Patterns and ecosystem consequences of shark declines in the ocean

Abstract

Whereas many land predators disappeared before their ecological roles were studied, the decline of marine apex predators is still unfolding. Large sharks in particular have experienced rapid declines over the last decades. In this study, we review the documented changes in exploited elasmobranch communities in coastal, demersal, and pelagic habitats, and synthesize the effects of sharks on their prey and wider communities. We show that the high natural diversity and abundance of sharks is vulnerable to even light fishing pressure. The decline of large predatory sharks reduces natural mortality in a range of prey, contributing to changes in abundance, distribution, and behaviour of small elasmobranchs, marine mammals, and sea turtles that have few other predators. Through direct predation and behavioural modifications, top-down effects of sharks have led to cascading changes in some coastal ecosystems. In demersal and pelagic communities, there is increasing evidence of mesopredator release, but cascading effects are more hypothetical. Here, fishing pressure on mesopredators may mask or even reverse some ecosystem effects. In conclusion, large sharks can exert strong top-down forces with the potential to shape marine communities over large spatial and temporal scales. Yet more empirical evidence is needed to test the generality of these effects throughout the ocean.

Recovery of marine animal populations and ecosystems

“…The best strategies for recovery include: (i) raising public and political awareness; (ii) taking legal action and enforcing management plans; (iii) addressing cumulative human impacts; (iv) maintaining or restoring biodiversity and ecosystem complexity; and (v) planning for the long term, as recovery of long-lived species and complex ecosystems might need decades, if not centuries after major threats have been removed or reduced. These lessons learned from past recoveries of many marine mammals, birds, reptiles, some fishes and habitats could be applied to species that are currently undergoing strong population declines but have received little protection so far, such as many sharks [28,50,87], and to species that are increasingly exploited with little management in place, such as many invertebrates [88]. Whereas much trial and error was previously involved in finding the right management and conservation actions, current and future recovery plans could be better informed and take hold before populations or habitats reach critically low levels.…”

“…In addition, natural fluctuations in populations and ecosystems can mask or alter human-induced changes. Sometimes, these can be teased apart [26,27], but in other cases they are confounded or act in combination [5,27,28]. Nevertheless, increasing understanding of the natural dynamics of ecological systems and their past histories of change can help to identify meaningful reference points that might assist in assessing recovery.…”

“…Recovery potential seems lower whenever strong competitors inhabit the niche previously occupied by the depleted or lost habitat or species [58,84]. By contrast, the strong decline in large predatory sharks might have positively contributed to recent recoveries of many marine vertebrates, for which they are often a major or sole predator [28]. Changes in ecosystem productivity or environmental conditions have also been shown to influence recovery [23,85].…”

Movements of the reef manta ray (Manta alfredi) in the Red Sea using satellite and acoustic telemetry

“…Mobulid species are also often characterized by small population sizes that, when combined with low reproductive rates, result in greater vulnerability to harvest than most other marine fishes (Dewar et al 2008;Ferretti et al 2010;Marshall et al 2011;Deakos 2012). However, despite the presence of significant directed fishing effort and a susceptibility to human exploitation, we lack basic ecological information on life history parameters, movements, and habitat use of mantas (Couturier et al 2012;Walter et al 2013).…”

“…Global elasmobranch populations have experienced major declines over the last several decades (Dulvy et al 2008;Ferretti et al 2010) with an estimated 17 % of shark and ray species listed as critically endangered, endangered, or vulnerable to extinction (IUCN 2015). Within the family Mobulidae, one species is listed as endangered and three as vulnerable to extinction, including both recognized Manta species.…”