A world without sharks : forensic methods, trade networks, and impacts of predator removal

Abstract

Despite endured success as one of the oldest predators in existence, modern sharks are severely threatened by direct and indirect exploitation. Major challenges facing sustainable management of sharks include lack of species-specific monitoring and enforcement (what is traded), lack of geographic exploitation data on which to base management decisions (where these are sourced from), lack of coordination and regulation strategies for shark trade supply chains (what interventions are effective), and lack of understanding regarding the impacts of shark removal on wider ecosystem dynamics (what ecological role do sharks play).

This dissertation addresses these questions by i) developing tools that have potential to increase and improve available data, particularly for species and regions, ii) evaluating the structure and evolution of trade to discern implications for intervention and iii) understanding impacts of predator removal from ecosystems, particularly the role of sharks in nitrogen cycling. Chapter 1 provides context for the theoretical framework and methods employed to resolve these challenges. Chapter 2 develops a convolutional neural network trained on skin morphology as a rapid assessment tool for species identification monitoring and enforcement; Chapter 3 establishes a method for comparing and assessing shark fin samples using stable isotopes, particularly in terms of elucidating collated spatial patterns of trade patterns and trophic ecology. Chapter 4 maps evidence of sequential exploitation and characterizes the highly robust yet adaptable structure of the shark trade network over time. The connectivity and speed at which the network can rewire outpaces local regulations, highlighting negative returns of intervention and reaffirming a need for coordinated global multilateral strategy. Chapter 5 takes a theoretical mathematical modeling approach and presents a dynamic role of sharks as “nitrogen capacitors” for coral reefs, stabilizing systemic nutrients which may have cascading effects on coral and algal ecosystem states. Shark biomass and the rate (not just scale) of exploitation was also found to drive critical transitions between stable and unstable equilibria. Chapter 6 summarizes key findings and recommends integrated future research directions, particularly in terms of development of nonlinear tools for better forecasting of complex dynamic behavior.

Overall, these multidisciplinary approaches contribute integrated insights to a complex problem and provide actionable implications for marine management.