Sex and Size Influence the Spatiotemporal Distribution of White Sharks, With Implications for Interactions With Fisheries and Spatial Management in the Southwest Indian Ocean

Abstract

Human activities in the oceans increase the extinction risk of marine megafauna. Interventions require an understanding of movement patterns and the spatiotemporal overlap with threats. We analysed the movement patterns of 33 white sharks (Carcharodon carcharias) satellite-tagged in South Africa between 2012 and 2014 to investigate the influence of size, sex and season on movement patterns and the spatial and temporal overlap with longline and gillnet fisheries and marine protected areas (MPAs). We used a hidden Markov model to identify ‘resident’ and ‘transient’ movement states and investigate the effect of covariates on the transition probabilities between states. A model with sex, total length and season had the most support. Tagged sharks were more likely to be in a resident state near the coast and a transient state away from the coast, while the probability of finding a shark in the transient state increased with size. White sharks moved across vast areas of the southwest Indian Ocean, emphasising the need for a regional management plan. White sharks overlapped with longline and gillnet fisheries within 25% of South Africa’s Exclusive Economic Zone and spent 15% of their time exposed to these fisheries during the study period. The demersal shark longline fishery had the highest relative spatial and temporal overlap, followed by the pelagic longline fishery and the KwaZulu-Natal (KZN) shark nets and drumlines. However, the KZN shark nets and drumlines reported the highest white shark catches, emphasising the need to combine shark movement and fishing effort with reliable catch records to assess risks to shark populations accurately. White shark exposure to shark nets and drumlines, by movement state, sex and maturity status, corresponded with the catch composition of the fishery, providing support for a meaningful exposure risk estimate. White sharks spent significantly more time in MPAs than expected by chance, likely due to increased prey abundance or less disturbance, suggesting that MPAs can benefit large, mobile marine megafauna. Conservation of white sharks in Southern Africa can be improved by implementing non-lethal solutions to beach safety, increasing the observer coverage in fisheries, and continued monitoring of movement patterns and existing and emerging threats.

How continuing mortality affects recovery potential for prohibited sharks: The case of white sharks in South Africa

“…White sharks were preemptively protected from exploitation in South Africa in 1991 even though their population size and status at that time were largely unknown (Compagno, 1991;Cliff et al, 1996). Since then, ongoing mortality have been occurring from the beach protection programs along the KwaZulu-Natal coastline (Cliff and Dudley, 2011), in recreational and subsistence fisheries along southern Africa's coastlines, as well as in commercial fisheries (Kock et al, 2022). To our knowledge, an annual time series of catches and mortalities of white shark from the KZNSB program has yet to be published.…”

“…To our knowledge, an annual time series of catches and mortalities of white shark from the KZNSB program has yet to be published. Kock et al, 2022 cites a total of 1317 white shark captures between 1978 and 2018 where 209 (16%) were released alive, which would suggest average mortalities of 28 animals per year. Cliff and Dudley (2011) excluded the early part of the time series (when white sharks were killed when captured) and calculated average values of 33 animals (SD = 12.3) in the 1990s, and 25 animals (SD = 7.8) in the 2000s with 15% and 10%, respectively, released alive.…”

“…Current complications for on-going management of white sharks in South Africa include: (1) a lack of reported interactions leading to poorly quantified incidental mortalities from suspected bycatch in commercial, artisanal and recreational fisheries along the southern African coastline (da Silva et al, 2015), (2) the relatively unknown impact of ongoing mortality from the early 1990s to present from the KZNSB Bather Protection Program (Cliff and Dudley, 2011;Kock et al, 2022), (3) uncertainty in regional abundance, with locationspecific estimates ranging from ~350 to > 2000 animals (Cliff et al, 1996;Ryklief, 2012;Towner et al, 2013;Hewitt, 2014;Andreotti et al, 2016), and (4) apparently divergent trends in abundance suggested by recent monitoring from different locations (c.f. 2022; Hammerschlag et al, 2019;and DFFE, 2020).…”

“…Here, we consider F to represent removals due to incidental human-caused mortality rather than directed fishing mortality. This would include any mortality from bycatch in fisheries off the southern African coast (da Silva et al, 2015;Kock et al, 2022) and mortalities from the KZNSB program (Cliff and Dudley, 2011).…”

“…Concentrating fishing pressure on juveniles leads to lower survival at age and thus reduced total reproductive output in the simulations. Given that the white shark population in the southwestern Indian Ocean is concentrated off South Africa (Bonfil et al, 2005;Kock et al, 2022), we consider the population to be closed and do not attempt to estimate immigration or emigration rates.…”