| More than 70% surface area of the Planet Earth is ocean.The ocean provides abundant resources for human boom.With the development of the worldwide commercial marine fisheries,more and more economic fish species are facing overfishing and loss of their biomass and genetic diversity,which is a problem especially for the offshore fishery.It is imperative to consult an appropriate fishing plan to implement effective management and conservation.Identification of species population structure is necessary for managing species effectively.On the one hand,ecological and biological characteristics of fishes could shape the population structure,such as early life strategy,pelagic eggs or larval duration,dispersal of adults and spawning and recruitment.On the other hand,topographical and hydrographic features with past climatic events such as,closure of Tethys and strait of Gibraltar,Mid-Atlantic Ridge and sea level change during the last glacial period could act as barriers to reduce gene flow and connectivity of some species.Water depth less than 200 meters is epipelagic,and from 200 meters to 1000 meters is mesopelagic,1000 m to 4000 m is bathypelagic.Beyond 4000 is the abyssopelagic.The deep-sea environment is relatively stable comparing with the offshore area and lock of seasonal currents and diversiform climate.So most species survived at deep-water has homogeneous genetic structure,such as the black scabbardfish and Polyprion americanus.The Portuguese dogfish could migrate thousands kilometers in the northeast of the Atlantic,and due to the closure of the Strait of Gibraltar Mediterranean Sea and the Mediterranean in making the Atlantic species gene communication interruption,thus forming two distinct groups.The mid-Atlantic ridge is a rugged steep ridge in central Atlantic,which divides the Atlantic sea into two large East and West basins.Studies on Atlantic bivalve have proved that the mid-Atlantic ridge could be a barrier gene exchange.Different from studying inshore species,challenges in obtaining samples at bathyal depths hinder researchers performing analysis of population structure and examining the connectivity of populations of Hydrolagus affinis.The biological studies,such as diet as well as life history are also missing for this species.In this article,we used the target enrichment and next-generation sequencing to investigate the patterns of genetic variation of small-eyed rabbitfish H.affinis,which is one of cartilage fishes with a broad bathymetric and geographical distribution.We collected 22 samples from the Northwest Atlantic(NWA),the Mid-Atalntic(MA)and the Northeast Atlantic(NEA).We obtained a complete data set with 583 single nucleotide polymorphisms(SNPs)with an outgroup taxon.The low pairwise Fst value(0.00213-0.01417)between different locations indicates that no significant spatial differences exist.STRUCTURE analysis revealed weak population structure,and the same results was validated by reconstruction of phylogenetic tree using RAx ML software and principal component analysis(PCA).However the result of MIGRATE-N analysis does indicate limited gene flow between different populations,and the relative contribution of migrants between different populations was different,suggesting that the Mid-Atlantic mountain range could act as a barrier to gene flow.The Merluccius is an important economic coastal teleost fishes and widely distributed in the coastal waters of the Atlantic,Pacific.During 2007-2009,the annual catch of fishes was more than 2 million tons.Nearly half a century of overfishing,lead the biomass of hakes reduced by 80% comparing before commercial fishing.Most of the existing hake species have similar morphological characteristics and overlapping distribution,so it is difficult to distinguish for classification management and protection.Marine realm warmer temperatures are causing rapid northwards expansion including offshore hake,and result in overlapping distribution areas between the 34 and the 42° N parallels.In last century,some unscrupulous traders mixed the offshore hake to silver sales to the European market,until 1990's fishers set up to catch offshore hakes.According to the whole otolith morphometrics and growth rates,two stocks of silver hake have been tentatively identified and the line dividing these two stocks occurs somewhere between Nantucket Shoals and the northern edge of Georges Bank.However,the stocks of offshore hake are not clear.From the RAx ML genes trees and STRUCTURE analysis of this study,we can discriminate the difference silver hake and offshore hake,the result was verified by PCA.We can also divide the two stocks of silver hake,but strong gene flow blurred the George boundaries.In addition,from the IMa2 and MIGRATE-N software,the asymmetry gene flow between the Gulf of Mexico and the Atlantic coast revealed that the Gulf of Mexico is the shelter of offshore hake.Therefore,the Gulfs should be set as a priority for conservation. |
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