Vulnerability assessment and simulation of potential conservation and management measures for silky and hammerhead sharks caught in eastern Pacific ocean pelagic fisheries

Silky and hammerhead sharks are frequently caught—either as a target or incidental catch (i.e., bycatch)in the industrial and artisanal pelagic fisheries in the eastern Pacific Ocean (EPO). These species are slow growing, long -lived, and have low reproductive output, leading to concerns about their long-term sustainability in the EPO. In 2016, the IATTC implemented Resolution C-16-05, which called for, among other things, a workplan to complet e stock assessments for four species : silky shark ( Carcharhinus falciformis), scalloped hammerhead ( Sphyrna lewini ), great hammerhead ( Sphyrna mokarran), and smooth hammerhead ( Sphyrna zygaena). However, a lack of reliable long -term time series of abundance has hampered stock assessments for silky shark, wh ich was attempted by the IATTC in 2014 and expanded to a Pacific-wide stock assessment in 2018. To address this critical data need, the IATTC has conducted research to develop shark sampling programs in Central America . In the meantime, the IATTC has used the EASI -Fish ecologically risk assessment approach developed by IATTC staff for data-limited species and fisheries, to assess the vulnerability of these species under 43 hypothetical scenarios involving practical conser vation and management measures (CMMs )—used in isolation and concert —to guide future research and management efforts. Several of the 43 CMM scenarios resulted in a significant reduction in the vulnerability status of all four species, although none resulted in a species being reclassified as “least vulnerable”. The CMMs having the greatest positive impact was similar for all four species, imposing EPO-wide closures of 120 or 180 days, especially for the industrial longline fishery, due to its large spatial effort footprint that overlaps significantly with the distribution of the four species. Although other scenarios such as banning wire traces, imposing a 100 cm total length minimum retention length for all sharks, and even prohibiting landing of all sharks was predicted to greatly reduce at-vessel mortality, this positive effect on vulnerability was mostly negated due to high post -release mortality of these species. These results highlighted that the most effective mitigation measure for these sharks is to avoid interaction with EPO fisheries. However, there are significant socioeconomic factors to consider, as temporary fishery closures, especially for industrial and artisanal longline fisheries, are likely to great ly reduce the catch of target species (e.g., tuna and billfish) or move effort to the eastern region of the western and central Pacific Ocean where these fisheries may continue to impact the species that the measure was designed to protect...