Browsing by Author "Kock, Alison"
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- ItemOpen AccessDemographics of a seasonal aggregation of white sharks at Seal Island, False Bay, South Africa(2014) Hewitt, Adrian Michael; Griffiths, Charles L; Booth, Tony; Kock, AlisonWhite sharks (Carcharodon carcharias) are widely distributed, ecologically important marine apex predators that are considered to be vulnerable to extinction. Given their crucial roles in structuring marine ecosystems, their populations need to be effectively monitored. Photo-identification provides a standardised, non-lethal method of assessing the population dynamics of extant species and provides live-encounter mark-recapture and data that are essential for management and conversation. White sharks occur throughout South African coastal waters but their centre of abundance is the Western Cape, where large sharks predominate at aggregation sites like Seal Island, False Bay. This study documents inter- and intra-seasonal trends in mean sighting rates and sex ratios of white sharks at Seal Island, and provides mark-recapture data that are used to estimate proportions of "resident" and "transient" white sharks, their size-distribution and maturity composition.
- ItemOpen AccessEffects of protection and environmental factors on rock lobster, abalone, and giant periwinkle abundances in the Table Mountain National Park Marine Protected Area(2022) Gardner, Kathryn; Attwood, Colin; Kock, AlisonThe Table Mountain National Park Marine Protected Area (TMNP-MPA) was established in 2004 to restore and protect the Cape Peninsula's biodiversity, including the populations of exploited macro-invertebrate species. Scientific publications on MPAs support the idea that biodiversity protection is well served by MPAs, particularly no-take MPAs. Focusing on west coast rock lobster, abalone and giant periwinkle, historically the three most important invertebrate resources of the region, this study assesses the effectiveness of spatial regulations in the TMNP-MPA. Three comprehensive peninsula-wide SCUBA surveys were conducted in 2000, 2003 and 2019. Patterns in the abundance of the three species were analyzed using generalized-linear models (GLM) to explore the effects of the TMNP-MPA and no-take zones within, whilst accounting for habitat, depth, species interactions, and the physical protection offered by False Bay. Overall, the rock lobster abundance around the Cape Peninsula decreased since the implementation of the TMNP-MPA, however, to a lesser degree than the dramatic overall decline of the rock lobster population in South Africa over this time. This decrease is possibly due to increased illegal harvesting of adults and juveniles and increased predation from recovering fish populations within TMNP-MPA. However, rock lobster abundance did respond positively to the length of protection in no-take zones, suggesting that the enforcement of no-take zones within the MPA was effective. The beneficial effects of the no-take zones are likely insufficient to compensate for increased harvesting in adjacent areas within the MPA but prevented a greater decline in rock lobster. No change in abalone abundance was detected over the years in the TMNP-MPA or within no-take protected zones. The regional abalone population has declined drastically over a similar period due to poaching. Effective policing has likely prevented the abalone within the MPA from following suit. Giant periwinkle abundance increased since the establishment of TMNP-MPA. However, it is unknown if this pattern reflects the regional giant periwinkle population, so it cannot be determined if the increase is a direct result of the MPA. However, the giant periwinkle abundance had not benefited from the no-take zones, suggesting general harvest restrictions in TMNP-MPA are affording them sufficient protection, or that the greater population is increasing. A combination of environmental factors, such as warmer water temperatures and reduced nutrients, could explain the decreased abundance of rock lobster in False Bay, compared to those along the Peninsula's west coast. Predation by rock lobster could explain the opposite pattern found in both the giant periwinkle and abalone that were more common in False Bay. Expected abundance patterns based on the habitat preferences of each species, associated with water depth, the presence of sand, and kelp and urchin density were found. The creation of the TMNP-MPA, with its many and varied no-take and harvest zones provided an excellent opportunity to test the effectiveness of no-take spatial protection in a complex marine community and in the vicinity of a heavily exploited coastal zone near a metropolitan setting. This is especially valuable for macro-invertebrates, which are not commonly the focus of MPA studies in South Africa. This study supports the use of MPAs as a protection measure for overexploited macro-invertebrates. The extent of the success of this protection is evident by way of comparison with the magnitude of declines elsewhere. As both rock lobster and abalone fisheries are heavily plagued by poaching, the impact of no-take zones and the enhanced policing brought by the TMNP-MPA as a whole, likely helped to buffer the declines of rock lobster and abalone, respectively.
