Determining the role of environmental covariates on planktivorous elasmobranch population trends within an isolated marine protected area

Abstract

Studies have found predictable relationships between the behavior of planktivores and environmental conditions, suggesting that they may be especially sensitive to environmental change. Most studies to date, however, are based on limited observations, include few of the environmental covariates that influence planktivore behavior, and often do not occur over long enough durations to determine the effects of environmental change. As such, exploration of long-term data sets on planktivores is necessary to disentangle the potential impacts of oceanographic and environmental variability. To elucidate the relationship between 3 groups of planktivorous elasmobranchs (whale sharks Rhincodon typus, mobula rays Mobula spp., and manta rays Manta birostris) and environmental variability, we employ hierarchical modeling to examine trends in elasmobranch counts from underwater visual censuses (UVCs), conducted over the last 27 yr (1993-2019) by divemasters from Cocos Island, Costa Rica. Data from UVCs were integrated with primarily remotely sensed environmental and oceanographic data. Models revealed that for planktivorous elasmobranchs, environmental variables (chlorophyll a, lunar cycle, and salinity) are correlated with occurrence and predicted relative abundances. In lunar phases of lower illuminations, there were significant predicted increases in relative abundances of mobula rays, increased chlorophyll a decreased whale shark relative abundance, and increases in salinity correlated with increases in mobula relative abundance but did not correlate with increased observations of mantas or whale sharks. Our findings highlight the need to consider environmental conditions when evaluating the efficacy of marine protection and demonstrate the potential ranging effects of environmental variability on planktivores at Cocos.