Shifting elasmobranch community assemblage at Cocos Island—an isolated marine protected area
Easton R. White,∗ § Mark C. Myers,† Joanna Mills Flemming,‡ and Julia K. Baum∗
Fishing pressure has increased the extinction risk of many elasmobranch (shark and ray) species. Although many countries have established no-take marine reserves, a paucity of monitoring data means it is still unclear if reserves are effectively protecting these species.
Conservation efforts directed at sharks and rays have increased substantially over the past decade with the recognition that fishing has greatly reduced the abundance of many of these species (Robbins et al. 2006; Dulvy et al. 2014a).
A consequence of coupling this high vulnerability with excessive fishing pressure is that many elasmobranch species are now considered at a heightened risk of extinction. Currently, 20% of shark and 16% of ray species are listed as threatened (critically endangered, endangered, or vulnerable) by the International Union for Conservation of Nature (IUCN) (Dulvy et al. 2014a). In addition, many elasmobranchs assessed globally lack regional assessments because of a paucity of data. This is true for many reef shark and ray species, especially those inhabiting remote islands or areas with little monitoring or management.
Given the challenges of controlling fishing pressure, marine protected areas (MPAs) are instead increasingly being used as a conservation measure for sharks (Knip et al. 2012), but to date there has been little evaluation of their efficacy at protecting these large mobile fishes. Comprehensive monitoring of elasmobranch populations relies on fisheries-dependent or research survey data; thus, estimating population trends is difficult in locations with few regulations, no formal data collection, or no fishing activity, including marine reserves (Graham et al. 2010).
Insufficient planning or lack of resources for monitoring and enforcement (Claudet & Guidetti 2010) also commonly creates situations where MPAs are merely paper parks, protected on paper but not in practice (Rife et al. 2013). Studies of the effectiveness of MPAs at conserving elasmobranchs tend to focus on species or life stages with limited ranges that can be protected in even small MPAs (Robbins et al. 2006; Knip et al. 2012).
As one of the world’s oldest MPAs, Cocos Island National Park (Isla del Coco; N 05°3108, W 87°0418), a small (23.85 km2), uninhabited island 550 km from mainland Costa Rica in the eastern tropical Pacific (Fig. 1), presents an opportunity to evaluate the effectiveness of MPAs at conserving elasmobranchs. The waters surrounding Cocos Island provide exceptional habitat for marine organisms due to the island’s isolation, associated reef and seamount complexes, and position at the confluence of several major oceanic currents (Garrison 2005). Sea surface temperatures at Cocos Island range from 24 to 30 °C and are affected every 4–9 years by El Niño Southern Oscillation (ENSO) events (Garrison 2005; Sibaja-Cordero 2008), which substantially influence the distribution and abundance of several marine species (Lea & Rosenblatt 2000).
Although Cocos Island has been protected for over 20 years, with a permanent ranger station in place since 1992, funding for monitoring and enforcement has been limited. Since 2003, however, in conjunction with the Costa Rican Coast Guard, the Mar- Viva Foundation, a regional nonprofit nongovernmental organization (NGO), has patrolled the island (Arias et al. 2014). However, illegal fishing of large elasmobranchs still occurs within the park’s waters (Arias et al. 2014).
Eight of 12 species declined significantly over the past 2 decades. We documented decreases in relative abundance for 6 species, including the iconic scalloped hammerhead shark (Sphyrna lewini) (−45%), whitetip reef shark (Triaenodon obesus) (−77%), mobula ray (Mobula spp.) (−78%), and manta ray (Manta birostris) (−89%) rays, and decreases in the probability of occurrence for 2 other species.
Several of these species have small home ranges and should be better protected by an MPA, which underscores the notion that declines of marine megafauna will continue unabated in MPAs unless there is adequate enforcement effort to control fishing.
The study examined standardized time series data of shark and ray observations at Cocos Island, collected systematically by divemasters from a single dive company, Undersea Hunter, on every dive they made between 1993 and 2013. Because there are no fisheries data or systematic research surveys for Cocos Island, these diver-collected data may be used to estimate trends in elasmobranch relative abundance and probability of occurrence. Moreover, these data is meant to be of higher quality than many citizen science data sets because relatively few highly experienced divers recorded the data and the species studied were large and could be easily identified (Ward-Paige & Lotze 2011; Vianna et al. 2014).
At least one shark or ray was seen on almost every dive (99% of 23,391 dives), with a total of 1,411,187 individuals recorded between 1993 and 2013. Whitetip reef sharks were by far the most common elasmobranch observed at Cocos Island. They were observed on 97% of dives (average
25.6 per dive). Scalloped hammerheads and marble rays were also commonly seen (on 77% and 84% of dives, average 34.0 and 8.5 individuals/dive). At least one of the 6 sharks species for which only presence or absence was noted was reported on 20% of dives.