- ItemOpen AccessEnvironmental predictors of Carcharodon carcharias presence at two popular beaches in False Bay, South Africa using acoustic telemetry(2018) Van Beuningen, Dave; O'riain, Justin; Kock, Alison; Irion, DylanBy understanding when white sharks (Carcharodon carcharias) are likely to be at certain popular beaches, it is possible to predict when the risk of overlap between water users and white sharks is highest, and to convey this information to the public so they can make informed decisions about using these areas. Previous studies have shown that white shark presence near popular recreational beaches in False Bay, South Africa, is influenced by a range of environmental variables. These studies have relied on land-based observers (shark observers), whose ability to detect sharks is subject to the depth at which the sharks swim and a suite of environmental conditions that influence water visibility, including cloud cover, wind speed and ambient light levels. In this study, I use passive acoustic telemetry on 56 tagged white sharks to determine whether the same or other environmental variables explain variation in white shark presence at the same beaches. A total of 13 803 and 1 481 white shark detections were recorded between April 2005 and December 2007 at Muizenberg and Fish Hoek beaches, respectively. This represented 32 and 16 individual white sharks with a median number of 32.5 (range 5.5 – 57.8) and 7 (range 4 – 14.8) detections per shark at Muizenberg and Fish Hoek beach, respectively. The low number of detections at Fish Hoek resulted in the data being highly zero-inflated with the result that the subsequent modelling of the data with environmental covariates did not converge, and hence I focused solely on Muizenberg beach. The probability of detecting a white shark at Muizenberg beach was modelled using binomial generalised additive mixed models (GAMMs) with water temperature, wind speed, wind direction, cloud cover, lunar phase, tide height, barometric pressure, year, season and time of day as predictor variables. Water temperature was a significant predictor of white sharks at Muizenberg beach during summer, autumn and winter while wind speed, time of day and barometric pressure were significant predictors of shark presence during the summer and autumn months. There was significant inter-annual variability in white shark detections and a strong seasonal relationship, with presence being highest during spring and lowest during winter. Encouragingly, the findings from this study support some of the key findings of previous studies using observational data, including the significant positive effects of increasing temperature, year, season and time of day.
- ItemOpen AccessThe influence of environmental variables on white shark (carcharodon carcharias) sightings at False Bay beaches.(2012) Weltz, Kay; Kock, Alison; Attwood, ColinIn response to an increase in shark attacks in the Western Cape over the last decade, a shark warning system called Shark Spotters that records white sharks (Carcharodon carcharias) in the surf zone was implemented in 2004 in Cape Town, South Africa.
- ItemOpen AccessLiving with great white sharks in Cape Town(2013) Kock, AlisonCoordinated by Alison Kock, Research Manager, Shark Spotters. For surfers, conservationists, lifeguards and anyone interested in the topic of living with great white sharks in Cape Town.
- ItemOpen AccessPopulation structure, site fidelity, and fine-scale habitat use of the broadnose sevengill shark, Notorynchus cepedianusat Pyramid rock, False Bay, South Africa(2012) Raw, Robert Nicolas Vause; Griffiths, Charles L; Kock, AlisonThe aims of this study were to investigate elements of the demographics, fine-scale habitat use and site fidelity of sevengill sharks. The study was conducted at an aggregation hotspot within a Marine Protected Area, near Miller’s Point in False Bay, South Africa.
