Spatial Variation in Fourfinger Threadfin

A total of 66 Eleutheronema tetradactylum individuals were collected through commercial fishing in Southern China from five locations: Zhangzhou (ZZ), Naozhou (NZ), Jianghong (JH), Dongxing (DX), and Wenchang (WC) (Table 1, Fig. 1), across a spatial scale of 1200 km. The samples from NZ were collected twice (Nov. 2020 and Nov. 2021), whereas the sampling from other four locations were collected in Nov. 2021. Fourfinger threadfin individuals were sampled by commercial fishery boats. Due to the high price in local fish markets and vulnerable state of this endangered species, the number of samples in this study was relatively low. Nonetheless, some previous studies demonstrated the efficacy of otolith microchemistry in revealing fish life history and population structure based on small sample sizes^{29 (link),30 (link)}. Although previous studies reported E. tetradactylum in the East China Sea and the South China Sea^{31 (link),32 (link)}, our survey on the important fishing ports along the coasts of China from Yangtze River estuary to Guangxi only found E. tetradactylum at the coasts of Southern China. Based on the von Bertalanffy growth curve of E. tetradactylum fitted by the previous study³³ , we used the total length of each specimen in this study to estimate the age of E. tetradactylum samples, and all individuals were of the same age (4 +).Table 1

Sampling locations and sample characteristics of fourfinger threadfin, Eleutheronema tetradactylum, collected from China.

Location	Sampling date	Population code	Number	Total length (cm, mean ± SD)
Zhangzhou	2021.11	ZZ	7	62.2 ± 2.36
Naozhou 2021	2021.11	NZ-21	13	60.3 ± 4.12
Naozhou 2020	2020.11	NZ-20	15	61.3 ± 3.97
Jianghong	2021.11	JH	15	59.0 ± 3.57
Dongxing	2021.11	DX	7	61.0 ± 2.00
Wenchang	2021.11	WC	9	62.8 ± 4.32

Figure 1

The locations where the fourfinger threadfin, Eleutheronema tetradactylum, were collected. (A) The different colored areas on the land represent the different basins along the southern coast of China, Oujiang Basin (OJ), Mindong Basin (MD), Minjiang Basin (MJ), Minnan Basin (MN), Hanjiang Basin (HJ), Zhujiang Basin (ZJ), Yuexi Basin (YX), Guinan Basin (GN). Red dots show the sample sites. (B) The green arrow symbol is the schematic representation of currents. The map was drawn using open source software QGIS 2.14 (http://www.qgis.org/). The base map was downloaded from the Natural Earth (open access) at https://www.naturalearthdata.com/. The boundary of different basins was extracted from HydroBASINS at https://www.hydrosheds.org/products/hydrobasins.

DX was located on the westernmost coast of China, and JH and NZ were located on the west and east sides of Leizhou Peninsula in Southern China, respectively. DX and JH were both adjacent to the Beibu Gulf, and NZ was located on the western Guangdong where water exchange between the coastal waters of western Guangdong and Beibu Gulf took place through the Qiongzhou Strait. The Guangdong Western Coastal Current (GDWCC) moves westward during most of the year but shifts eastward during the monsoon season^{34 (link),35 (link)}. WC was located on the east side of Hainan island. ZZ was adjacent to the Taiwan Strait, and distant from other sites (~ 1200 km distance from DX, ~ 900 km distance from WC). During winter the Guangdong Coastal Current (GDCC) from Guangdong moves westward to the southeast of Hainan island^{34 (link)}.
As estuaries are the transition zones linking freshwater and marine environments, their environmental and hydrological processes are largely governed by the degree of freshwater inflow^{36 (link),37 (link)}. Therefore, the freshwater inflow may be an important environmental variable affecting fish recruitment^{36 (link),38 (link),39 (link)}. Freshwater inflow to an estuary is usually related to runoff directly, although there are estuaries that receive freshwater from springs^{36 (link)}. In this study, runoff in coastal basins throughout the coast of Southern China was used as a proxy for freshwater inflow to assess the interannual variation in habitat availability (the boundary of the basin is shown in Fig. 1). We accessed ERA5-Land ECMWF Climate Reanalysis to extract the regional average monthly runoff data from January of 2016 to December of 2020⁴⁰ . To make the data comparable across years and basins, we used the average values of runoff of each basin (Fig. 2).Figure 2

Average monthly runoff of the basins throughout coast of Southern China from January of 2016 to December of 2020. The 12 facets represent the months from January to December. Horizontal line in box: median value; bottom and top of box: 25th and 75th percentiles; whiskers: 5th and 95th percentiles.