PS 20-191 - Environmental influences on planktivorous elasmobranchs at Cocos Island, Costa Rica

Background/Question/Methods

Many species of charismatic marine megafauna are globally threatened. Planktivorous elasmobranchs are no exception to this. A key life history trait of planktivorous elasmobranchs is their seasonal aggregations in tropical environments. These aggregations make it difficult to separate population trends from fluctuations. In order to make this separation, we use data obtained over the last 30 years by a small group of divers at Cocos Island, Costa Rica, one of the oldest marine reserves in the world. We expand past work at Cocos by focusing on planktivorous elasmobranchs, to determine whether the observed fluctuations in elasmobranch sightings correspond with environmental variability or if they can be used to better understand long-term population trends. White et al. 2015 found that at Cocos, the odds of occurrence for whale sharks has increased by 4.5% each year and peaks in whale shark presence occurred every 3 years. However, the reasons for this remain unknown. Is this because of environmental conditions and food availability? In other locations, aggregations, abundance, behavior, and movement of planktivorous elasmobranchs is based on predictable food-pulses of zooplankton. In this study, we employ a set of quantitative approaches to determine if this is the case at Cocos.

Results/Conclusions

We used a set of generalized linear mixed models to analyze over 30,000 records of relative abundance or frequency of occurrence for three planktivorous elasmobranchs (whale sharks, manta rays, and mobula spp.) In our models we included a variety of environmental factors as covariates (i.e., sea surface temperature, ocean niño index, lunar phase and illumination, chlorophyll a, salinity). We found that, in general, for planktivorous elasmobranchs, environmental conditions have clear influences on their occurrence and relative abundances. This is significant for long-term monitoring efforts, as it shows environmental conditions should be taken into account when evaluating the efficacy of marine protection. Furthermore, we show that occurrences of planktivorous elasmobranchs correspond with environmental conditions which correspond with predictable food pulses. This is of particular interest as the predictable nature of these pulses could make their protection suitable.