

Vital Heim
Principal Investigator

At Lab Since: September 2016
Principal Investigator since July 2018


From: Frutigen, Switzerland

Current position: PhD student in Zoology at the University of Basel, Switzerland.
Principal Investigator at the Bimini Biological Field Station, Bimini, Bahamas

Current project: My project investigates fishery interactions and spatial dynamics of scalloped, Sphyrna lewini, and great hammerhead, Sphyrna mokarran, sharks to assess the efficacy of hypothetical time-area closures for a better conservation management.

Furthermore, I aim to understand the dynamics of predator-prey interactions and how the absence of a top-consumer affects the mesoconsumers in an ecosystem using S. mokarran and the southern stingray, Hypanus americanus, as a model.

Education: 2012 - 2015: BSc in Biology, University of Bern, Switzerland

Thesis title: "Hypoxia in pancreatic neuroendocrine tumor (pNET) cell lines - Influence on the expression of alternately spliced VEGF isoforms"

2017 - 2018: MSc in Animal Biology, University of Basel, Switzerland

Thesis title: "The effects of dive tourism and food provisioning on the behavior and local space use of the great hammerhead shark, Sphyrna mokarran, in Bimini, the Bahamas"

Research interests: I have strong interests in the behavior, predator-prey interactions, migration patterns, spatial dynamics and physiology of sharks and how these factors influence their role in ecosystem functioning.

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