Unveiling The Secrets Of Planktivorous Sharks

Recent studies have illuminated predictable relationships between these majestic creatures and their surroundings, hinting at their heightened sensitivity to environmental changes. However, the existing body of research is limited in scope, often lacking comprehensive observations, essential environmental covariates, and the temporal depth required to fully grasp the effects of environmental shifts. To unravel the mysteries of planktivorous elasmobranchs, researchers delved into a treasure trove of data spanning 27 years (1993–2019) from underwater visual censuses (UVCs) conducted by divemasters at Cocos Island, Costa Rica.

Situated approximately 550 kilometers off the Pacific coast of Costa Rica, Cocos Island stands as an uninhabited gem in the vast ocean expanse. Celebrated for its remarkable biodiversity and unspoiled marine landscapes, it has earned a reputation as an ecological haven. The encompassing waters of Cocos form a marine protected area, holding the prestigious title of the world’s oldest Marine Protected Area (MPA). Notably, Cocos Island has become a sought-after destination for diving enthusiasts, particularly renowned for live-aboard shark dives that offer a thrilling and immersive experience in its pristine underwater realm. “We were given access to data from Undersea Hunter, a dive company that regularly dives at Cocos. Essentially, after each dive, trained dive masters recorded the number of whale sharks, mantas, and mobulas that they observed! In the end, we had data from a total of 35,706 at 17 sites around Cocos,” explains lead author PhD student Julia Saltzman Florida State University. “To understand how these species are affected by environmental variation, we then integrated this data with open-source environmental data, most of which is remotely sensed through satellites. This open-source data gave us the ability to have salinity, sea surface temperature, lunar phase, and more associated with each of the conducted dives.” Saltzman went on to explain that the study fills a gap in our understanding of how the abundance of planktivores is influenced by environmental variation on a long-term scale since their abundance in past studies has been related to food availability and environmental conditions. They particularly focused on three groups: whale sharks (Rhincodon typus), mobula rays (Mobula spp.), and manta rays (Manta birostris).

The models uncovered compelling correlations between environmental variables and the occurrence and predicted relative abundances of planktivorous elasmobranchs. Notably, chlorophyll a levels, the lunar cycle, and salinity emerged as key influencers. “We found that lunar phases with lower illuminations coincide with predicted increases in mobula ray abundance, while increased chlorophyll a is associated with decreased whale shark relative abundance, and salinity variations correlate with changes in mobula ray relative abundance, but not with observations of mantas or whale sharks,” says Saltzman. “This is interesting, because these three different planktivores, despite having similar foraging strategies, are impacted by their environment differently.”

These findings, Saltzman presses, underscore the critical importance of considering environmental conditions when evaluating the efficacy of marine protection efforts. “We showed that when we do not account for environmental variation in ecological models, our estimate of how abundances change shifts. In other words, we show that when evaluating the efficacy of an MPA, it is important to see how environmental variation affects your predicted abundance. [It’s important for] scientists to consider if the phenomena they are observing are due to environmental variation, effective marine conservation strategies, or both.”

As we strive to protect and preserve our oceans, understanding these dynamic relationships becomes paramount.

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