Rates of shark depredation of line-caught fish on the Protea Banks, KwaZulu-Natal

Master Thesis


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University of Cape Town

This study estimates rates of shark depredation in the charter boat fishery on Protea Banks, KwaZulu-Natal. Previous estimates based on fisher surveys suggested that shark depredation is a concern locally and may distort fishing mortality estimates. Methods involved quantitative data collection by an onboard observer from November 2013 to January 2014. Catch composition data were collected to enable comparisons with the commercial and recreational catch returns used in monitoring and assessment. Results revealed an average depredation rate of 8.4% that varied depending on the species fishers targeted. Depredation was highest when catching pelagic species (18.6%) and lowest when catching reef species (1.9%). Depredation rates were highest in November (19.6%) and lowest in January (5.3%). Observed rates were highest on the Banks itself and immediately offshore (9.9%), but no depredation was observed inshore of the Banks. The most commonly identified sharks involved in depredation incidents were the dusky shark (Carcharhinus obscurus) and the blacktip shark (Carcharhinus limbatus). Multi-dimensional scaling showed commercial catch composition to be significantly different from recreational and charter catch composition, mainly due to abundance of tuna in the recreational and charter sectors. No significant relationship was found between catch composition and shark depredation. Depredation is estimated to cost charter fishing operators 8% of their revenue. Depredation rates are at a level that could impact effective stock assessment and should be considered when making management decisions.

Includes bibliographical references.