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Hussey N.E.,Bangor University | Hussey N.E.,University of Windsor | Dudley S.F.J.,KwaZulu Natal Sharks Board | Dudley S.F.J.,University of KwaZulu - Natal | And 4 more authors.
Canadian Journal of Fisheries and Aquatic Sciences | Year: 2011

Understanding the role of predators is challenging but critical for ecosystem management. For community dynamics, predator-specific size-based variation in diet, trophic position, and habitat use are rarely accounted for. Using two applied tools (stable isotopes and stomach content data), we examined inter- and intra-species ontogenetic variability in diet (stomach contents), trophic position (TP SIA for δ 15N and TP SCA for stomach contents), and habitat use (δ 13C) of two large sharks, the scalloped hammerhead (Sphyrna lewini) and the dusky (Carcharhinus obscurus). Stomach contents identified size-based and gender-specific shifts in diet indicating resource partitioning for and between species. Calculated TP for the two sharks varied by method, either TP SIA or TP SCA and with species, size, and gender, but were complicated by differing baselines and broad functional prey groups, respectively. TP increased with size for S. lewini, but was low in large C. obscurus compared with small sharks. Size-based δ 13C profiles indicated habitat partitioning by sex in S. lewini and a movement to shelf edge foraging in large C. obscurus. These results demonstrate that predators exert proportional size-based effects on multiple components of the marine system that are further complicated by species- and gender-specific strategies.


Olin J.A.,University of Windsor | Olin J.A.,Louisiana State University | Hussey N.E.,University of Windsor | Grgicak-Mannion A.,University of Windsor | And 4 more authors.
PLoS ONE | Year: 2013

The application of stable isotopes to characterize the complexities of a species foraging behavior and trophic relationships is dependent on assumptions of δ15N diet-tissue discrimination factors ({increment}15N). As {increment}15N values have been experimentally shown to vary amongst consumers, tissues and diet composition, resolving appropriate species-specific {increment}15N values can be complex. Given the logistical and ethical challenges of controlled feeding experiments for determining {increment}15N values for large and/or endangered species, our objective was to conduct an assessment of a range of reported {increment}15N values that can hypothetically serve as surrogates for describing the predator-prey relationships of four shark species that feed on prey from different trophic levels (i.e., different mean δ15N dietary values). Overall, the most suitable species-specific {increment}15N values decreased with increasing dietary-δ15N values based on stable isotope Bayesian ellipse overlap estimates of shark and the principal prey functional groups contributing to the diet determined from stomach content analyses. Thus, a single {increment}15N value was not supported for this speciose group of marine predatory fishes. For example, the {increment}15N value of 3.7‰ provided the highest percent overlap between prey and predator isotope ellipses for the bonnethead shark (mean diet δ15N = 9‰) whereas a {increment}15N value < 2.3‰ provided the highest percent overlap between prey and predator isotope ellipses for the white shark (mean diet δ15N = 15‰). These data corroborate the previously reported inverse {increment}15N-dietary δ15N relationship when both isotope ellipses of principal prey functional groups and the broader identified diet of each species were considered supporting the adoption of different {increment}15N values that reflect the predators' δ15N-dietary value. These findings are critical for refining the application of stable isotope modeling approaches as inferences regarding a species' ecological role in their community will be influenced with consequences for conservation and management actions. © 2013 Olin et al.


Atkins S.,Endangered Wildlife Trust | Atkins S.,University of Witwatersrand | Cliff G.,KwaZulu Natal Sharks Board | Cliff G.,University of KwaZulu - Natal | Pillay N.,University of Witwatersrand
Biological Conservation | Year: 2013

Humpback dolphins (Sousa plumbea) in South Africa are classified as Vulnerable and one quantifiable threat is accidental mortality in the shark nets in KwaZulu-Natal. We investigated the spatial, temporal and life history patterns of this bycatch to guide mitigation strategies to decrease humpback dolphin capture. A total of 203 individuals were caught between 1980 and 2009. We analysed patterns of captures in relation to the area of capture (location) and year and month of retrieval from the nets. We also analysed the distribution of the sex and size (body length) of humpback dolphins. Most catches (61%) occurred at Richards Bay in the northern part of their distribution in South Africa. Annual catch rate fluctuated considerably and there was little seasonality. The sex ratio was male-biased (1.55:1) and in particular skewed towards adolescents (56%) which constituted the majority of the catch. We suggest that mitigation strategies be focused at Richards Bay, throughout the year. Of the existing shark net mitigation strategies, changing fishing gear from nets to baited hooks (drumlines) could be useful to decrease humpback dolphin capture rates. © 2012 Elsevier Ltd.


