Bull sharks (Carcharhinus leucas) are globally distributed top predators that play an important ecological role within coastal marine communities. However, little is known about how the spatial and temporal scales of their habitat use are associated with their ecological role. In this study, a population of sub-adult and adult bull sharks were investigated within a remote subtropical marine community in the southwest Indian Ocean off the coast of southern Mozambique. The main objectives of the study were to test a minimally invasive remote biopsy sampling method; to investigate the feeding ecology of bull sharks; and to investigate the temporal and spatial scales of bull shark residency patterns and migration dynamics. Biopsy tests on free-swimming bull sharks showed that the devised sampling technique provided a minimally invasive and consistent method (biopsy retention rate = 87%) to obtain muscle tissue samples sufficiently large enough (310±78mg, mean ± SD) for stable isotope analysis. Results from the stable isotope analysis showed that adult bull sharks appeared to exhibit a shift towards consistentlyhigher trophic level prey from an expanded foraging range compared to sub-adults, possibly due to increased mobility linked with size. Additionally, bull sharks had significantly broader niche widths compared to top predatory teleost assemblages with a wide and enriched range of δ13C values relative to the local marine community, suggesting that they forage over broad spatial scales along the east coast of southern Africa. Results from the passive acoustic telemetry investigation, conducted over a period between 10 and 22 months, supported these findings showing that the majority of tagged adult sharksexhibited temporally and spatially variable residency patterns interspersed with migration events. Ten individuals undertook coastal migrations that ranged between 433 and 709km (mean = 533km) with eight of these sharks returning to the study site. During migration, individuals exhibited rates of movement between 2 and 59km.d-1 (mean = 17.58km.d-1) and were recorded travelling annual distances of between 450 and 3760km (mean = 1163km). These findings suggest that adult bull sharks are not the sedentary species once thought to be but rather, consistently move over broad spatial scales on the east coast of southern Africa and play an important predatory role shaping and linking ecological processes within the southwest Indian Ocean.

The bull shark, Carcharhinus leucas, is an important euryhaline predator that is exploited throughout its range for meat and fins. This species is taxonomically paired with the pigeye shark, C. amboinensis. Validity of this group has not been directly tested using molecular markers. Genetic variation and phylogeny was examined with concatenated datasets of mitochondrial genes (cytb, cox1 and 12stRNA16s) for nineteen species, from three families within the order Carcharhiniformes. Whilst there were indications of species pairings within the genus, there was no evidence to suggest that the bull and pigeye sharks should be considered a species group. Phylogenetic analysis failed to resolve Carcharhinus, but confirmed Negaprion as a sister taxon and placed Prionace glauca within Carcharhinus. It remains unclear if the family Sphyrnidae, arose from a Rhizoprionodon or Scoliodon ancestor and future revision of this complex group of sharks is required. Like other large carcharhinid species, C. leucas exhibits a low reproductive rate and long generation times. It is susceptible to localised depletions and such declines have been documented. The global stock structure was assessed by analysing the mitochondrial control region in 245 individuals sampled from eight populations in three ocean basins. The bull shark exhibits relatively high haplotype diversity (0.896 ± 0.010) when compared to other globally-distributed sharks and the nucleotide diversity was similar to others from the genus (0.00465 ± 0.00014). There was significant stock structure found among populations (ΦST=0.736, p<0.00001) and among ocean basins (ΦCT=0.527, p=0.00653). Geneflow between the US Atlantic coastline and Gulf of Mexico was sufficient to consider the area to be a single panmictic population. Coalescent analyses suggest an Indian Ocean origin with population divergences associated with warm interglacials and increased habitat with drop in sea level during the recent Wisconsin glaciation. Seven discrete stock management units for bull sharks are proposed.

This study explored spatiotemporal patterns in movement, diet, and baseline phycotoxin concentrations in immature bull sharks (Carcharhinus leucas) of the Indian River Lagoon (IRL), an estuary of national significance that has been considerably impacted by multiple toxic harmful algal blooms (HABs). Long-term spatial use of the system was assessed for 29 acoustically tagged sharks over a 4 year period (2017–2020). Tissue samples for diet and toxin analysis were collected from a separate cohort of 50 individuals between 2018 and 2020. UPLC-MS/MS was used to screen tissues for 14 algal toxins. Young bull sharks were found to be mainly piscivorous and displayed high residency to the IRL as well as to specific regions of the IRL, with small activity spaces. Multiple phycotoxins were detected in screened tissues, indicating that young bull sharks in the IRL may be compromised by trophic transfer of HABs while they reside in this important nursery. / Includes bibliography. / Thesis (M.S.)--Florida Atlantic University, 2021. / FAU Electronic Theses and Dissertations Collection

