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

Aims: This study was to establish the feeding habits of silky shark Carcharhinus falciformis in waters off Côte d’Ivoire. Study design: All stomachs of the specimens were collected using a standard method and analysed at laboratory. Place and Duration of Study: The study of the diet of C. falciformis was carried out at the laboratory of the Oceanological Research Centre (ORC) between April 2019 and August 2020. Methodology: To carry out this study, measurements and extractions of the stomachs of silky sharks were performed using a tape measure and dissection kit. Diet was described using the Index of Relative Importance (%IRI). The food overlap between sexes, size groups and marine seasons were analysed using the Morisita index. Results: A total of 265 specimens ranging in size from 79 to 254 cm total length were used. Of the stomachs examined, 163 individuals had food contents (61.51%), whilst 102 were empty (38.49%). The results indicated that silky shark has epipelagic and mesobathypelagic feeding habits, preying upon a wide number of prey taxa (33 species) but with a diet dominated by teleost fishes, especially Thunnus albacares (60.62%IRI) and Katsuwonus pelamis (33.18%IRI). The Morisita index and the Spearman rank correlation coefficient showed similarity of the diet between sexes (Cλ > 0.60; Rs = 0.776 and p = 0.001 and marine seasons (Cλ > 0.60; Rs = 0.658 and p = 0.001). The juveniles and adults did not show any overlap. The Amundsen graphical (Fig. 3) analysis suggested that silky shark displays specialist feeding behaviour in this area. Conclusion: Diet consisted mainly of T. albacares and K. pelamis, the proportions of which varied according to size and marine seasons. Carcharhinus falciformis is considered to be a specialist predator.

Monitoring of the status of eleven bony and cartilaginous fish populations in the Malpelo Sanctuary of Fauna and Flora, Colombian Pacific

To determine trends over time in the relative abundance of species prioritized in the Malpelo Fauna and Flroa Sanctuary Management Plan, underwater visual censuses were done from April 2009 to August 2019. Data were collected for five species of bonefish: bluefin trevally (Caranx melampygus), longfin yellowtail (Seriola rivoliana), yellowfin tuna (Thunnus albacares), leather bass (Dermatolepis dermatolepis) and sailfin grouper (Mycteroperca olfax), and six cartilaginous fish: spotted eagle ray (Aetobatus laticeps), silky shark (Carcharhinus falciformis), Galapagos shark (Carcharhinus galapagensis), whitetip reef shark (Triaenodon obesus), whale shark (Rhincodon typus) and scalloped hammerhead shark (Sphyrna lewini). A negative non-significant trend in abundance through time was observed for most of the evaluated species, except A. laticeps and T. obesus, which showed positive non-significant trends. The hammerhead shark (S. lewini) was the only species that showed a statistically significant decrease in abundance over time. The results of this study highlight the urgent need to establish more effective fisheries management and regulation measures that promote the conservation of important fish species, not just inside the sanctuary but also within the Tropical Eastern Pacific.

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

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.