Thesis (Ph. D.)--Simon Fraser University, 2001. / Includes bibliographical references.

Thesis (Ph. D.)--Rhodes University, 2006. / Title from PDF t.p. (viewed on Apr. 12, 2007). Includes bibliographical references (p. 184-207).

Thesis (Ph.D. (Ichthyology and Fisheries Science))--Rhodes University, 2006.

Habitat use of mesophotic coral reefs by sharks is largely unknown. However, it is well established that mesophotic reefs are the site of spawning aggregations for many species of teleost fish. These aggregations represent seasonal concentrations of potential prey biomass that may influence the habitat use of predatory species such as large sharks. I employed acoustic monitoring to examine the movements of three shark species [lemon shark (Negaprion brevirostris), tiger shark (Galeocerdo cuvier), and Caribbean reef shark (Carcharhinus perezi)] to determine 1) the comparative spatiotemporal patterns of mesophotic reef habitat use by the three shark species and 2) the spatiotemporal relationship between these sharks and grouper spawning aggregations at a fish spawning aggregation (FSA) site (Hind Bank and Grammanik Bank) along the southern reef shelf edge off St. Thomas, United States Virgin Islands (USVI). Tiger and lemon sharks were detected across nearly the entire acoustic array, which spanned ~ 1060 km2. When present, Caribbean reef sharks used a much smaller activity space, composed exclusively of mesophotic reef habitat located within FSA sites. Individuals from all three species were typically detected for stretches of several consecutive days, while periods without detections usually lasted less than one week. Lemon sharks were present at the FSA site more often during the grouper spawning season (Dec-May) than the non-spawning season (Jun-Nov), but showed no preference toward specific areas within the FSA site, which varied by location and grouper species composition. In contrast, there was no relationship between the presence of tiger and Caribbean reef sharks at the FSA site and the grouper spawning season. My results suggest that despite different habitat use 5 patterns and varying degrees of fidelity, this mesophotic reef serves as an important habitat to all three shark species.

This thesis develops the first open population model for any shark species worldwide using the Cormack-Jolly-Seber (CJS) model. In conjunction with a tagging study, five auxiliary studies were conducted to investigate stock structure, post-release mortality, tag shedding, and tag-reporting rates. The results from each of the studies were used to correct for any violations of the models assumptions to provide the first unbiased estimates of survival and abundance for the raggedtooth shark (Carcharias taurus) in South Africa. The C. taurus population exhibited complex stock structuring, by size and sex. Competitive shore anglers fished an estimated 37, 820 fishing days.year⁻¹ (95% C.I. = 28, 281 - 47, 359 days.year⁻¹) for sharks, and caught 1764 (95% C.I. = 321 – 3207) C. taurus. Although released alive, post-release mortality ranged from 3.85% for young-of-the-year sharks to 18.46% for adult sharks. Between 1984 and 2004, a total of 3471 C. taurus were tagged. In all, 302-tagged sharks (8.7%) were recaptured. Both juvenile (< 1.8 m TL) and adult sharks (> 1.8 m TL) displayed philopatric behaviour for specific parts of their ranges, including gestating and parturition areas. Significant differences were observed in the percentage of recaptures between the different tag types, tagging programs, individual taggers and capture methods used to tag sharks. The annual tag retention rate for juvenile sharks, 94.19% (95% C.I. = 80.68% - 100.00%) was significantly higher than for adult sharks, estimated at 29.00% (95% C.I. = 6.76% - 64.39%). Tag reporting rates, from fishermen varied both spatially and temporally from 0.28 (95% C.I. = 0.00 – 0.63) to 0.77 (95% C.I. = 0.56 – 0.97). Associated tag wound damage and biofouling growth indicated that B-type tags were a suitable tag type for use on C. taurus, whereas C-type tags were not. The CJS bias-adjusted estimate for juvenile survival was 0.456 (95% C.I. = 0.367 – 0.516) and for adult sharks, 0.865 (95% C.I. = 0.795 – 0.915). From 1984 to 2004 the mean bias-adjusted population size for juvenile sharks was estimated at 3506 (95% C.I. = 2433 – 4350) and for adult sharks, 5899 (95% C.I. = 7216 – 11904). Trends in abundance over the 20-year study period indicated a stable, healthy population.

