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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

Modeling age and growth of the bigeye thresher (Alopias superciliosus) in the Atlantic Ocean

Publication . Fernandez-Carvalho, Joana; Coelho, R.; Erzini, Karim; Santos, Miguel N.

The bigeye thresher (Alopias superciliosus) is a pelagic shark captured as bycatch in pelagic longline fisheries. Important information on its biology is still missing, especially from the Atlantic Ocean. In all, 546 vertebrae collected by fishery observers between 2007 and 2009 were used to estimate age and growth parameters for this species in the Atlantic Ocean. The size composition was 102–265 cm fork length (FL) for females and 94–260 cm FL for males. The estimated ages ranged from 0 to 25 years for both sexes. From the 5 growth models used, the 3-parameter von Bertalanffy growth model, reparameterized to estimate length at birth (L0), produced the best results. The estimated parameters were asymptotic maximum length (Linf)=284 cm FL, growth coefficient (k)=0.06/year, and L0=109 cm FL for females and Linf=246 cm FL, k=0.09/year, and L0=108 cm FL for males. Although differences between hemispheres indicate slower growth rates in the South Atlantic Ocean, these differences may also have been caused by the lower sample size and larger specimen sizes for the Southern Hemisphere. The estimated growth coefficients are among the lowest found for the Alopiidae, highlighting the bigeye thresher’s slow growth and consequent low resilience to fishing pressure.

2015-09-22Journal article

Open Access

Population dynamics and fisheries assessment of the bigeye thresher (Alopias superciliosus) in the Atlantic: a comparison between North Atlantic and South Atlantic stocks

Publication . Carvalho, Joana Fernandez de; Erzini, Karim; Santos, Miguel Neves dos; Coelho, Rui

The bigeye thresher shark, Alopias supercilious is an oceanic pelagic shark, occasionally caught as bycatch in pelagic longline fisheries targeting tunas and swordfish in the Atlantic Ocean. It is particularly vulnerable to fishing pressure, with overexploitation occurring even at low levels of fishing, due to their slow growth, extremely low fecundity and migratory nature crossing both national and international waters. The present study focused on several aspects of the biology and fisheries of this species, including fisheries impact, gear modification for bycatch mitigation, life history, distribution patterns and habitat utilization. The bigeye thresher was amongst the shark species with the highest hooking mortality rates (around 50%) by the Portuguese pelagic longline fleet. When testing possible gear modification to reduce the bycatch, the use of circle hooks or mackerel bait (instead of J-style hook and squid bait) does not seem to be an effective mitigation measure. Life history parameters, specifically age, growth and size at maturity were estimated, with the observed growth coefficients (k values) being the lowest ever presented for the species and within the Alopiidae family. Three nursery areas were proposed along the Atlantic Ocean, specifically in the tropical northeast Atlantic and equatorial waters closer to the African continent, in the tropical northwestern Atlantic in areas closer to the Caribbean Sea and Florida, and in the southwest Atlantic closer to the Rio Grande Rise. Finally, satellite telemetry was used to study habitat use and vertical migrations of this species, and when compared to the depth of operation of the longline fishing gear resulted that most of the overlaps between habitat and gear deployment occur during the night and seem to affect more the juveniles. The results presented in this thesis are being integrated into demographic models and being used for stock assessment and ecological risk assessment analysis for pelagic elasmobranchs. Furthermore, the new information can be used to evaluate the impact of recent recommendations prohibiting the retention of some vulnerable elasmobranch species, and assist fishery managers to adopt more informed and efficient conservation measures for this species in the Atlantic.

2015-07-24Doctoral thesis

Open Access

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

An overview of the hooking mortality of elasmobranchs caught in a swordfish pelagic longline fishery in the Atlantic Ocean

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

Hooking (or “at-haulback”) fishing mortality was analysed in elasmobranchs captured by Portuguese longliners targeting swordfish in the Atlantic Ocean. Information was collected by on-board fishery observers who monitored 834 longline fishing sets between August 2008 and December 2011, and recorded information on 36,067 elasmobranch specimens from 21 different taxa. The hooking mortality proportions were species-specific, with some species having relatively high percentages of live specimens at time of haulback (e.g., blue shark, crocodile shark, pelagic stingray, manta, devil and eagle rays), while others had higher percentages of dead specimens (e.g., smooth hammerhead, silky shark, bigeye thresher). For the most captured species, logistic GLMs were carried out to compare the mortality rates between sexes, specimen sizes and the regions of operation of the fleet. The sex-specific proportions of hooking mortality were significantly different for blue and crocodile sharks, with the males of both species having higher proportions of hooking mortality than the females. Specimen size was significant for predicting the hooking mortality for blue and shortfin mako sharks: in both cases, the larger specimens had lower odds of dying due to the fishing process. There were differences in the hooking mortality depending on the region of operation of the fleet, but those differences were also species-specific. For blue and crocodile sharks, the hooking mortality was higher in the Equatorial and southern Atlantic areas (when compared to the NE Atlantic region), while the opposite was observed for the shortfin mako, with lower mortality rates in the NE tropical area compared with the other regions. The results presented in this paper can be integrated into future ecological risk assessment analysis for pelagic elasmobranchs. Furthermore, the new information can be used to evaluate the impact of recent recommendations prohibiting the retention of some vulnerable elasmobranch species.

2012-11-21Journal article

Open Access