1 INTRODUCTION
Marine fish are generally assumed to have high dispersal potential
because of their high dispersal capability at both larval and adult
stages, and the absence of physical barriers to dispersal (Caley et al.,
1996; Hellberg, 2009). Theoretically, the higher dispersal capability of
a species, the lower its inter-population genetic structure. Therefore,
in marine fish species, especially migratory species, the signal of
population differentiation is weak and difficult to detect because of
high levels of gene flow (Gandra, Assis, Martins, & Abecasis, 2020).
Understanding the distribution of genetic diversity of important
commercial species is vital to implementing protection policies and
management regulations (Araki & Schmid, 2010). Genetic diversity
(within and among populations) greatly influences species adaptive
potential to environmental changes, and ultimately determines their
long-term resilience to ecological disturbances (Pauls, Nowak, Bálint,
& Pfenninger, 2013). Additionally, knowledge of genetic structure is
useful for understanding fish spawning migration routes, areas, and
seasons. Such information can help fisheries managers define the
spatiotemporal scales over which they can implement effective stock
management and conservation plans (Bradbury, Laurel, Snelgrove, Bentzen,
& Campana, 2008). Therefore, from a resource management perspective,
understanding the patterns of genetic diversity and levels of gene flow
among populations is a fundamental issue. The effectiveness of marine
protected areas depends on both their ability to self-recruit
(reproductive potential) and the spillover of adults and export of
larvae to nearby fished areas (Harrison et al., 2012; Le Port et al.,
2017). Over the last 30 years, genetic studies have become an essential
tool for stock management and conservation of estuarine, coral reef, and
coastal populations because genetic studies are useful for estimating
genetic diversity and the ability to survive anthropogenic activities
(e.g., overfishing, habitat degradation, eutrophication, invasive
species, and pollution) (Gaither, Toonen, Robertson, Planes, & Bowen,
2010; Gill & Kemp, 2002; Machado, da Silva Cortinhas, Proietti, &
Haimovici, 2020; Ruzzante, Taggart, & Cook, 1998; Ryman, Utter, &
Laikre, 1995). Mitochondrial DNA (mtDNA) markers are widely employed to
detect population structure in marine species because they have large
number of copies, high mutation rates, generally maternal inheritance,
and almost nonexistent recombination. The primary advantages of mtDNA
are the inheritance pattern and nonexistent recombination: clonal
inheritance through the maternal line allows tracing of speciation
events over multiple generations, and male dispersal does not homogenize
the population (Prugnolle & de Meeus, 2002). These factors make mtDNA
markers particularly appropriate indicators of population genetic
structure in marine organisms such as zooplankton and migratory fishes,
which generally have high gene flow (Machado et al., 2020; L. Wang et
al., 2013; Xu, Li, Wang, & Du, 2019).
Threadfin porgy, Evynnis cardinalis (Lacepède 1802), occurs in
the Indo-West Pacific from Japan, Korea, and China to Vietnam and
Indonesia (Z. Chen & Qiu, 2005). This species is primarily distributed
from 30–60 m depth but also can occur to 100 m depth (Iwatsuki &
Carpenter, 2014). This species is found over a wide range of bottom
types, but is more common close to reefs or on rough bottoms. Small
individuals are very common at some localities in shallow, sheltered
bays; larger fish often occur in deeper water (Eggleston, 1974).Evynnis cardinalis is one of the main commercial fishing targets
in bottom trawl fisheries in the northern South China Sea and is
considered to have three geographical stocks in the northern South China
Sea: the Taiwan Strait, South China Sea, and Beibu Gulf stocks.Evynnis cardinalis is a migratory fish that is captured
year-round with significant seasonal differences among fishing areas. In
Beibu Gulf, E. cardinalis undergoes seasonal migration and is
found toward the northeastern shallow area of the gulf in late autumn
and winter. Spawning takes place in northwest Weizhou Island in the
spring, and the recruits disperse widely in the northern area of the
gulf and begin to disperse south in late spring (Iwatsuki & Carpenter,
2014). However, this species exhibits life history characteristics,
including late maturity and longevity, that predispose it to impacts
from heavy exploitation. In the northern South China Sea, the Beibu Gulf
stock of E. cardinalis declined 58% from 2001 to 2005, whereas
the Taiwan Strait stock declined 62% from 1993 to 2008. Therefore, a
recent IUCN Red List assessment reported E. cardinalis as
endangered, and this species is mainly threatened by overexploitation
(Iwatsuki & Carpenter, 2014). Previous studies on this species in Beibu
Gulf have focused on feeding habits, growth and mortality, ecological
distribution, phylogeny, and stock density (Cai, Chen, Xu, & Zhang,
2017; Z. Chen & Qiu, 2005; Z. Z. Chen & Qiu, 2003; D. Zhang, Shao, Su,
& Jiang, 2007; K. Zhang et al., 2020; Y. Zhang, Dai, Yan, Yang, & Lu,
2014). However, no information is available to date on the genetic
structure of E. cardinalis in the northern South China Sea, and
this prevents reliable stock assessments and protection policy
formulation. The objective of this study was to
provide a population genetic
analysis using a portion of the mitochondrial cytochrome c oxidase
subunit I (COI) gene to assess the population genetic diversity pattern
and historical demography, and estimate E. cardinalis expansion
time in Beibu Gulf.