There are no data available on the global population size of the Smooth Hammerhead. Genetic studies reveal structure between the Atlantic and Indo-Pacific, however contrasting results may indicate female philopatry and male mediated gene flow (Testerman 2014).

Population trend data are available from four sources: (1) stock assessment in the Northwest Atlantic and Gulf of Mexico (Jiao et al. 2011), and from standardized catch-per-unit-effort (CPUE) in: (2) the Northwest Atlantic and Gulf of Mexico (J. Carlson unpubl. data); (3) the South Pacific (Reid et al. 2011) and, (4) the Indian Ocean (Dicken et al. 2018). The trend data from each source were analyzed over three generation lengths using a Bayesian state-space framework (a modification of Winker et al. 2018). 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 generations (see the Supplementary Information).

First, the Northwest Atlantic stock assessment for 1981–2005 indicated that the stock was overfished from the mid-1980s with overfishing occurring continuously from 1994–1998, and the risk of overfishing generally low after 2001 when catches dramatically reduced (Jiao et al. 2011). A previous stock assessment found significant historical declines and that abundance had stabilized since the late 1990s (Hayes 2008); however, both stock assessments report a high degree of uncertainty mostly due to poor catch data that includes very low sample sizes, unreliable data due to the scarcity of Smooth Hammerhead in fisheries catches, and issues with accurate species identification (Miller 2016). The trend analysis of the Northwest Atlantic modeled abundance for 1981–2005 (25 years) revealed an annual rate of reduction of 7.0%, consistent with an estimated median reduction of 99.6% over three generation lengths (72.3 years), with the highest probability of >80% reduction over three generation lengths.

Second, more recent data (1992–2017) are available from Northwest Atlantic and Gulf of Mexico comprising the pelagic longline observer time-series underlying the Jiao et al. (2011) stock assessment for Smooth Hammerhead (J. Carlson unpubl. data). The time-series indicate this population has slowly begun to increase after the implementation of management after 2005 (NMFS 2006). The trend analysis of the CPUE for 1992–2017 (26 years) revealed an annual rate of increase of 0.8% with a median increase and the highest probability of an increase over three generation lengths (72.3 years).

Third, hammerhead catches in the New South Wales (Australia) beach meshing program, which comprised predominantly Smooth Hammerhead, fluctuated over 60 years with strong declines from 1990–2010 (Reid et al. 2011). The trend analysis of the CPUE for 1950–2009 (60 years) revealed an annual rate of reduction of 3.0%, consistent with an estimated median reduction of 17.9% over three generation lengths (72.3 years), with the highest probability of <20% reduction over three generation lengths.

Fourth, the Smooth Hammerhead CPUE in the South Indian Ocean Natal Shark’s Board bather protection netting program varied markedly over 1978–2014 with a slight, but not significant, increasing trend (Dicken et al. 2018). The analysis of the CPUE for 1978–2014 (37 years) revealed an annual rate of increase of 0.6% with a median increase and the highest probability of an increase over three generation lengths (72.3 years).

Further to the above data and trend analysis, standardized CPUE data from the Portuguese longline fishery in the Eastern Central and Southwest Atlantic for 2008–2016 indicated fluctuations for 2008–2010, a decrease till 2013 then an increase back to 2008 catch levels in 2016 (Santos and Coelho 2019). We considered a recent analysis of fishery-dependent longline CPUE from the South Atlantic. These time-series are complex, reflecting access by different fleets and nationalities over time, but the best estimates of trends come from phase B spanning 1998 to 2008 during which twenty fleets (100 vessels) fished for tunas, swordfishes, and sharks. During this 10-year period there was a 61.7% decline in CPUE of hammerheads (Sphyrna spp.) (Table S11, Barreto et al. 2016), however, because the data were not considered robust due to the low catch rates and large confidence intervals (R. Barreto unpubl. data), they were not analyzed over three generations.

