Genetic data indicate genetic structuring between ocean basins (western Atlantic, Gulf of Mexico, and Caribbean Sea distinct from eastern Atlantic and Indo-Pacific), potentially within ocean basins, and likely global male-biased dispersal and female philopatry (Keeney and Heist 2006, Sodré et al. 2012, Spaet et al. 2015, Almojil et al. 2018, Johnson et al. 2018, Bester-van der Marewe et al. 2019). Population trend data are available from three sources: stock assessments in (1) the Gulf of Mexico (SEDAR 2018) and (2) the Northwest Atlantic (SEDAR 2020); and standardized catch-per-unit-effort (CPUE) in (3) the Western Indian Ocean (M. Dicken and H. Winker unpubl. data 2020). The trend data from each source were analyzed over three generation lengths using a Bayesian state-space framework (Sherley et al. 2020, Winker et al. 2020). This analysis yields an annual rate of change, a median change over three generation lengths, and the probability of the most likely IUCN Red List Category percent change over three generation lengths (see the Supplementary Information).

First, the stock assessment from the northern Gulf of Mexico estimated that in 2016 the stock was not overfished and that overfishing was not occurring; the stock appeared capable of supporting the current fishing pressure (SEDAR 2018). The population declined from 1981 to the early 2000s, and then began to slightly increase, possibly in response to more limited catches in the latter years (SEDAR 2018). The spawning stock fecundity was estimated to have reduced from virgin levels in 1981 by 1–32%, but most likely by <5% (SEDAR 2018). The trend analysis of the northern Gulf of Mexico abundance for 1981–2016 (36 years) revealed annual rates of reduction of 0.2%, consistent with an estimated median reduction of 6.6% over three generation lengths (45 years), with the highest probability of <20% population reduction over three generation lengths.

Second, the stock assessment from the North Atlantic (from Maine to Florida) estimated that in 2018 the stock was not overfished and that overfishing was not occurring, although it had experienced overfishing in some previous years (SEDAR 2020). Although the spawning stock fecundity declined by 54% over 38 years from 1981 to 2018, it did not fall below the minimum stock size threshold (MSST) which is used as a benchmark for stock status (SEDAR 2020). The trend analysis of the spawning stock fecundity for 1981–2018 (38 years) revealed annual rates of reduction of 2.5%, consistent with an estimated median reduction of 59.2% over three generation lengths (36 years), with the highest probability of 50–79% population reduction over three generation lengths.

Thirdly, the standardized CPUE from the Kwazulu-Natal Sharks Board (KZNSB) bather protection program (South Africa) fluctuated but steadily declined from 1981 to 2019 (M. Dicken and H. Winker unpubl. data 2020). The trend analysis of the CPUE for 1981–2019 (39 years) revealed annual rates of reduction of 1.8%, consistent with an estimated median reduction of 20.3% over three generation lengths (27 years), with the highest probability of <20% reduction over three generation lengths. In contrast, both recreational and commercial fishermen as well as SCUBA divers have noticed a marked increase in the abundance of Blacktip Sharks off the KwaZulu-Natal coast since ~2006 (M. Dicken, KZN Sharks Board, unpubl. data 2016). Indeed, the Blacktip Shark is considered a common species off the KwaZulu-Natal coast and is one of the most commonly caught species in both the KZNSB program (Dudley and Simpfendorfer 2006) and competitive shore angling fishery (Pradervand 2004, Pradervand et al. 2007).

Additionally, standardized CPUE is available for combined Blacktip Shark and Australian Blacktip Shark (C. tilstoni) from longline fishery independent surveys in northwest Australia (Braccini et al. 2020). The CPUE for 2003–2017 fluctuated, but overall was mostly stable, although there was a high level of uncertainty as the catches were relatively low. The area of the surveys was closed to commercial fishing from 1993 to 2005 onwards and the fluctuating, but overall stable pattern is suggested to reflect either no major detrimental fishing effects on the population even prior to the closures or naturally low abundance that precludes detecting abundance patterns (Braccini et al. 2020). The lack of targeted fishing or incidental catch since ~2009 means there is no longer a threat in that region. Elsewhere in Australia, stock assessments for the Blacktip Shark species complex that consists of Blacktip Shark, Australian Blacktip Shark, and Spottail Shark (C. sorrah) suggest that while the northern Australian stock was over-exploited in the past, on the North and West Coast and on the East Coast the complex has recovered from previous fishing and is currently sustainable (Bradshaw et al. 2013, Grubert et al. 2013. Johnson et al. 2018). Two of the species in the Blacktip Shark species complex are difficult to visually distinguish which has hampered species-specific stock status determination (Johnson et al. 2018)

