Shifting elasmobranch community assemblage at Cocos Island—an isolated marine protected area

Abstract

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. We examined data collected by a small group of divers over the past 21 years at one of the world's oldest marine protected areas (MPAs), Cocos Island National Park, Costa Rica. We used mixed effects models to determine trends in relative abundance, or probability of occurrence, of 12 monitored elasmobranch species while accounting for variation among observers and from abiotic factors. 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%), 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. In addition, probability of occurrence at Cocos Island of tiger (Galeocerdo cuvier), Galapagos (Carcharhinus galapagensis), blacktip (Carcharhinus limbatus), and whale (Rhincodon typus) sharks increased significantly. The effectiveness of MPAs cannot be evaluated by examining single species because population responses can vary depending on life history traits and vulnerability to fishing pressure.

Trophic overlap in mobulid rays: insights from stable isotope analysis

“…Both bycatch and targeted fisheries appear to contribute to global declines in mobulid abundance (Ward-Paige et al 2013, White et al 2015, Croll et al 2016. Targeted fisheries can be managed with legislation banning the capture of mobulids, but bycatch remains a more challenging and persistent threat due to the ubiquity of mobulid bycatch in artisanal and commercial fisheries of all types (Croll et al 2016).…”

“…While these growing targeted fisheries have catalyzed focused conservation and scientific attention for mobulids (Ward-Paige et al 2013, White et al 2015, bycatch of these species likely impacted populations long before large-scale targeted fisheries began. Mobulid rays are vulnerable to incidental capture in gill nets, purse seines, trawls, and even long lines (Croll et al 2016).…”

Ecosistemas coralinos del Parque Nacional Isla del Coco, Costa Rica: estructura y comparación 1987-2014

“…Esto se ve reflejado en la estructura trófica de la isla, que muestra una pirámide invertida, con una predominancia de altas biomasas de depredadores tope en los arrecifes que ronda las 1.2±8.5 tn ha -1 , como lo ha demostrado este estudio, y que puede llegar hasta 6 tn ha -1 al muestrear sitios más oceánicos (Friedlander et al, 2012). Estas cantidades de tiburones y otros peces grandes son comunes en ecosistemas marinos con poca afectación antropogénica, y estos grupos constituyen presiones selectivas que mantienen los sistemas productivos y cadenas tróficas equilibradas (Friedlander et al, 2012;Espinoza, Cappo, Heupel, Tobin, & Simpfendorfer, 2014;White, Myers, Mills-Flemming, & Baum, 2015). Además, se ha determinado que la cobertura de coral y la cercanía con arrecifes también son factores que favorecen la riqueza y abundancia de tiburones, inclusive de especies que no son residentes en sistemas arrecifales, por lo cual la protección de los ecosistemas arrecifales tiene importantes efectos en las comunidades de peces, aún si son especies altamente móviles (Espinoza et al, 2014).…”

“…Una de ellas, el tiburón tigre, hasta hace pocos años empezó a ser más común. En contraste, especies como el tiburón piloto que tradicionalmente ha sido común ya casi no se observa y las densidades son bajas (Sibaja-Cordero, 2008;Friedlander et al, 2012;White et al, 2015).…”

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“…Given the 21 ecological value and the economic importance of Darwin and Wolf for the dive tourism industry, 22 the current protection should ensure the long-term conservation of this hotspot of unique global 23 value. 34 Overfishing has reduced biomass of most sharks and other large predatory fishes worldwide by 35 over 90% (Baum et al, 2003;Myers & Worm, 2003;Ward-Paige et al, 2010), and even remote 36 locations have been severely impacted (Dulvy et al, 2008;Sibaja-Cordero, 2008; Graham, 37 Spalding & Sheppard, 2010;White et al, 2015). One in four species of cartilaginous fishes is 38 now threatened with extinction due primarily to overexploitation and habitat loss (Dulvy et al, 39 2014).…”