Decline of coastal apex shark populations over the past half century - PubMed (original) (raw)

Decline of coastal apex shark populations over the past half century

George Roff et al. Commun Biol. 2018.

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.

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Conflict of interest statement

The authors declare no competing interests.

Figures

Fig. 1

Fig. 1

Regional setting and historical changes in catch and effort for the Queensland Shark Control Program. a Timing of the establishment of shark control programmes across the Queensland coastline (map created under Creative Commons Attribution 4.0 International from Geoscience Australia). b Cumulative effort for nets and drumlines and catch between 1962 and 2017. c Historical photograph of contractors measuring sharks removed from QSCP nets on the Gold Coast in the early years of the programme (3 November 1963), reprinted from Paterson (1990) Biological Conservation, 52(2), 147–159 (ref. ) with permission from Elsevier

Fig. 2

Fig. 2

Catch per unit effort (CPUE) in nets and drumlines with fits from Bayesian negative binomial generalised additive mixed effects models (± 95% credibility intervals), and the annual zero-catch probability (± 95% credibility intervals). Percentages represent the % decline over the 1962–2017 dataset. Photographs courtesy of Juan Oliphant (

http://oneoceandiving.com/

)

Fig. 3

Fig. 3

Initial catch per unit effort (defined as the average CPUE of the first five years of operation) for each beach within regions, and model fits from Bayesian generalised additive mixed effects models (± 95% credibility intervals) for nets and drumlines. Symbols courtesy of the Integration and Application Network (

ian.umces.edu/symbols/

)

Fig. 4

Fig. 4

Long-term changes in size structure and sexual maturity. Linear regressions (± 95% confidence intervals) for change in size for a hammerheads (1962–2017, shading indicates period in which species specific data are available), great and scalloped hammerheads (Sphyrna mokarran and Sphyrna lewini, 1997–2017), b whaler sharks (Carcharhinidae, 1962–2015), c tiger sharks (Galeocerdo cuvier, 1962–2017), and binomial probability models for sexual maturity in male and female, d great and scalloped hammerheads (S. mokarran and S. lewini, 1997–2017) and e tiger sharks (G. cuvier, 1962–2017). ns not significant. ***p < 0.001, **p < 0.01, *p < 0.05. Symbols courtesy of the Integration and Application Network (

ian.umces.edu/symbols/

)

Fig. 5

Fig. 5

Regional movement patterns among coastal and oceanic ecosystems. Representative movement tracks derived from satellite and acoustic tracking studies of sharks adjacent to the eastern Australian coastline for a tiger sharks (Galeocerdo cuvier, n = 10) and bull sharks (Carcharhinus leucas, n = 17), b white sharks (Carcharodon carcharias, n = 6), and c maximum distance derived from movement studies for whaler sharks (Carcharhinidae), tiger sharks (G. cuvier), and white sharks (C. carcharias) caught in the QSCP program

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