Molecular ecology and conservation genomics of scalloped (sphyrna lewini) and carolina hammerheads (s. gilberti)

Abstract

Scalloped hammerheads (Sphyrna lewini) are a circumglobally distributed shark that has experienced declines in abundance throughout its range. Management of scalloped hammerheads in the U.S. Atlantic is a challenge due to the presence of Carolina hammerheads (S. gilberti), a recently discovered, co-distributed, cryptic species. The species are indistinguishable based on external morphology and can only be identified with genetics or precaudal vertebrae counts. Little is known about Carolina hammerheads, but they are thought to be less abundant than scalloped hammerheads and may have a more restricted range. Scalloped hammerhead stocks are considered overfished in the U.S. Atlantic and Gulf of Mexico (Gulf) and are managed with great hammerheads (S. mokarran) and smooth hammerheads (S. zygaena) as the hammerhead shark complex. Carolina hammerheads are not currently considered in management plans, and data is needed regarding habitat use, distribution, and relative abundance of the species. In each chapter, molecular data is used to investigate questions relevant to the conservation and management of scalloped and Carolina hammerheads. Chapter two investigates hybridization between the species. First generation hybrids and backcrosses were found throughout the region in which the species area co-distributed. Carolina hammerheads were typically the mother of hybrids, and most instances of backcrossing were between an F1 and pure scalloped hammerhead. Chapter three describes the distribution and relative abundance of Carolina and scalloped hammerheads in the U.S. Atlantic and Gulf. Carolina hammerhead were more abundant than scalloped hammerheads in South Carolina, but less abundant in the U.S. Atlantic overall and were not present in the Gulf. Chapter four evaluates patterns of genetic variation between management units in the western North Atlantic. Significant structure was vi present between the North and central Atlantic, as well as within the central Atlantic. Chapter five examines the relative reproductive importance of scalloped and Carolina hammerheads nurseries in the U.S. Atlantic and Gulf. Reproductive potential was roughly equivalent between nurseries, but site fidelity was high, suggesting nurseries are not used equivalently by individual sharks. Results presented in this dissertation provide important information on the reproductive ecology, distribution, and relative abundance of scalloped and Carolina hammerheads in the U.S. Atlantic. Carolina hammerhead abundance was low in the U.S. Atlantic compared to scalloped hammerheads and should be considered in future decisions regarding the management of the hammerhead shark complex. This dissertation presents some of the first research conducted on Carolina hammerheads since the species was described, but many questions remain. Additional work is needed to understand adult habitat, movement patterns, and how the species may interact with the commercial fishery.