Molecular and Morphological Identification and Seasonal Distribution of Eggs of Four Decapterus Fish Species in the Northern South China Sea: A Key to Conservation of Spawning Ground

Gang Hou,Jinrun Wang,Wangsu Huang,Zuozhi Chen,Hui Zhang,Jinlong Zhou

doi:10.3389/fmars.2020.590564

Abstract

Although species of Decapterus form important pelagic fisheries in the northern South China Sea, information on their spawning grounds is limited because identification of fish eggs based on their morphology is difficult. We identify eggs of four Decapterus species (D.macrosoma, D.maruadsi, D.macarellus, and D.tabl) with DNA barcodes from the fishery resources surveys in Spring and Autumn 2018 in Xisha islands, and Spring and later summer-Autumn 2019 along the continental shelf of the northern South China Sea, and describe egg morphology. Of 1405 fish eggs with obtained COI sequences, 81 were successfully attributed to four Decapterus species; eggs of each are spherical, have a smooth chorion and narrow perivitelline space, and can be partly differentiated by diameter, melanophore drops on the oil globule, and the notum of the embryo. Seasonal distributions of eggs reveal spawning grounds, with that of D.maruadsi located mainly off the Pearl River estuary in spring; eggs of D. maruadsi rarely co-occur with those of D.macrosoma. Spawning grounds of D.macrosoma are probably further south, where water temperature and salinity are higher. Spawning periods of these four Decapterus species overlap slightly. Spawning habitat D.maruadsi has been lost, and the spawning season of D.macrosoma has extended. Identification of Decapterus eggs using DNA barcodes can assist with the identification of eggs using traditional morphological approaches. Spatial and temporal information on the distributions of Decapterus eggs can be used for improved conservation of spawning grounds and fisheries management in the northern South China Sea.

Highlights

Pelagic fish play important roles in marine ecosystems, occupy a large proportion of commercially important global fisheries, and are considered increasingly important as a source of protein (Lainez del Pozo, 2013; Food and Agricultural Organization [FAO], 2019)
All sequences of Decapterus eggs were of high quality, with no evidence of insertions/deletions, heterozygous sites, or stop codons; each has been uploaded to NCBI (Genbank accession numbers: MT609955–MT610035)
The 81 sequences were 619 nucleotides in length after alignment and trimming of noisy sites; a BOLD Blast search identified four candidate species belonging to four species of genus Decapterus; all egg sequences reached a match of similarity 98–100%, with matching results for the nearest neighbor ranging similarity 90.78–100%

Summary

Introduction

Pelagic fish play important roles in marine ecosystems, occupy a large proportion of commercially important global fisheries, and are considered increasingly important as a source of protein (Lainez del Pozo, 2013; Food and Agricultural Organization [FAO], 2019). Scads in the genus Decapterus are commercially important pelagic fish. The small Japanese scad D. maruadsi is commercially important in China and, from 1998 to 2006, was the first number catch in the northern South China Sea (SCS). The average catch of this species decreased by 28%, relegating it to the third number catch in 2007–2018, with catch varying considerably between years (China Agriculture Press), 1997–2019). A fourth species, Roughear scad D. tabl, found on the edge of the continental shelf from 200 to 360 m, is uncommon (Froese and Pauly, 2010)

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