Global catches, exploitation rates, and rebuilding options for sharks

Abstract

Adequate conservation and management of shark populations is becoming increasingly important on a global scale, especially because many species are exceptionally vulnerable to overfishing. Yet, reported catch statistics for sharks are incomplete, and mortality estimates have not been available for sharks as a group. Here, the global catch and mortality of sharks from reported and unreported landings, discards, and shark finning are being estimated at 1.44 million metric tons for the year 2000, and at only slightly less in 2010 (1.41 million tons). Based on an analysis of average shark weights, this translates into a total annual mortality estimate of about 100 million sharks in 2000, and about 97 million sharks in 2010, with a total range of possible values between 63 and 273 million sharks per year. Further, the exploitation rate for sharks as a group was calculated by dividing two independent mortality estimates by an estimate of total global biomass. As an alternative approach, exploitation rates for individual shark populations were compiled and averaged from stock assessments and other published sources. The resulting three independent estimates of the average exploitation rate ranged between 6.4% and 7.9% of sharks killed per year. This exceeds the average rebound rate for many shark populations, estimated from the life history information on 62 shark species (rebound rates averaged 4.9% per year), and explains the ongoing declines in most populations for which data exist. The consequences of these unsustainable catch and mortality rates for marine ecosystems could be substantial. Global total shark mortality, therefore, needs to be reduced drastically in order to rebuild depleted populations and restore marine ecosystems with functional top predators.

Introduction

Sharks, skates, rays and chimaeras together comprise the chondrichthyan fishes (Class Chondrichthyes), a group of about 1000 species that has persisted for at least 400 million years, rendering them one of the oldest extant vertebrate groups on the planet. Recently, however, the global growth of fishing, coupled with Chondrichthyes' relatively slow growth and reproductive rates, have resulted in the progressive depletion of populations around the world. This trend has been particularly pronounced for sharks, largely due to their inherent vulnerability, and an increasing demand, particularly for their fins, in the Asian market [1], [2], [3], [4]. As such, many shark species are comparable to great whales, which also have late maturity, slow growth and low reproductive rates, and experienced escalating global fishing pressure until a global whaling moratorium came into effect in 1986 [5]. Similar to whales, quantifying the precise extent of sharks' decline, the risk of species extinction, and the consequences for marine ecosystems have been challenging and controversial, mostly due to data limitations [4], [6], [7], [8].

A key problem is the incomplete reporting of shark catches to the United Nations Food and Agriculture Organization (FAO), which tracks the status of fisheries worldwide. Caught sharks are often not landed and are instead discarded at sea [7], [9], with such discards not usually reported to national or international management agencies unless there are trained observers on board. Compounding this problem is the practice of shark finning, where the animal's fins are removed prior to the body being discarded at sea [9]. Due to the high value of the fins in Asian markets this practice is globally widespread. Some jurisdictions, such as Canada, the United States, Australia, and Europe have gradually introduced anti-finning legislation over the last 10 years, yet the practice continues in most other parts of the world [2]. Therefore it is very likely that reported catches represent only a fraction of total shark mortality. For example, Clarke et al. [9] used trade auction records from Hong Kong to estimate that the total mass of sharks caught for the fin trade. Estimates ranged between 1.21 and 2.29 Mt (million metric tons) yr⁻¹ with a median estimate of 1.70 Mt yr⁻¹ in the year 2000. This amounted to more than four times the reported shark catch from FAO at that time [9].

Notwithstanding these problems, the FAO, among other management bodies, has long recognized the conservation challenges associated with sharks and their relatives, and it launched an International Plan of Action for Sharks in 1999 (IPOA-Sharks, which also includes skates, rays, and chimaeras). This plan aims to enhance the conservation and management of sharks and their sustainable use, while improving data collection and the monitoring and management of shark fisheries [10]. The IPOA-Sharks further recommends that all states contributing to fishing mortality on sharks should participate in its management, and should have developed a National Shark Plan by 2001. However, progress remains disappointing so far, with limited adoption and implementation of IPOA goals at the national level [2], [11].

The objective of this paper is to provide an up-to-date assessment of the current status of shark populations including estimated global catches, current exploitation rates (herein defined as the total catch divided by the estimated biomass), and potential extinction risks at current levels of exploitation. Based on this review, possible management solutions for conserving and rebuilding shark populations are discussed. The authors intend to provide critical baseline information for the further development of national and international action plans that help ensure the conservation of sharks and their relatives.