- ItemOpen AccessResidency, habitat use and sexual segregation of white sharks, Carcharodon carcharias in False Bay, South Africa(Public Library of Science, 2013) Kock, Alison; O'Riain, M Justin; Mauff, Katya; Meÿer, Michael; Kotze, Deon; Griffiths, CharlesWhite sharks ( Carcharodon carcharias ) are threatened apex predators and identification of their critical habitats and how these are used are essential to ensuring improved local and ultimately global white shark protection. In this study we investigated habitat use by white sharks in False Bay, South Africa, using acoustic telemetry. 56 sharks (39 female, 17 male), ranging in size from 1.7-5 m TL, were tagged with acoustic transmitters and monitored on an array of 30 receivers for 975 days. To investigate the effects of season, sex and size on habitat use we used a generalized linear mixed effects model. Tagged sharks were detected in the Bay in all months and across all years, but their use of the Bay varied significantly with the season and the sex of the shark. In autumn and winter males and females aggregated around the Cape fur seal colony at Seal Island, where they fed predominantly on young of the year seals. In spring and summer there was marked sexual segregation, with females frequenting the Inshore areas and males seldom being detected. The shift from the Island in autumn and winter to the Inshore region in spring and summer by females mirrors the seasonal peak in abundance of juvenile seals and of migratory teleost and elasmobranch species respectively. This study provides the first evidence of sexual segregation at a fine spatial scale and demonstrates that sexual segregation in white sharks is not restricted to adults, but is apparent for juveniles and sub-adults too. Overall, the results confirm False Bay as a critical area for white shark conservation as both sexes, across a range of sizes, frequent the Bay on an annual basis. The finding that female sharks aggregate in the Inshore regions when recreational use peaks highlights the need for ongoing shark-human conflict mitigation strategies.
- ItemOpen AccessThe trophic dynamics of the broadnose sevengill shark (Notorynchus cepedianus) in False Bay, South Africa, using multiple tissue stable isotope analysis(2017) De Necker, Leigh; O'Riain, M Justin; Kock, Alison; Lewis, Matthew Charles; Barnett, AdamDespite their important ecological role, there is limited quantitative information on the trophic ecology of large, apex predator sharks. This is largely a consequence of their occupying naturally low population densities, being highly mobile and elusive and ranging over large distances. Stable isotopes provide a low cost, non-lethal method for investigating the short and long-term diet of a predator, which when combined with prey data can be used to understand the trophic interactions and the potential regulatory effects they may have on the structure and function of marine ecosystems. In this study, I used non-lethal stable isotope analysis to investigate the trophic dynamics of sevengill sharks, Notorynchus cepedianus, within False Bay, South Africa. A total of 39 muscle biopsies (33 female, six male), and 28 blood plasma samples (25 female, three male) were collected from sevengill sharks. These were analysed together with 161 prey samples from 32 different species, including cephalopods, crustaceans, teleosts, chondrichthyans and marine mammals. In addition, seven white shark, Carcharodon carcharias, muscle samples were collected from False Bay and analysed for comparison with those of sevengills. Sevengills in False Bay had the highest δ15N values of all species sampled in this study (including white sharks) and appear to feed predominantly on a variety of coastal prey species from various functional groups. A stable isotope mixing model revealed that inshore chondrichthyans were their most important prey, with Cape fur seals and inshore teleost species also being important prey groups. There was no apparent seasonal shift in the diet of sevengills, despite clear seasonal aggregation in coastal kelp forests during the summer months. δ15N decreased significantly with sevengill size, with immature females having higher δ15N values than mature females, while δ13C increased with sevengill size. These trends are hypothesised to be linked to dietary shifts associated with the relative use of different habitat types when individuals become sexually mature. There is a need to combine stable isotope data with movement patterns and habitat use to better understand the relationship between isotope ratios and habitat use. Ongoing research on sevengill and white shark movement patterns in False Bay, together with the results presented in this study, will provide important information on the trophic and ecological role that two top predatory sharks play in False Bay. Keywords: Notorynchus cepedianus, sevengill shark, stable isotopes, trophic ecology