Genetic studies support the presence of multiple subpopulations throughout its range, with structure detected between and within island groups in the Pacific (Vingnaud et al. 2014). How many subpopulations exist is currently unknown because of limited sampling of most locations.

The Blacktip Reef Shark in common in tropical and subtropical waters but there is little data available on population status and trends. Severe localized depletions in reef sharks, that include the Blacktip Reef Shark, have been recorded from the Indian Ocean (Graham et al. 2010). Based on diving survey data, Graham et al. (2010) showed that the number of reef sharks in the Chagos Archipelago (Indian Ocean) declined from a mean of 4.2 sharks per dive in 1970 to 0.4 in 2006, which represents a population reduction of 94% over three generations (44 years). Species-specific data were available for the period between 1996 and 2006 and showed a decline in the Blacktip Reef Shark to a level where it was rarely encountered in 2006 (Graham et al. 2010). Towed-diver surveys performed in the Western Central Pacific between 2004 and 2010 showed that reef shark densities are higher at remote locations with very limited or no human presence (Nadon et al. 2012). Based on a modelling approach, they estimated that reef sharks (including the Blacktip Reef Shark) in this region have declined to 3–10% of baseline levels, with baseline considered absence of humans (Nadon et al. 2012). Approximately 6% of all the sharks recorded during towed surveys (4,620 sharks) were the Blacktip Reef Shark, which was not present at all sites. Based on this information it is suspected that reef shark populations, including the Blacktip Reef Shark, have been reduced in the Western Central Pacific, particularly in islands such as the Hawaiian Main Island, Mariana Islands, and Samoa that have high human population densities (Chin et al. 2011, Nadon et al. 2012). Given the high level of population structuring in this species, localised studies are unlikely to provide a good indication of overall population trend. In many parts of south and east Asia (e.g. Sri Lanka, Myanmar) this is the only reef shark species that remains, following the loss of species such as the Grey Reef Shark and Whitetip Reef Shark, suggesting that population declines are not as severe as reported for these other species.

To account for differences in the trend of Blacktip Reef Shark between locations spatial variation in an index of population abundance was constructed using data from the Global Finprint project (MacNeil et al. 2020) that sampled in nations containing 88.6% of the coral reefs within the species’ historic range. This is the largest and most recent data set available to assess the status of this species. The analysis estimate depletion by reef by comparing local abundance indices to a level inferred to represent no depletion (median of positive values) (see Supplementary Information). Reef level depletion estimates were aggregated by jurisdiction and weighted by jurisdictional coral reef area (relative to total global coral reef area) to produce an estimate of global depletion. The coral reef area weighted population depletion of Blacktip Reef Sharks was estimated as 51.2% (standard error 43.3–59.6%) and assumed to have occurred over the past three generation lengths (44 years) (see Supplementary Information). The error estimates include both the IUCN Red List Vulnerable and Endangered categories. However, the ability of the Global Finprint data to estimate depletion for this species is limited by complex ecological patterns not accounted for in the methodology. As a result it over-estimates the level of decline at some reefs where population levels are thought to be healthy. As such, it is estimated that the Blacktip Reef Shark population has undergone a population reduction of 30–49% over the past three generation lengths (44 years).