Cette thèse porte sur deux espèces de grands requins, le requin bouledogue Carcharhinus leucas et le requin tigre Galeocerdo cuvier. Les objectifs sont d'étudier la structure génétique de leurs populations, la taille efficace des populations identifiées et les modes de reproduction de ces deux espèces. Une différenciation génétique importante a été identifiée entre les populations de requin bouledogue de l'Ouest de l'océan Indien et de l'Ouest du Pacifique, reflétant soit une absence de flux de gènes contemporains, soit des flux de gènes uniquement assurés par les mâles. À l'inverse, les populations de requin tigre de ces deux régions sont homogènes génétiquement. Une plus faible diversité génétique a été identifiée chez le requin tigre que chez le requin bouledogue, peut-être liée à une diminution forte des effectifs datant de moins de 3 000 ans. Autour de La Réunion, les populations des requins bouledogue et tigre suivent des dynamiques différentes, liées à leurs modes de reproduction. Chez le requin bouledogue, les individus semblent fidèles à des zones côtières particulières (philopatrie) pour s'accoupler et/ou mettre bas, et les portées sont fréquemment issues de plusieurs pères (polyandrie). À l'inverse, les zones d'accouplement et de mise bas du requin tigre restent mal connues, et cette espèce semble exclusivement monoandre, caractéristiques liées à sa nature semi-océanique. Ces travaux montrent les capacités de dispersion importantes de ces deux espèces. Leurs populations présentent des dynamiques différentes induisant une vulnérabilité différente aux pressions anthropiques. / This PhD thesis focuses on two large shark species, the bull shark Carcharhinus leucas and the tiger shark Galeocerdo cuvier. The aims are to study the genetic structuring of their populations, the effective population size of the delimited populations and the reproductive modes of both species. A strong genetic differentiation was highlighted between bull shark populations from the Western Indian Ocean and the Western Pacific, due to either an absence of contemporary gene flow or to an absence of female gene flow only. On the opposite, tiger shark populations seem genetically homogenous, with important genetic connectivity between both regions. Within each region, no genetic differentiation among localities was highlighted for both species. A weaker genetic diversity was identified for the tiger shark, probably linked to the occurrence of a recent bottleneck occurring less than 3,000 years ago. Around Reunion Island, bull and tiger shark populations present different dynamics, linked to their reproductive modes. Bull shark individuals from both sexes seem to exhibit some fidelity to specific coastal sites (philopatry) to mate and/or deliver embryos, and litters are frequently issued from several fathers (polyandry). On the opposite, mating and pupping areas of the tiger shark remain poorly known, and this species seems exclusively monoandrous, probably linked to its semi-oceanic nature.This work highlights the high dispersal abilities of both species. Their populations present different dynamics, leading to different sensitivities to anthropogenic pressures. These results point out the need to adopt management plans specific to each species

Post-pelagic juvenile and subadult Kemp's ridley sea turtles (Lepidochelys kempii) (20-40 cm straight carapace length) utilize nearshore waters of the northwestern Gulf of Mexico as nursery or developmental feeding grounds. This study utilizes 10 years of entanglement netting data to characterize long-term abundance and distribution of Kemp's ridley sea turtles at index habitats in this region. Netting surveys were conducted during April-October 1993-2002, primarily at Sabine Pass, Texas and Calcasieu Pass, Louisiana. Additionally, this study takes an ecosystem-based approach to understanding factors influencing Kemp's ridley in-water abundance and distribution via the development of a conceptual model incorporating data on nesting dynamics, environmental conditions, prey availability, and predation pressure. Overall monthly mean ridley catch-per-unit-effort (CPUE) peaked in the beginning of summer (April-June), probably in response to rising water temperatures and seasonal occurrence of blue crab prey. Annual mean ridley CPUE across all study areas peaked in 1994, 1997, 1999 and 2002, suggesting a 2-3 year cycle in abundance that may be related to patterns in clutch size or hatch success at the Rancho Nuevo, Mexico nesting beach. However, ridley CPUE in nearshore waters remained relatively constant or decreased slightly even as number of hatchlings released from Rancho Nuevo increased exponentially. Annual declines in Texas strandings since 1994 and subsequent increases in Florida counterparts since 1995 suggest a shift in ridley distribution from the western to eastern Gulf in recent years. Significant declines in ridley CPUE at Sabine Pass since 1997 coincided with a concurrent reduction in blue crab size, but a similar trend was not detected at Calcasieu Pass. Kemp's ridley occurrence at study sites was not significantly related to shrimping activity/by-catch. There also were no biologically significant relationships between Kemp's ridley CPUE and abiotic factors, nor were ridleys deterred from utilizing areas frequented by bull sharks. Overall, nesting dynamics and prey availability were conceptual model components appearing to have the greatest influence on nearshore ridley occurrence.