Decline of coastal apex shark populations over the past half century

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.

Life-history traits inform on population trends when assessing the conservation status of a declining tiger shark population

“…The nets 105 and drumlines are checked by contractors 15-20 days of each month, who also record the 106 length and taxonomic identity of captured sharks. A previous analysis of this same data-set 107 found declines in four major groups of sharks caught in the QSCP, and declines were 108 consistent across regions, suggesting a common trend across the eastern coast of Australia 109 in these shark groups (Roff et al 2018). This included a 75% decline in tiger sharks 110 A recent analysis of the QSCP data found declines in four major groups of sharks across 119 regions, suggesting a common trend across the eastern coast of Australia in these shark 120 groups (Roff et al 2018).…”

“…Contact chris.brown@griffith.edu.au While some uncertainty exists in the historical data used here, data collection was 430 standardized in the QSCP in the early 1990's. This implies trends in recent decades when 431 declines have been observed are robust (see Roff et al (2018) for discussion). Our 432 proposed statistical method may also help smooth over temporal and regional variability in 433 trends by estimating the long-term trend that is consistent across regions, and by 434 controlling for differences in gear type.…”

“…As a case-study we modelled multi-decadal trends in a declining tiger shark population 92 from the east coast of Australia (Holmes et al 2012;Roff et al 2018). The data are from the 93 Queensland Shark Control Program (QSCP) a unique spatially replicated annual time-series 94 of shark catch and effort that covers 1962-2017.…”

“…The term , , was an offset term that accounted 159 for variation in the number of drum lines and nets across time and regions. This model is 160 similar to that used in Roff et al (2018), except that we ignored site level variation by 161 summing over drum/net sites within a region.…”

Resource‐use dynamics of co‐occurring chondrichthyans from the First Coast, North Florida, USA

“…Although the ecological roles of sharks are generally understood (Hammerschlag et al ., ; Roff et al ., ), their functional roles can be diverse, warranting the need for context‐ ( e.g ., time of year, sex, size class, species) and location‐specific assessments. Such work is pertinent considering the concern over the health of shark populations worldwide, owing largely to exploitation by commercial fisheries (Dulvy et al ., ; Roff et al ., ), coastal development and habitat removal (Knip et al ., ) and emerging threats associated with climate change (Rosa et al ., ; Rummer and Munday, ). Nonetheless, extensive assessments of resource‐use dynamics for sharks in many geographic regions are yet to be undertaken, precluding the management of key habitats, prey groups and other potentially facilitative interactions that support shark biomass.…”