Distribution patterns and habitat use of the smooth hammerhead shark (Sphyrna zygaena) in the Atlantic Ocean
The smooth hammerhead shark, Sphyrna zygaena, is a cosmopolitan pelagic shark mostly captured as bycatch by industrial longline fleets targeting swordfish in the inter-tropical Atlantic Ocean. Listed by the IUCN as “Vulnerable”, S. zygaena is currently under international protection. Nevertheless, the current information on life history, movement patterns, essential habitat and population dynamics is still scarce over most of its range. This study aimed to improve the knowledge on distribution patterns and habitat use of this species in the Atlantic Ocean. To this end, fishery observer data, collected between 2003 and 2016 from the Portuguese pelagic longline fishery in the Atlantic Ocean were analyzed. Datasets included information on the catch and effort (used to calculate CPUEs, catch per unit of effort), size and sex of the smooth hammerhead shark. A total effort of 2 523 288 hooks yielded 638 sharks, ranging in size from 123 cm to 275 cm fork length (FL). Results confirmed the wide latitudinal range of distribution of this species in the Atlantic Ocean, with higher CPUEs found closer inshore within the Tropical North and Equatorial regions. The larger sharks seemed to occur in the open ocean habitat and the smaller specimens in more coastal areas, while the sex ratio distribution showed a predominance of males in the sampled area of the Atlantic Ocean (overall sex ratio of 1.4 males for each female). Differences in CPUE and size distributions were also detected spatially and temporally. We highlight the increasing trend in the mean CPUE along with a decrease in the mean specimen size in the Equatorial region from 2012 onwards. Furthermore, eight smooth hammerheads were tagged with Pop-up Satellite Archival Tags (PSATs) in the inter-tropical region of the Northeast Atlantic Ocean, between 2012 and 2016, with successful transmissions received from seven tags (total of 319 tracking days). The findings of this work confirmed the ability of the species to travel significant distances, as the longest migration ever documented for the smooth hammerhead shark (> 6600 km) was recorded. The smooth hammerhead did not exhibit a diel vertical movement behavior, swimming mostly at surface waters (0-50 m) above 23 °C. However, differences in the vertical habitat utilization were found when comparing adults and juveniles, with the juveniles staying in deeper colder waters during nighttime. To assess the overlap between the species vertical distribution and the fishing depth of the Portuguese pelagic longline fishery, Minilog Temperature and Depth Recorders (TDRs) were deployed on 60 fishing sets. The overlap between species habitat and fishing gear deployment is taking place mainly during the night and is higher for juveniles. The results presented in this work provide a better understanding of the smooth hammerhead shark spatio-temporal dynamics, population structure, habitat use, and habitat overlap and potential impacts with pelagic longline fishing gear in the Atlantic Ocean. These results can now be used to provide better scientific advice and further improve the current species conservation measures.