Browsing by Author "Nagel, Cicely"

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    A comparative study on the role of colour and contrast discrimination in the predatory behaviour of oceanic blacktip and bronze whaler sharks
    (Stellenbosch : Stellenbosch University, 2024-03) Nagel, Cicely; Cherry, Michael; Andreotti, Sara; Stellenbosch University. Faculty of Science. Dept. of Botany and Zoology.
    ENGLISH ABSTRACT: Despite the low number of yearly fatalities caused by sharks, negative shark-human interactions have been at the centre of national and international debates. Specifically, controversial policies such as shark culling have been adopted in South Africa and Australia to reduce shark populations in the proximity of recreational beaches. This project was designed to determine whether some colours might attract sharks more than others, to help minimize the chances of negative shark-human interactions. Sharks, together with rays, skates and chimaeras are part of the class Chondrichthyans, one of the oldest most prolific vertebrate lineages extant today. Despite their ecological importance and historical longevity, chondrichthyan vision remains poorly studied, especially in regard to their ability to perceive colours. Colour vision in sharks has been debated within the literature for decades, with behavioural experiments providing conflicting results. Recent microspectrophotometry analysis has provided evidence that sharks do not possess multiple cone pigments within their eyes. This suggests that they are colour-blind, and that colour preferences reflect differences in contrast with their background. Utilising a novel in-field behavioural approach, the present study aims to determine whether oceanic blacktip sharks (Carcharhinus limbatus) and bronze whaler sharks (Carcharhinus brachyurus) possess the ability to discriminate between five differently coloured bait tubes (white, yellow, silver, red, and black), in terms of how these colours contrast against a background of sea water. Results show that both C. limbatus and C. brachyurus were able to discriminate between the five coloured tubes, demonstrating a preference for the brighter colours (white, yellow, and silver) over the darker colours (red and black). Shark reactions can be explained in terms of luminance contrast between different coloured bait tubes and sea water. This study is the first behavioural study on C. limbatus and C. brachyurus simulating visual predation and provides empirical evidence on which colours are best avoided when engaging in activities in shark-inhabited areas. Both C. limbatus and C. brachyurus are classified as potentially dangerous to humans, having been involved in 35 and 16 unprovoked attacks, respectively, worldwide since 1540. These findings provide an insight into the visual ecology of sharks, showing that visual cues are important in shark predatory behaviour. Based on the results of this study, to reduce the chances of negative interactions between these two species and humans, dive gear, wetsuits, and bathing suits should avoid high-contrast colours such as white, yellow, and silver.