Among the Pelagic Sharks, it was estimated that the relative abundance of scalloped hammerhead sharks at Cocos Island has declined by 45% since 1993 (Fig. 4a) Silky sharks are also estimated to have declined: the odds of occurrence of silky sharks declined by 91% over the 21 years (Fig. 4c).
In contrast, the odds of occurrence of tiger sharks increased by 79%/year (Fig. 4b), driven largely by the fact that this species was not observed by divemasters at Cocos until 2000 and have since increased such that they are now typically observed on 12% of dives. It is possible that within this system of strong fishing pressure, tiger sharks have an advantage over other elasmobranch species because of their relatively high intrinsic rate of increase (Hutchings et al. 2012) and high post-hooking survival rate (Gallagher et al. 2014).
Reef-Associated Shark counts, such as the Whitetip reef shark, decreased by 77% (Fig. 4d). Although whitetip reef sharks are year-long residents of Cocos Island, their relative abundance showed seasonal trends. The odds of occurrence for silvertips declined by 87% over the study period (Fig. 4g). Silvertip sharks were more commonly observed at lower current strengths and when water visibility was poor. The sharpest declines in silvertip occurrence came after 2007.
It was initially hypothesized that reef-associated sharks (blacktip, Galapagos, and silvertip), because of their high site fidelity, would be better protected by the Cocos Island MPA. Results are consistent with this hypothesis for both blacktip and Galapagos sharks, but silvertip sharks declined over time.
Bottom-Feeding Rays Were estimated that both species of rays declined at Cocos Island over the past 2 decades. Marble ray relative abundance decreased by an estimated 73% from 1993 to 2012 (Fig. 4i). Eagle ray relative abundance also decreased by an estimated 34%
In the remaining group, the Planktivores, the odds of occurrence for whale sharks has increased by 4.5%/year (Fig. 4j). Whale sharks were less commonly seen in years of high El Niño activity and more commonly seen during the wet season. In addition, peaks in whale shark presence occurred every 3 years. Mobula and manta rays were observed only occasionally at Cocos Island. Mobula ray relative abundance decreased by 78% (Fig. 4k).
The scalloped hammerhead is considered endangered within the eastern tropical Pacific, where it is caught as bycatch in at least Mexico, Costa Rica, and Ecuador (Baum et al. 2007; Kyne et al. 2012). Scalloped hammerhead sharks are known to move among the major offshore islands in the region: Cocos, Galapagos, and Malpelo (Bessudo et al. 2011). Although each of these islands is designated as an MPA, scalloped hammerheads are still caught both illegally within these protected areas and legally outside them (Kyne et al. 2012). Thus, substantial declines in these species are not surprising.
Silky shark, the other large pelagic shark that declined significantly, is the most commonly caught shark species in the eastern Pacific’s tuna purse seine fisheries (Watson et al. 2009). Although silky sharks are listed as near threatened globally, they are considered vulnerable in the eastern tropical Pacific because of directed fishing for their fins and bycatch (Watson et al. 2009; IUCN 2014). From 1994 to 2004, capture rates of silky sharks as bycatch in purse seine fisheries in this region are estimated to have fallen by 50% (Minami et al. 2007). Although we examined silky shark presence instead of counts, our results indicate a similar dramatic decline.
Moreover, while many divers are excited by the increasing number of some larger elasmobranch species (i.e., tiger, blacktip, Galapagos, and whale sharks), these shifts reflect the changing community assemblage that has occurred at Cocos Island over the past 21 years and are not necessarily an indication of the MPA’s effectiveness (Baskett et al. 2007). It is unclear if the current dynamics of the Cocos Island elasmobranch community are simply indicative of a long transient response following creation of the MPA (White et al. 2013). Although management efforts have increased in the past decade, illegal fishing still occurs within the island’s waters (Arias et al. 2014). It is unclear if the Cocos Island MPA is even properly designed to protect such large and wide-ranging species. Conservation efforts at Cocos Island cannot be focused simply on expanding the protected area (Arias et al. 2014); rather, efforts should be put toward increasing enforcement and management (Kelaher et al. 2015).
Costa Rica’s efforts to increase their MPA coverage are admirable, but the establishment of MPAs cannot be the end point. Explicit plans and dedicated funding for monitoring and enforcement must be in place to prevent the creation of a network of paper parks. These plans need to include using both theory about MPAs and empirical data (White et al. 2011). Further, there must be stronger penalties for noncompliance with MPA rules to offset the potential gains of illegal fishing (Arias et al. 2014).
Still, Cocos Island is touted as an example of a successful MPA, and it is known by divers as one of the best locations in the world to view sharks and rays in large numbers (Friedlander et al. 2012; Edgar et al. 2014).What remains unclear, however, is if Cocos Island National Park represents a conservation success or merely a paper park (Rife et al. 2013)
We recommend that monitoring and enforcement of Costa Rica’s MPAs be increased substantially and that international environmental NGOs and foundations contribute to these efforts. Such efforts are urgently required if Cocos Island is to recover its elasmobranch populations and claim its status as a truly successful MPA.
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