Defense Date

11-20-2020

Document Type

Thesis

Degree Type

Master of Science

Degree Name

Marine Science

First Advisor

Bradley Wetherbee

Second Advisor

Bernhard Riegl

Third Advisor

Matthew Johnston

Keywords

Sphyrnidae, hammerhead, Eusphyra blochii, Sphyrna, corona, media, gilberti, tiburo, tudes

Abstract

Historically, sharks were not considered to be of high commercial value due to low demand and modest catch rates in comparison to bony fishes. However, with recent heightened demand for shark products, their economic value has increased profoundly, which has led to overharvesting of many species. Sharks are considered as a K-selected species, which includes life history traits such as slow growth, late maturity, few offspring, and slow reproductive rates. Given the slow intrinsic rate of increased characteristic of most species of sharks, few species can support heavy fishing pressure and are vulnerable to drastic population declines as a result of fishing. For many species of sharks, even modest rates of mortality in fisheries (either targeted or as bycatch) may lead to population declines and require decades for recovery from overfishing. Hammerhead sharks (family Sphyrnidae) in particular have experienced large population declines over the past several decades, estimated as high as 86% throughout the world oceans, primarily due to overfishing. The vulnerability of hammerhead sharks to overfishing results from a combination of factors, including relatively slow rates of population growth and low post-release survivorship of for individuals captured and released in fisheries. Research on hammerhead sharks has focused almost exclusively on three large species: scalloped hammerhead (Sphyrna lewini), great hammerhead (Sphyrna mokarran), and smooth hammerhead (Sphyrna zygaena). The other six species of hammerheads – winghead shark (Eusphyra blochii), scalloped bonnethead (Sphyrna corona), Carolina hammerhead (Sphyrna gilberti), scoophead (Sphyrna media), bonnethead (Sphyrna tiburo), and smalleye hammerhead (Sphyrna tudes) – have received much less attention and the biology is poorly understood in comparison to the three large species. Limited information about the biology of the six lesser known species, particularly in regards to life history characteristics, interactions with fisheries, and status of their populations has hindered assessments of the status of their populations and curtailed the ability to enact well-informed policy for management of their populations. This thesis reviews information on the biology of six lesser known species of hammerhead sharks and documents the current knowledge of interactions with fisheries around the world, with the overall goal of collating and summarizing knowledge for these species and contributing information useful for improved management of their populations.

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