In the Arabian Seas region, the Blacktip Shark is a common component of fishery catches (Moore et al. 2012, Moore and Pierce 2013, Jabado et al. 2015, Spaet and Berumen 2015). In some areas, it is the most common species landed, for e.g., in the Gulf of Aden and off the Socotra Islands, Yemen (Shaher 2007). Declines have been noted in some parts of the region, for e.g., in Oman, it was one of the most commonly caught species representing up to 7% of shark landings by number in 2002, but there was a noticeable decline in landings in 2003 to 1% of shark landings by number (Henderson et al. 2007). In Pakistan, it was commonly taken in artisanal fisheries but has declined by 20–30% since 1999 (M. Khan pers. comm. 9 February 2017). In India, it was one of the most commonly caught shark species in the 1990s. However, in the last 10–15 years there has been a 60% decline in landings despite stable or increasing fishing effort (S.J. Kizhakudan pers. comm. to K.K. Akhilesh 09 February 2017) with reports that stocks of Blacktip Sharks (Carcharhinus spp.) have collapsed (Mohamed and Veena 2016). Fisheries in the Arabian Seas region have experienced increased demand for sharks since the 1970s due to the shark fin trade and as a result, effort is increasing in traditional shark fisheries in many areas (Henderson et al. 2007, Jabado et al. 2015). Simultaneously, there has been a significant increase in coastal fishing effort and power in some parts of the Arabian Seas region leading to a reduction in the number of shark catches with a population reduction of 30–49% suspected for the Blacktip Shark over the past three generation lengths (~39 years) (Jabado et al. 2017).

Elsewhere, species-specific population trend data is limited for the Blacktip Shark. However, catches for all combined whaler sharks (Carcharhinidae), Carcharhinus spp., Hardnose Shark (Carcharhinus macloti), Spottail Shark, Tawny Nurse Shark (Nebrius ferrugineus), and houndsharks (Triakidae) have been reconstructed for 1950–2016 based on landings data from the Large Marine Ecosystems; LMEs 30–41, 44, and 45 (i.e., Agulhas Current, Somali Coastal Current, Arabian Sea, Red Sea, Bay of Bengal, Gulf of Thailand, South China Sea, Sulu-Celebes Sea, Indonesian Sea, North Australia, Northeast Australia, East Central Australia, West-Central Australia, and Northwest Australia) (Pauly et al. 2020). Although landings data are not a direct measure of abundance, these can be used to calculate population reduction where landings have decreased while fishing effort has remained stable or increased. Across the LMEs, there is no reason to suspect that overall fishing effort has decreased. In fact, as human coastal population continues to grow and as fishing technology and market access improves, fishing effort and power is continuing to increase globally, with some of the highest increases in Southeast Asia (Anticamara et al. 2011, Watson et al. 2013). Catch levels of the whaler sharks grew rapidly through the 1960s and 1970s to a peak of 141,117 t in 1983. Catch then declined to 71,772 t in 2016 representing a 49% decline over 33 years (Pauly et al. 2020). This is equivalent to a 54% reduction when scaled to an average three generation length across the region of the Blacktip Shark (38 years). This level of reduction is not species-specific but is informative for understanding the broader levels of decline in sharks in the region.

Across the regions, the Blacktip Shark is estimated to be sustainably fished in areas where management measures are in place, i.e., Gulf of Mexico, North Atlantic, Australia, and South Africa. Population declines of <20% were estimated in the Gulf of Mexico and South Africa, and although there were 50–79% declines in North Atlantic, the spawning stock fecundity in 2018 did not indicate overfishing. Stock assessments in Australia suggest that while the stock was over-exploited in the past, it has recovered and is currently sustainable. Elsewhere, there is limited information on population trends, however, high demand for the fins, and the presence of intensive fisheries mean that like many other carcharhinids, it will have undergone population reductions. Across the Arabian Seas region, it is suspected to have undergone a population reduction of 30–49% over the past three generation lengths (~39 years). The declines across the LMEs equate to a 54% reduction in all carcharhinids over the past three average generation lengths of the Blacktip Shark (38 years) which is not species-specific but is informative for understanding the broader levels of decline in sharks. Overall, it is suspected that the Blacktip Shark has undergone a population reduction of 30–49% over the past three generations lengths (27–49 years) due to levels of exploitation.