Freddy Arocha

Methodological assessment of 2b-RAD genotyping technique for population structure inferences in yellowfin tuna (Thunnus albacares)

Global population genetic structure of yellowfin tuna (Thunnus albacares) is still poorly understood despite its relevance for the tuna fishery industry. Low levels of genetic differentiation among oceans speak in favour of the existence of a single panmictic population worldwide of this highly migratory fish. However, recent studies indicated genetic structuring at a much smaller geographic scales than previously considered, pointing out that YFT population genetic structure has not been properly assessed so far. In this study, we demonstrated for the first time, the utility of 2b-RAD genotyping technique for investigating population genetic diversity and differentiation in high gene-flow species. Running de novo pipeline in Stacks, a total of 6772 high-quality genome-wide SNPs were identified across Atlantic, Indian and Pacific population samples representing all major distribution areas. Preliminary analyses showed shallow but significant population structure among oceans (FST=0.0273; P-value<0.01). Discriminant Analysis of Principal Components endorsed the presence of genetically discrete yellowfin tuna populations among three oceanic pools. Although such evidence needs to be corroborated by increasing sample size, these results showed the efficiency of this genotyping technique in assessing genetic divergence in a marine fish with high dispersal potential.

The Recovery of Atlantic Swordfish: The Comparative Roles of the Regional Fisheries Management Organization and Species Biology

Regional fisheries management organizations (RFMO) are often criticized for ineffectual management of high-seas fisheries resources. However, in the case of the two Atlantic swordfish stocks occurring in the North and South Atlantic, those stocks have rebuilt to the BMSY objective of the responsible RFMO, the International Commission for the Conservation of Atlantic Tunas (ICCAT). The relative contribution of the management actions of the RFMO and biological characteristics of swordfish stocks are evaluated in relation to the recovery of the stocks. It is concluded that while swordfish have characteristics that promote stock resilience (including relatively fast growth, and spatially- and temporally-dispersed spawning), positive management actions combined with a period of relatively good recruitment were essential to achieve the rebuilt outcome. The challenges that the RFMO faces to maintain the stocks in the rebuilt condition are described, and some possible additional measures discussed.

Comparative population genetics and evolutionary history of two commonly misidentified billfishes of management and conservation concern

BackgroundMisidentifications between exploited species may lead to inaccuracies in population assessments, with potentially irreversible conservation ramifications if overexploitation of either species is occurring. A notable showcase is provided by the realization that the roundscale spearfish (Tetrapturus georgii), a recently validated species, has been historically misidentified as the morphologically very similar and severely overfished white marlin (Kajikia albida) (IUCN listing: Vulnerable). In effect, no information exists on the population status and evolutionary history of the enigmatic roundscale spearfish, a large, highly vagile and broadly distributed pelagic species. We provide the first population genetic evaluation of the roundscale spearfish, utilizing nuclear microsatellite and mitochondrial DNA sequence markers. Furthermore, we re-evaluated existing white marlin mitochondrial genetic data and present our findings in a comparative context to the roundscale spearfish.ResultsMicrosatellite and mitochondrial (control region) DNA markers provided mixed evidence for roundscale spearfish population differentiation between the western north and south Atlantic regions, depending on marker-statistical analysis combination used. Mitochondrial DNA analyses provided strong signals of historical population growth for both white marlin and roundscale spearfish, but higher genetic diversity and effective female population size (1.5-1.9X) for white marlin.ConclusionsThe equivocal indications of roundscale spearfish population structure, combined with a smaller effective female population size compared to the white marlin, already a species of concern, suggests that a species-specific and precautionary management strategy recognizing two management units is prudent for this newly validated billfish.

Reproductive biology and space–time modelling of spawning for sailfish Istiophorus platypterus in the western Atlantic Ocean

ABSTRACT This study presents data and estimates reproductive parameters of the sailfish Istiophorus platypterus (Shaw, 1792) in the western Atlantic, useful for future stock assessments of the species in the Atlantic Ocean. During the period 2006–2011, a total of 1132 fish were sampled by scientific observers on board Venezuelan and Brazilian longline vessels and from artisanal fisheries based out of Cumaná, Puerto La Cruz and Playa Verde, Venezuela and Cabo Frio, Rio de Janeiro, Brazil. The ovaries of 543 females were classified into five ovarian development phases based on microscopic analysis of the most advanced group of oocytes. Generalized additive models were applied to the gonadal index and sex ratio data in order to assess spatio-temporal trends of the reproductive activity and identify the spawning areas. Space–time effect was a significant factor contributing to changes in sex ratio and reproductive activity. Spawning activity was observed in two distinct areas. On the south-east Brazilian coast spawning peaked during December–February, while in the south-eastern Caribbean Sea spawning occurred between March and September, with its peak in March and April. Using a Bayesian approach, size at 50% maturity was estimated to be 146.12 cm (credibility interval of 95%, 138.45–152.09 cm). Batch fecundity ranged from 436,800 to 2,492,500 hydrated oocytes per female. This study confirms multiple aggregation spawning for this species in the western Atlantic Ocean.