A first assessment of the distribution and abundance of large pelagic species at Cocos Ridge seamounts (Eastern Tropical Pacific) using drifting pelagic baited remote cameras

Understanding the link between seamounts and large pelagic species (LPS) may provide important insights for the conservation of these species in open water ecosystems. The seamounts along the Cocos Ridge in the Eastern Tropical Pacific (ETP) ocean are thought to be ecologically important aggregation sites for LPS when moving between Cocos Island (Costa Rica) and Galapagos Islands (Ecuador). However, to date, research efforts to quantify the abundance and distribution patterns of LPS beyond the borders of these two oceanic Marine Protected Areas (MPAs) have been limited. This study used drifting-pelagic baited remote underwater video stations (BRUVS) to investigate the distribution and relative abundance of LPS at Cocos Ridge seamounts. Our drifting-pelagic BRUVS recorded a total of 21 species including elasmobranchs, small and large teleosts, dolphins and one sea turtle; of which four species are currently threatened. Depth of seamount summit was the most significant driver for LPS richness and abundance which were significantly higher at shallow seamounts (< 400 m) compared to deeper ones (> 400m). Distance to nearest MPA was also a significant predictor for LPS abundance, which increased at increasing distances from the nearest MPA. Our results suggest that the Cocos Ridge seamounts, specifically Paramount and West Cocos which had the highest LPS richness and abundance, are important aggregation sites for LPS in the ETP. However, further research is still needed to demonstrate a positive association between LPS and Cocos Ridge seamounts. Our findings showed that drifting pelagic BRUVS are an effective tool to survey LPS in fully pelagic ecosystems of the ETP. This study represents the first step towards the standardization of this technique throughout the region.


131
Nine seamounts along the Cocos Ridge were surveyed in April 2018 (Fig 1).
132 Seamount selection was based on the depth of each seamount summit and available time in 133 the field. We prioritized the shallowest seamounts we could detect in the study area to 134 increase the probability of LPS detection rates in subsurface waters (10 and 25 m deep) 135 (Table 1). The depth of each seamount was previously researched using the Seamount

316
The number of LPS ranged from 1 species at NW Darwin to 8 species at West Cocos 317 ( Fig 4A; S1 Table). Elasmobranchs were detected at all seamounts except for Medina 1 318 and NW Darwin, whereas large teleosts were not detected at Paramount and Las Gemelas 329 single deployment, respectively (S1 Table). Few individuals of S. lewini were also detected 330 in Medina 3 and Gemelas 2 (S1 Table). Large teleosts were more abundant at Las Gemelas 331 1 and Medina 1 (Fig 4B), where our BRUVS reported schools of 27 and 28 individuals of 332 C. hippurus in a single deployment, respectively (S1 Table). However, the highest 333 abundance of tunas (Tunnus albacares) and billfishes (Istiompax indica, Istiophorus 334 platypterus and Kajikia audax) was reported at West Cocos (S1 Table). Other marine 335 megafauna species were more abundant at West Cocos and Las Gemelas 1 (Fig 4B) where 336 our BRUVS recorded groups of up to 6 and 10 T. truncatus, respectively (S1 Table). The 337 black sea turtle Chelonia mydas agassizi and T. truncatus were observed only at seamounts 20 338 close to Cocos Island (S1 Table).

348
Differences between morning and afternoon deployments were not significant either   440 Our results can be used as a first approach to guide future studies using drifting-pelagic 441 BRUVS in the region.

442
Species accumulation curves in this study indicate that with higher survey effort 443 more species from all groups would be detected. Survey effort in open-water ecosystems is 444 expected to be higher than in demersal habitats due to lower densities of organisms [33].
445 However, our results reveal a variation at a group level in species accumulation rates that is 465 Differences in species accumulation rates among groups may be related to differences in 466 species richness by group, the specific responses of each group to the different BRUVS 467 stimuli (olfactory, visual or auditory) and to the schooling behavior of each group [40].
468 Therefore, we recommend considering such differences when selecting optimal survey 469 effort using drifting-pelagic BRUVS.