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MEPS prepress abstract   -  DOI:

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

Julia Saltzman*, Easton R. White

*Corresponding author:

ABSTRACT: Several studies have found predictable relationships between the behavior of planktivores and environmental conditions, suggesting they may be especially sensitive to environmental change. Most of the studies to date, however, are based on limited observations and include few of the many environmental covariates which influence planktivore behavior. Furthermore, these studies often do not occur over long enough durations to make inferences about the effects of environmental change. As such, exploration of long-term datasets on planktivores is necessary to disentangle the potential impacts of oceanographic and environmental variability. Here, to elucidate the relationship between three species/groups of planktivorous elasmobranchs (whale sharks Rhincodon typus, mobula spp., and manta rays Manta birostris) and environmental variability, we employ a hierarchical modeling approach to examine trends in counts of elasmobranchs from Underwater Visual Censuses (UVCs). Data from UVCs was integrated with primarily remotely sensed environmental and oceanographic data. UVCs occurred over the last 27 years (1993-2019) by a small group of dive masters at Cocos Island, Costa Rica. These models revealed that for planktivorous elasmobranchs, several environmental variables, such as chlorophyll a, lunar cycle, and salinity are correlated with their occurrence and predicted relative abundances. We found 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 take environmental conditions into account when evaluating the efficacy of marine protection and demonstrate the potential ranging effects of environmental variability on planktivores at Cocos.