Dicken M.L.,KwaZulu Natal Sharks Board | Dicken M.L.,Nelson Mandela Metropolitan University
African Journal of Marine Science | Year: 2014

Understanding socio-economic aspects of the diving industry at Sodwana Bay, including data on participant motivation and expenditure, is crucial for the effective management of the St Lucia and Maputaland marine protected areas, South Africa. Between July 2011 and July 2012 a total of 59 553 dives was conducted by 15 780 divers (95% CI = 15 295-16 277). Data were collected by means of the administration of a semi-structured survey questionnaire to 750 dive participants. Participant responses indicated that the direct value of diving to the iSimangaliso Wetland Park was R75 484 784 (95% CI = R73 071 709-R78 682 514). A total of 1 000 Monte Carlo simulations was used to estimate confidence intervals. The majority of dives at Sodwana were on coral-covered sandstone reefs (95.2%), with shark diving accounting for only 4.8% of dives. Although sharks were not the primary attraction for divers to visit Sodwana, 84.2% of respondents stated that they were interested in shark diving and that more opportunities to dive with sharks would encourage them to revisit Sodwana more often. Attaching an economic value to sharks as a dive attraction to Sodwana and highlighting their potential for the growth of the dive industry may act as leverage for their protection against fishing within iSimangaliso. © 2014 Copyright © NISC (Pty) Ltd.


O'Donoghue S.H.,University of KwaZulu - Natal | Drapeau L.,Paris West University Nanterre La Défense | Peddemors V.M.,KwaZulu Natal Sharks Board
African Journal of Marine Science | Year: 2010

The annual movement of South African sardine Sardinops sagax up the east coast of South Africa, known as the 'sardine run', was investigated using data from aerial surveys for the period 1988-2005 and compared with remotely sensed sea surface temperature (SST) and chlorophyll a data. Sardine sighting rates were highest within the Waterfall Bluff Bight off the Eastern Cape Coast, where conditions appeared to be most favourable. Sardine and predator sightings decreased significantly northwards of Mdoni on the KwaZulu-Natal (KZN) coast, whereas the proportion of nearshore sightings increased. The causal mechanism for this inshore concentration is suggested to be the influx of warm Agulhas Current water from the Durban Eddy that forces sardine shoreward. Cape gannet Morus capensis, common dolphin Delphinus capensis and sardine distributions were associated, and there was an association between SST and sardine and predator distributions. There was a marked increase in bottlenose dolphin Tursiops aduncus sightings upon commencement of the sardine run, with these dolphins being considered to be a 'migratory' stock that enters KZN waters every winter. © NISC (Pty) Ltd.


O'Donoghue S.H.,University of KwaZulu - Natal | Drapeau L.,Paris West University Nanterre La Défense | Dudley S.F.J.,KwaZulu Natal Sharks Board | Dudley S.F.J.,University of KwaZulu - Natal | Peddemors V.M.,University of KwaZulu - Natal
African Journal of Marine Science | Year: 2010

The nearshore presence of sardine Sardinops sagax on the KwaZulu-Natal (KZN) coast was investigated using sightings data collected by the KZN Sharks Board from 1997 to 2007. The spatio-temporal distribution of sardine was described in relation to that of their predators and to environmental conditions, and subjected to generalised linear model (GLM) and generalised additive model (GAM) analyses. Variables describing spatio-temporal conditions performed best in the models (r2 = 0.52) with seasonal effects, specifically June and July, making the greatest contribution towards sardine presence. The contribution of the years 2003, 2006 and 2007, and the KZN North Coast, was significantly lower. The predator variables were highly significant (r2 = 0.48) with Cape gannets Morus capensis, followed by the sharks/gamefish and common dolphins Delphinus capensis, being most closely associated with sardine presence. Environmental variables were not as influential in the GLM models (r2 = 0.23), but some variables were useful in describing conditions favouring sardine presence, namely calm current conditions, light north-westerly land breezes and stable atmospheric conditions. Increasing sea surface temperature (SST), moderate north to south currents, large swells and turbid water had a negative impact upon sardine presence. North-easterly and north-westerly winds and north to south currents had a cooling effect upon nearshore SSTs, whereas south-easterly winds and increasing air temperatures caused nearshore warming. Results are discussed in the context of developing an understanding of the mechanisms that govern fine-scale movements of sardine shoals during the KZN sardine run, with a view to predicting such movements. © NISC (Pty) Ltd.


Hussey N.E.,Bangor University | Wintner S.P.,KwaZulu Natal Sharks Board | Wintner S.P.,University of KwaZulu - Natal | Dudley S.F.J.,KwaZulu Natal Sharks Board | And 5 more authors.
Journal of Animal Ecology | Year: 2010

1. Life-history theory predicts that organisms will provide an optimal level of parental investment for offspring survival balanced against the effects on their own survival and future reproductive potential. 2. Optimal resource allocation models also predict an increase in reproductive output with age as expected future reproductive effort decreases. To date, maternal investment in sharks has received limited attention. 3. We found that neonatal dusky sharks (Carcharhinus obscurus) are not independent from maternal resource allocation at the point of parturition but instead are provisioned with energy reserves in the form of an enlarged liver that constitutes approximately 20% of total body mass. 4. Analysis of long-term archived data sets showed that a large proportion of this enlarged liver is utilized during the first weeks or months of life suggesting that the reported weight loss of newborn sharks signifies a natural orientation process and is not necessarily related to prey abundance and/or indicative of high mortality rates. 5. Interrogation of near-term pup mass in two carcharhinids, the dusky and spinner shark (Carcharhinus brevipinna), further revealed an increase in reproductive output with maternal size, with evidence for a moderate decline in the largest mothers. 6. For the dusky shark, there was a trade-off between increasing litter size and near-term pup mass in support of optimal offspring size theory. 7. For both the dusky and spinner shark, there was a linear increase in near-term pup mass with month, which may indicate variable parturition strategies and/or that carcharhinids are able to adjust the length of the gestation period. 8. The identification of optimal size-specific reproductive output has direct implications for improving the reproductive potential of exploited shark populations and for structuring future management strategies. © 2009 British Ecological Society.


