POPULATION DYNAMICS AND FISHERIES ASSESSMENT OF THE BIGEYE THRESHER SHARK ALOPIAS SUPERCILIOSUS IN THE ATLANTIC OCEAN: A COMPARISON BETWEEN NORTH AND SOUTH ATLANTIC FISHING STOCKS

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Pan-Atlantic distribution patterns and reproductive biology of the bigeye thresher, Alopias superciliosus

Publication . Fernandez-Carvalho, Joana; Coelho, Rui; Mejuto, Jaime; Cortés, Enric; Domingo, Andrés; Yokawa, Kotaro; Liu, Kwang-Ming; García-Cortés, Blanca; Forselledo, Rodrigo; Ohshimo, Seiji; Ramos-Cartelle, Ana; Tsai, Wen-Pei; Santos, Miguel N.

The bigeye thresher (Alopias supercilious) is occasionally caught as bycatch in pelagic longline fisheries targeting tunas and swordfish. Still, it is one of the least known and studied of all pelagic sharks, which hinders assessment of the status of its populations. As part of an ongoing cooperative program for fisheries and biological data collection, information collected by fishery observers and through scientific projects from several nations that undertake fishing activities in the Atlantic (Japan, Portugal, Spain, Taiwan, Uruguay and US) was compiled and analyzed. Datasets include information on location, size, sex and, in some cases, maturity stage. A total of 5590 bigeye thresher records collected between 1992 and 2013 were compiled, with sizes ranging from 70 to 305 cm fork length (FL). Considerable variability was observed in size, with tropical regions recording a smaller mean size compared to other regions. The distribution of juvenile and adult specimens also showed considerable variability, and the sex ratios varied between regions and size classes. Median sizes at maturity were estimated at 208.6 cm FL for females and 159.2 cm FL for males. Pregnant females were recorded in the tropical northeast and southwest Atlantic, with these regions possibly serving as nursery areas. The biological and distributional patterns presented in this study provide a better understanding of different aspects of this species in the Atlantic, which can help managers adopt more informed and efficient conservation measures.

2015Journal article

Restricted access

Age and growth of the smooth hammerhead, Sphyrna zygaena, in the Atlantic Ocean: comparison with other hammerhead species

Publication . Rosa, Daniela; Coelho, Rui; Fernandez-Carvalho, Joana; Santos, Miguel N.

The smooth hammerhead Sphyrna zygaena (Sphyrnidae) is a pelagic shark occasionally caught as bycatch in pelagic longline fisheries, but is one of the least studied of all pelagic sharks. Age and growth of S. zygaena was studied along a wide Atlantic region covering both the northern and southern hemispheres. Data from 304 specimens, caught between October 2009 and September 2014, ranging in size from 126 to 253 cm fork length (FL), were analysed. Growth models were fitted using the three-parameter von Bertalanffy growth function (VBGF) re-parameterized to calculate L0 (size at birth). Growth models were fitted to the sample data and data from several back-calculation models. The model fit to the quadratic modified Dahl-Lea back-calculated data seems to be the most appropriate to describe growth in this species, with resulting growth parameters of Linf = 285 cm FL, k = 0.09 year−1 for males and Linf = 293 cm FL, k = 0.09 year−1 for females. Compared with other species of the same genus, estimated growth coefficients for S. zygaena seem to fall in the low to middle range. Although further work is still needed, this study adds to knowledge of the vital life-history parameters of smooth hammerheads in the Atlantic Ocean, which can be used in the management and conservation of this species.

2017Journal article

Open access

Habitat use and diel vertical migration of bigeye thresher shark: overlap with pelagic longline fishing gear

Publication . Coelho, Rui; Fernandez-Carvalho, Joana; Santos, Miguel N.

Pelagic longliners targeting swordfish and tunas in oceanic waters regularly capture sharks as bycatch, including currently protected species as the bigeye thresher, Alopias superciliosus. Fifteen bigeye threshers were tagged with pop-up satellite archival tags (PSATs) in 2012-2014 in the tropical northeast Atlantic, with successful transmissions received from 12 tags for a total of 907 tracking days. Marked diel vertical movements were recorded on all specimens, with most of the daytime spent in deeper colder water (mean depth = 353 m, SD = 73; mean temperature = 10.7 °C, SD = 1.8) and nighttime spent in warmer water closer to the surface (mean depth = 72 m, SD = 54; mean temperature = 21.9 °C, SD = 3.7). The operating depth of the pelagic longline gear was measured with Minilog Temperature and Depth Recorders (TDRs), and the overlap with habitat utilization was calculated. Overlap is taking place mainly during the night and is higher for juveniles. The results presented herein can be used as inputs for Ecological Risk Assessments for bigeye threshers captured in oceanic tuna fisheries, and serve as a basis for efficient management and conservation of this vulnerable shark species.

2015-12Journal article

Open access