In the Southeast Pacific, landings of Smooth Hammerheads in Peruvian small-scale gillnet fisheries (1996–2010) peaked in 1998 and 2003 at ~1,500 tonne annually, but for the remainder of the period were fairly stable at 200–300 tonne annually (Gonzalez-Pestana et al. 2016). Without any effort data, it is not possible to ascertain if this is a reflection of stock abundance trends (Miller 2016). Population trend data from the North Pacific and Western Central Pacific is limited. In the North Pacific, landings of Smooth Hammerhead from Taiwan comprise a small proportion (1.38%; ~78 tonne) of the annual catch and although catches have declined in recent years, so too has the fishing effort and it is not possible to determine a population trend (Miller 2016). In the Western Central Pacific, hammerhead catches are for the species-complex, with no indication of the proportion of Smooth Hammerheads. The CPUE (1995–2015) was highly variable and as Smooth Hammerhead are captured infrequently, little inference was possible from the temporal trend (Rice et al. 2015). An analysis of Smooth Hammerhead yield- and biomass-per-recruit on the Kerala coast, India in the Northern Indian Ocean for 2008–2009, indicated that the stock was over-exploited (Manjusha et al. 2011). Fishing pressure has significantly increased in recent decades across the distribution of the Smooth Hammerhead in the tropical Western Indian Ocean (Cooke 1997, Jabado et al. 2017), and has likely caused declines in this region.

Steep declines of hammerheads (Sphyrna spp.) have occurred in the Mediterranean Sea; Ferretti et al. (2008) compiled nine time-series of abundance indices from commercial and recreational fishery landings, scientific surveys and sighting records, to reconstruct long-term population trends of large sharks in the northwest Mediterranean Sea. Of the taxa for which there were enough data to investigate, hammerhead sharks declined the fastest; disappearing from coastal waters after 1963 followed by declines (across all fishing sectors) in pelagic waters by the early 1980s. Meta-analysis showed an average instantaneous rate of decline in abundance of -0.17 (time range 178 years) and biomass of -0.36 (time range 107 years), which equates to an estimated decline of 99.99% in abundance and biomass since the early 19th century (Ferretti et al. 2008). Walker et al. (2005) also report that hammerhead species have virtually disappeared from the central-southern Mediterranean Sea since 1986.

The Smooth Hammerhead trend analyses estimated historic declines in the Atlantic and South Pacific, and an increase in the Southern Indian Ocean. There are some signs of stabilization and possible recovery in response to management in the Northwest Atlantic and Gulf of Mexico, fluctuating but overall stable catches in the Eastern Central and Southwest Atlantic in 2016, over-exploited stocks in the Northern Indian Ocean, and likely declines in the tropical Western Indian Ocean due to intense fishing pressure. To estimate a global population trend, the estimated three generation population trends for each region (Atlantic, South Pacific, and Indian) were weighted according to the relative size of each region; the two sources of North Atlantic were used to generate two global trends. This resulted in a median reduction of 21.8–64.8%, with the highest probabilities of <20% and >80% reduction over three generation lengths (72.3 years). The second Northwest Atlantic data that includes the period after management changes led to a far lesser estimated global reduction, yet it needs to be considered that all time-series are from limited areas within each region and may not accurately represent the trend in Smooth Hammerhead across the entire region. There is uncertainty in some of the catch data and levels of exploitation; the regions with no trend data are likely areas where the species is under the most pressure, such as the tropical Western Indian Ocean. Yet, the Smooth Hammerhead is a generally more temperate species than the Scalloped Hammerhead and Great Hammerhead and thus, the majority of its distribution is not as exposed to the same level of threat from intense artisanal fisheries. Based on the considerations of possible recovery in areas with managed fisheries, potential declines in intensively fished data-poor regions, and less relative exposure to artisanal fisheries than the other two large hammerhead species, expert judgement elicitation inferred a global population reduction of 30–49% over three generation lengths (72.3 years). More robust species-specific data and monitoring of catches is required to improve certainty of catch estimates for a future assessment of this species.