Davidson B.,Saint James School of Medicine | Cliff G.,KwaZulu Natal Sharks Board | Cliff G.,University of KwaZulu - Natal
Fish Physiology and Biochemistry | Year: 2011

Female raggedtooth sharks (Carcharias taurus) migrate from the waters off the eastern Cape past KwaZulu-Natal and up to southern Mozambique and then back on an annual basis. They mate off the KwaZulu-Natal coast, gestate the pups off Mozambique, then deliver same off the eastern Cape. Prior to mating, they hypertrophise their livers and store large amounts of lipid, then towards the end of gestation subsist on this stored lipid as well as using it to feed their pups in utero. Raggedtooth sharks are aplacental, and hepatic lipids provide nutrients to the pups via continued ovulation throughout pregnancy. The fact of the liver hypertrophy was well documented, but whether the nature of the stored lipid or the amount of lipid per Kg of liver changed with season was unknown. Samples from raggedtooth females caught throughout the year were analysed for their lipid and fatty acid contents and significant differences noted between lipid, but not fatty acid, concentration with certain seasons. Liver mass decreased from spring to winter (16.3-9.9 kg) as did lipid concentration (572-326 mg/g). Within the fatty acids, 22:6n3 was ±17%, 20:5n3 ±7%, total n3 ±30% and total n6 ±7%. Also, both total polyunsaturates (±36%) and total monounsaturates (±33%) were greater than the total saturates (±28%). © 2010 Springer Science+Business Media B.V.


Dudley S.F.J.,KwaZulu Natal Sharks Board | Dudley S.F.J.,University of KwaZulu - Natal | Cliff G.,KwaZulu Natal Sharks Board | Cliff G.,University of KwaZulu - Natal
African Journal of Marine Science | Year: 2010

Shark catches in the protective nets set off the beaches of KwaZulu-Natal (KZN), South Africa, are strongly influenced by the sardine run, the winter influx of shoals of Sardinops sagax from the south-west. The effect of the sardine run, which is highly variable from year to year, is greatest in June and July at beaches south of Durban. Total annual shark catch and effort are presented for the period 1952-2005, and total monthly shark catch on the KZN south coast for the period May-August, 1965-2005. Measures to reduce catches of sharks associated with the sardine run have been introduced and have been increasingly successful. Reliable species-specific catch data are available for the period 1978-2005 only. For this period, the spatio-temporal distribution of each of 14 species of shark and the frequency of occurrence of sardine in their diets is documented. Occurrence varies according to species, as does the apparent influence of the sardine run on shark distribution. During June and July on the KZN south coast, sardine were found in the diet of all but two species and frequency of occurrence was 40% or greater in eight species. The presence of copper or bronze whaler sharks Carcharhinus brachyurus in KZN waters appears to be strongly associated with the sardine run, as does that of certain life-history stages of dusky sharks C. obscurus. Spinner sharks C. brevipinna and smooth hammerhead sharks Sphyrna lewini are caught in greater numbers in summer than in winter, but appear to shift their spatial distribution seasonally to feed on sardine. © NISC (Pty) Ltd.


Cliff G.,KwaZulu Natal Sharks Board | Cliff G.,University of KwaZulu - Natal | Dudley S.F.J.,KwaZulu Natal Sharks Board | Dudley S.F.J.,University of KwaZulu - Natal
Marine and Freshwater Research | Year: 2011

Large-scale shark-control programs at popular beaches in New South Wales and Queensland, Australia, and KwaZulu-Natal (KZN), South Africa, provide protection against shark attack. Although these programs have enhanced bathing safety, reducing the environmental impacts of decades of fishing for large sharks and the associated by-catch remains a challenge. Over the past three decades, there have been several interventions to reduce such impact in the KZN program. The first was the release of all live sharks, including those species known to be responsible for fatal shark attacks. Measures to reduce catches of sharks associated with the winter influx of shoals of sardines, Sardinops sagax, have been increasingly successful. In addition, extensive removal of nets has resulted in a major reduction in effort. Collectively, these initiatives reduced mortalities of sharks by 64%. Baited lines, termed drumlines, were introduced at 18 beaches, where they replaced some of the nets. The former had a far lower by-catch of rays, turtles and cetaceans and significantly lower catches of certain shark species. Replacement of some nets with drumlines is planned for the remaining beaches. Only two attacks, both non-fatal, have occurred at protected beaches in KZN over the past three decades, indicating that the program has maintained its public safety mandate while it has succeeded in reducing its impact on the environment. © CSIRO 2011.

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