Large-Scale Absence of Sharks on Reefs in the Greater-Caribbean: A Footprint of Human Pressures

Abstract

BackgroundIn recent decades, large pelagic and coastal shark populations have declined dramatically with increased fishing; however, the status of sharks in other systems such as coral reefs remains largely unassessed despite a long history of exploitation. Here we explore the contemporary distribution and sighting frequency of sharks on reefs in the greater-Caribbean and assess the possible role of human pressures on observed patterns.Methodology/Principal FindingsWe analyzed 76,340 underwater surveys carried out by trained volunteer divers between 1993 and 2008. Surveys were grouped within one km2 cells, which allowed us to determine the contemporary geographical distribution and sighting frequency of sharks. Sighting frequency was calculated as the ratio of surveys with sharks to the total number of surveys in each cell. We compared sighting frequency to the number of people in the cell vicinity and used population viability analyses to assess the effects of exploitation on population trends. Sharks, with the exception of nurse sharks occurred mainly in areas with very low human population or strong fishing regulations and marine conservation. Population viability analysis suggests that exploitation alone could explain the large-scale absence; however, this pattern is likely to be exacerbated by additional anthropogenic stressors, such as pollution and habitat degradation, that also correlate with human population.Conclusions/SignificanceHuman pressures in coastal zones have lead to the broad-scale absence of sharks on reefs in the greater-Caribbean. Preventing further loss of sharks requires urgent management measures to curb fishing mortality and to mitigate other anthropogenic stressors to protect sites where sharks still exist. The fact that sharks still occur in some densely populated areas where strong fishing regulations are in place indicates the possibility of success and encourages the implementation of conservation measures.

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.…”

Sizing up the ecological role of sharks as predators

“…Friedlander & DeMartini 2002, Robbins et al 2006, Sandin et al 2008, Heupel et al 2009, Ward-Paige et al 2010. These studies have typically used either underwater visual survey techniques or fishing data to examine the presence and abundance of shark species at different coral reef platforms.…”

Dermal denticles as a tool to reconstruct shark communities

“…For example, top predator biomass was found to be 5 to 15-fold higher at unfished islands in the Line Islands as compared to populated, fished islands . However, these records are sporadic, limited in detail or taxonomic resolution, and only date back half a century (Odum & Odum 1955, Baum & Myers 2004, Ward & Myers 2005, Ferretti et al 2008, Ward-Paige et al 2010b. Cryptic behavior, rarity, and diurnal and seasonal movement patterns prevent sharks from being meaningfully censused in many regions (Sale & Douglas 1981, MacNeil et al 2008, Ward-Paige et al 2010a, McCauley, et al 2012a.…”