Although tiger sharks are an important apex predator in many ecosystems, little is known about their reproduction. The goal of this study was to determine the size-at-maturity and the reproductive seasonality of tiger sharks in the western Atlantic Ocean. This was achieved using a combination of ultrasonography and measurements of plasma hormone concentrations; in particular, testosterone for males and estradiol and progesterone for females. Steroid hormone concentrations were measured using chemiluminescent assays (CLIA). Maturity was also examined through histology of reproductive organs in females and clasper calcification in males. Females were found to mature between 270 and 310 cm total length and males were found to mature between 260 and 300 cm total length. Mating was determined to occur in October/November, based on the presence of mating wounds on females and increased concentrations of testosterone in males. Some females were shown to exhibit increased plasma estradiol concentrations also during October/November; however, we do not believe that ovulation takes place until May or June based on ultrasonography data. This suggests a period of sperm storage although histological examination of the oviducal gland was not able to confirm this. Ultrasonography data, showing increasing embryo size over the course of a year, and data on minimum size of tiger sharks caught in longline surveys suggested that parturition occurs between June and September with pups being born as small as 56 cm fork length. The findings from this study show that some tiger sharks reach reproductive maturity at sizes smaller than what has been previously suggested. Additionally, the possibility of tiger sharks storing sperm suggests that their reproductive cycle is a minimum of two years long and could be up to three years in duration. This information is important for management of the species in the future. Additionally, this study adds to the limited knowledge about reproduction of elasmobranchs and how patterns of reproductive steroids can correlate with different reproductive events.

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

Habitat use of mesophotic coral reefs by sharks is largely unknown. However, it is well established that mesophotic reefs are the site of spawning aggregations for many species of teleost fish. These aggregations represent seasonal concentrations of potential prey biomass that may influence the habitat use of predatory species such as large sharks. I employed acoustic monitoring to examine the movements of three shark species lemon shark (Negaprion brevirostris), tiger shark (Galeocerdo cuvier), and Caribbean reef shark (Carcharhinus perezi)] to determine 1) the comparative spatiotemporal patterns of mesophotic reef habitat use by the three shark species and 2) the spatiotemporal relationship between these sharks and grouper spawning aggregations at a fish spawning aggregation (FSA) site (Hind Bank and Grammanik Bank) along the southern reef shelf edge off St. Thomas, United States Virgin Islands (USVI). Tiger and lemon sharks were detected across nearly the entire acoustic array, which spanned ~ 1060 km2. When present, Caribbean reef sharks used a much smaller activity space, composed exclusively of mesophotic reef habitat located within FSA sites. Individuals from all three species were typically detected for stretches of several consecutive days, while periods without detections usually lasted less than one week. Lemon sharks were present at the FSA site more often during the grouper spawning season (Dec-May) than the non-spawning season (Jun-Nov), but showed no preference toward specific areas within the FSA site, which varied by location and grouper species composition. In contrast, there was no relationship between the presence of tiger and Caribbean reef sharks at the FSA site and the grouper spawning season. My results suggest that despite different habitat use patterns and varying degrees of fidelity, this mesophotic reef serves as an important habitat to all three shark species.

As climate change continues, climactic extremes are predicted to become more frequent and intense, in some cases resulting in dramatic changes to ecosystems. The effects of climate change on ecosystems will be mediated, in part, by biotic interactions in those ecosystems. However, there is still considerable uncertainty about where and how such biotic interactions will be important in the context of ecosystem disturbance and climactic extremes. Here, I review the role of consumers in seagrass ecosystems and investigate the ecological impacts of an extreme climactic event (marine heat wave) and subsequent widespread seagrass die-off in Shark Bay, Western Australia. Specifically, I compare seagrass cover, shark catch rates, and encounter rates of air breathing fauna in multiple habitat types before and after the seagrass die-off to describe post-disturbance dynamics of the seagrass community, shifts in consumer abundances, and changes in risk-sensitive habitat use patterns by a variety of mesoconsumers at risk of predation from tiger sharks (Galeocerdo cuvier). Finally, I conducted a 16 month field experiment to assess whether xi loss of top predators, and predicted shifts in dugong foraging, could destabilize remaining seagrass. I found that the previously dominant temperate seagrass Amphibolis antarctica is stable, but not increasing. Conversely, an early-successional tropical seagrass, Halodule uninervis, is expanding. Following the die-off, the densities of several consumer species (cormorants, green turtles, sea snakes, and dugongs) declined, while others (Indo-Pacific bottlenose dolphins, loggerhead sea turtles, tiger sharks) remained stable. Stable tiger shark abundances following the seagrass die-off suggest that the seascape of fear remains intact in this system. However, several consumers (dolphins, cormorants) began to use dangerous but profitable seagrass banks more often following seagrass decline, suggesting a relaxation of anti-predator behavior. Experimental results suggest that a loss of tiger sharks would result in a behaviorally mediated trophic cascade (BMTC) in degraded seagrass beds, further destabilizing them and potentially resulting in a phase shift. My work shows that climactic extremes can have strong but variable impacts on ecosystems mediated in part by species identity, and that maintenance of top predator populations may by important to ecological resilience in the face of climate change.