Browsing by Author "Engelbrecht, Tamlyn"

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    The impact of White Shark (Carcharodon carcharias) sightings and attacks on recreational water use patterns in False Bay
    (2013) Engelbrecht, Tamlyn; O'Riain, Justin; Kock, AA
    White sharks (Carcharodon carcharias) are apex predators that play an important role in the structure and functioning of marine ecosystems. However, despite both their ecological importance and threatened conservation status this species is still subject to lethal control to reduce the risks of predation for recreational water users. The Shark Spotter program, pioneered in False Bay, South Africa, proposes a non-lethal alternative to reducing predation risk. This program aims to balance the needs of people with white shark conservation by actively reducing conflict between recreational water users and sharks. In this study I establish the extent of spatial overlap between white sharks and water users at two popular recreational beaches in False Bay (Fish Hoek and Muizenberg), and investigate how shark sightings (accompanied by warning flags and/or a siren) as well as attacks influence water use patterns amongst bathers, surfers and paddlers. In the period from 2006 to 2012, the total number of shark sightings recorded was 531 at Muizenberg and 322 at Fish Hoek, with a notable increase in sightings at both beaches in recent years. Shark sightings were rare in winter increasing into the spring and summer months when recreational use of the inshore was highest. Daily shark sightings peaked at midday to late afternoon at both beaches, coinciding with peak numbers of water users. The response of water users to warnings of shark presence by the Shark Spotters was only found to be significant in cases where the siren was sounded by the shark spotters, and in the absence of a siren warning flags had little impact on average numbers of water users. The occurrence of a fatal shark attack was found to a) increase response of water users to auditory warnings (when the siren is sounded) but not visual warnings (warning flags unaccompanied by a siren); and b) reduce the average number of water users present at both beaches for at least three months following the fatal incident. Annual averages of water users at Muizenberg beach also reflected this pattern, with a general reduction in water use in years with attacks compared to those without. These findings indicate that the Shark Spotter program is effective in mitigating conflict between water users and white sharks through auditory warnings and subsequent beach clearing in the event of a shark sighting; however the lack of response by water users in the absence of a siren and after long periods without an attack remains a challenge to the effectiveness of the program. The large spatial overlap between white sharks and water users, as well as the increasing number of shark sightings at both beaches, emphasises the need for continuous revision and improvement of mitigation strategies to prevent conflict between white sharks and water users in False Bay.
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    The spatial ecology and behaviour of the broadnose sevengill shark (Notorynchus cepedianus) in South Africa
    (2024) Engelbrecht, Tamlyn; O'riain, Mannus
    Top predators are an important component of healthy ecosystems, contributing to ecosystem stability and biodiversity through direct and indirect effects. However, significant knowledge gaps exist in our understanding of the occurrence, biology and spatiotemporal dynamics of marine predators, limiting our ability to conserve these species in the face of anthropogenic threats. The broadnose sevengill shark (Notorynchus cepedianus) is a marine top predator with a global distribution in coastal and shelf waters of temperate regions. This species faces exploitation across their range and are classified as “Vulnerable” on the IUCN Red List of Threatened Species. In this thesis, the distribution, growth rates, spatiotemporal dynamics and habitat use of N. cepedianus in the coastal waters of southern Africa were investigated. Firstly, the distribution and growth rate of N. cepedianus in southern Africa were investigated using data from the Oceanographic Research Institute's Cooperative Fish Tagging Project. Broad-scale patterns in coastal presence, movement and population connectivity were then investigated for both sexes and a range of sizes of N. cepedianus in South Africa using acoustic telemetry, and a national scale array of acoustic receivers (the Acoustic Tracking Array Platform). Finally, an array of acoustic receivers in False Bay, Cape Town, were used to investigate patterns in the presence and fine-scale movement patterns of N. cepedianus in False Bay. During data collection, several unexpected predations on N. cepedianus by killer whales (Orcinus orca) occurred in False Bay, providing a unique opportunity to investigate the impacts of a novel apex predator on the behaviour of N. cepedianus. Overall, the results showed that N. cepedianus has a broad distribution in temperate coastal waters of southern Africa, ranging from northern Namibia to the Eastern Cape Province of South Africa. Growth rates for N. cepedianus at reference lengths of 100 cm and 160 cm (PCL) were estimated to be 4.7 cm.year-1 and 4.0 cm.year-1 , respectively, which are slower than previous growth estimates for this species. Population connectivity was established between the west and south coasts of South Africa, but not between South Africa and Namibia. Tag recapture data and acoustic telemetry showed that N. cepedianus of all sizes display site fidelity to certain coastal sites, and most movements had a limited range (<50 km). However, a few individuals of both sexes and a range of sizes made large-scale coastal movements of up to 600 km, highlighting that a portion of the population roams more widely. Acoustic telemetry also confirmed size and sex segregation by N. cepedianus at coastal sites in southern Africa. These findings can be used in stock assessments and to guide spatial management for this species. Within False Bay, most female N. cepedianus showed high site fidelity to Miller's Point, a large, well-known aggregation site for this species, and were observed returning for up to four consecutive years after extended periods of absence. However, following O. orca predation events, tagged N. cepedianus fled the site within 24 hours for up to 290 days. There was a notable decrease in the number of N. cepedianus that returned to Miller's Point following each of the four confirmed predation events by O. orca, until eventually, the sharks abandoned the aggregation site completely. Displacement of a top predator (N. cepedianus) by an even higher-level predator (O. orca) is expected to have cascading effects on ecosystems, highlighting the need to monitor impacts to better understand the ecological role of sharks and the consequences of population declines.