Decline of coastal apex shark populations over the past half century

George Roff,Mark A Priest,Peter J Mumby,Christopher J Brown

doi:10.1038/s42003-018-0233-1

George Roff, Mark A Priest + Show 2 more

Open Access

https://doi.org/10.1038/s42003-018-0233-1

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Abstract

Overexploitation of large apex marine predators is widespread in the world’s oceans, yet the timing and extent of declines are poorly understood. Here we reconstruct a unique fisheries-independent dataset from a shark control programme spanning 1760 km of the Australian coastline over the past 55 years. We report substantial declines (74–92%) of catch per unit effort of hammerhead (Sphyrnidae), whaler (Carcharhinidae), tiger shark (Galeocerdo cuvier) and white sharks (Carcharodon carcharias). Following onset of the program in the 1960s, catch rates in new installations in subsequent decades occurred at a substantially lower rate, indicating regional depletion of shark populations over the past half a century. Concurrent declines in body size and the probability of encountering mature individuals suggests that apex shark populations are more vulnerable to exploitation than previously thought. Ongoing declines and lack of recovery of vulnerable and protected shark species are a cause for concern.

Highlights

Overexploitation of large apex marine predators is widespread in the world’s oceans, yet the timing and extent of declines are poorly understood
Analysis of the Queensland Shark Control Program (QSCP) catch data reveals a diverse range of sharks (45 spp.) spanning multiple trophic levels (Supplementary Table 1), ranging from small (~ 80 cm maximum total length [TLmax]) low trophic level sharks (e.g. Heterodontus portusjacksoni) to large (> 600 cm TLmax) apex sharks such as tiger sharks (Galeocerdo cuvier) and white sharks (Carcharodon carcharias)
The QSCP has been in operation since 1962 (Fig. 1c), records relating to species identification are considered reliable only from ~1996 onwards following a systematic review of the programme[9]

Summary

Introduction

Overexploitation of large apex marine predators is widespread in the world’s oceans, yet the timing and extent of declines are poorly understood. Following onset of the program in the 1960s, catch rates in new installations in subsequent decades occurred at a substantially lower rate, indicating regional depletion of shark populations over the past half a century. The QSCP has been operating since 1962 using a system of mesh nets and baited drumlines (Supplementary Figure 1) with an aim to “minimise the threat of shark attack on humans”[9] by reducing the local populations of large sharks to minimise the probability of encounters between sharks and swimmers[10]. The programme started in Cairns in 1962, and has since expanded to 11 regions in Queensland, spanning tropical and sub-tropical coastal ecosystems across 1760 km of the eastern Australian coastline[11] (Fig. 1a, Supplementary Figure 2). From the onset of the programme in 1962, increasing numbers of baited drumlines were installed in place of nets (Fig. 1b) due to logistical constraints and issues of bycatch (predominantly turtles and dugongs[11])

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