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.

Genomics versus mtDNA for resolving stock structure in the silky shark (Carcharhinus falciformis)

Conservation genetic approaches for elasmobranchs have focused on regions of the mitochondrial genome or a handful of nuclear microsatellites. High-throughput sequencing offers a powerful alternative for examining population structure using many loci distributed across the nuclear and mitochondrial genomes. These single nucleotide polymorphisms are expected to provide finer scale and more accurate population level data; however, there have been few genomic studies applied to elasmobranch species. The desire to apply next-generation sequencing approaches is often tempered by the costs, which can be offset by pooling specimens prior to sequencing (pool-seq). In this study, we assess the utility of pool-seq by applying this method to the same individual silky sharks, Carcharhinus falciformis, previously surveyed with the mtDNA control region in the Atlantic and Indian Oceans. Pool-seq methods were able to recover the entire mitochondrial genome as well as thousands of nuclear markers. This volume of sequence data enabled the detection of population structure between regions of the Atlantic Ocean populations, undetected in the previous study (inter-Atlantic mitochondrial SNPs FST values comparison ranging from 0.029 to 0.135 and nuclear SNPs from 0.015 to 0.025). Our results reinforce the conclusion that sampling the mitochondrial control region alone may fail to detect fine-scale population structure, and additional sampling across the genome may increase resolution for some species. Additionally, this study shows that the costs of analyzing 4,988 loci using pool-seq methods are equivalent to the standard Sanger-sequenced markers and become less expensive when large numbers of individuals (>300) are analyzed.