Behavior and Ecology of Silky Sharks Around the Chagos Archipelago and Evidence of Indian Ocean Wide Movement

Abstract

Silky sharks (Carcharhinus falciformis) represent a major component of global shark catch, both directly and as bycatch, and populations are declining as a result. An improved understanding of their movement ecology is needed to support conservation efforts. We deployed satellite and acoustic tags (2013–2018) and analyzed historical fisheries records (1997–2009), to investigate the spatial ecology of silky sharks in the central Indian Ocean and a large Marine Protected Area (MPA; 640,000 km2) around the Chagos Archipelago. We observed high fidelity to the MPA, and a sustained diurnal association with a seamount complex, with individuals moving off at night and returning at sunrise. Yet, we also observed large-scale divergent movements in two satellite tagged individuals and documented the furthest recorded displacement distance for a satellite tagged silky shark to date, with one individual moving from the MPA to the Kenyan coast—a displacement distance of 3,549 km (track distance ∼4,782 km). Silky sharks undertook diel vertical migrations and oscillatory diving behavior, spending > 99% of their time in the top 100 m, and diving to depths of greater than 300 m, overlapping directly with typical deployments of purse seine and longline sets in the Indian Ocean. One individual was recorded to a depth of 1,112 m, the deepest recorded silky shark dive to date. Individuals spent 96% of their time at liberty within water temperatures between 24 and 30°C. Historic fisheries data revealed that silky sharks were a major component of the shark community around the archipelago, representing 13.69% of all sharks caught by longlines before the fishery closed in 2010. Over half (55.88%) of all individuals caught by longlines and purse seiners were juveniles. The large proportion of juveniles, coupled with the high site fidelity and residence observed in some individuals, suggests that the MPA could provide considerable conservation benefits for silky sharks, particularly during early life-history stages. However, their high mobility potential necessitates that large MPAs need to be considered in conjunction with fisheries regulations and conservation measures in adjacent EEZs and in areas beyond national jurisdiction.

Understanding Persistent Non-compliance in a Remote, Large-Scale Marine Protected Area

“…This continued exploitation diminishes the ability of the MPA to attain intended conservation benefits, including providing vital refuge for shark species that are increasingly threatened with extinction (Davidson and Dulvy, 2017;Letessier et al, 2019). Previous research has shown that even short non-compliance events threaten local populations of sharks within the MPA (Tickler et al, 2019) and the concentration of non-compliance in shallow reef areas likely greatly reduces the protection afforded by the MPA for vital early life-history stages and reef-associated species (Curnick et al, 2020;Jacoby et al, 2020). The impact on local populations will be further exacerbated for multiple species we identified in illegal catches that exhibit high site fidelity, including silky sharks (Carlisle et al, 2019;Curnick et al, 2020).…”

Reviewing the Ecosystem Services, Societal Goods, and Benefits of Marine Protected Areas

“…The protection of groupers has also been verified in the Llevant de Majorca-Cala Rajada, Columbretes islands or Cabo de Palos MPAs (Hackradt et al, 2014; Table 2). In the case of silky sharks (Carcharhinus falciformis) in the Indian Ocean, despite their high mobility, the large proportion of juveniles, coupled with the high site fidelity observed in some individuals, in agreement with historical fishery data, suggests that MPAs could provide considerable conservation benefits for the species, particularly during early life history stages (Curnick et al, 2020).…”

Diet of the Silky Shark Carcharhinus falciformis (Müller & Henle, 1839) in Waters off Côte d’Ivoire (West Africa)

“…L. vulgaris and T. danae), which are characterized by being distributed at 0-500 m and 385-395 m depth, providing further evidence that this species undertakes vertical migrations in the water column. [37] reported that silky shark dived to depths greater than 300 m, with a maximum recorded depth of 1112 m.…”