Impacts of Winter and Spring Water Masses on Demersal Fish Community Structure Around Hainan Island

Simple Summary

Fish populations are highly sensitive to changes in the ocean environment. This study investigated how different “water masses”—large bodies of water with distinct temperature and salinity levels—affect fish diversity around Hainan Island. By conducting surveys in winter and spring, we analyzed which fish species were present and how their distribution shifted with seasonal water changes. We found that the movement of water masses significantly dictates where fish live, and which species are most abundant. For instance, fish communities in winter were very distinct across different areas, whereas in spring, the intrusion of warmer, saltier water brought more migratory species into mixed water zones. Additionally, the areas with the highest fish diversity moved northward from winter to spring. Understanding these seasonal patterns is vital for protecting marine biodiversity. This knowledge helps fishery managers and conservationists make better decisions to ensure sustainable fishing and healthy ocean ecosystems in the face of environmental changes.

Abstract

To elucidate water mass impacts on fish diversity in Hainan Island, bottom trawl surveys were conducted at 50 stations around the Island in December 2023 (winter) and April 2024 (spring). K-means clustering identified three water masses: Coastal Water (CW), Mixed Water (MW), and Offshore Water (OW). A total of 396 species were collected. Winter communities demonstrated clear habitat specificity, with distinct dominant species in each water mass (OW, MW, and CW). Conversely, the spring intrusion of warm, saline water facilitated the dominance of migratory species in the MW. Diversity centers shifted significantly northward and shoreward, transitioning from the MW region in winter to the CW region in spring. These findings highlight the critical influence of seasonal hydrodynamics on fish community structure, providing essential baselines for regional fisheries management and conservation.

1. Introduction

Hydrographic characteristics and water mass dynamics are fundamental drivers in shaping the spatial distribution and community structure of marine organisms [1,2]. To effectively analyze these hydrological environments, the K-means clustering algorithm has been widely adopted as a robust classification tool. Previous applications in adjacent regions have demonstrated the efficacy of this method. For instance, studies in Daya Bay [3], the Pearl River Estuary [4], and the broader South China Sea [5] showed that K-means clustering can reveal the coupling relationships between water mass properties and biological distributions. However, the specific environmental mechanisms governing demersal fish patterns around Hainan Island remain to be fully elucidated. Therefore, understanding these dynamics is indispensable for formulating effective resource restoration strategies and ensuring the long-term sustainability of marine ecosystems.

Hainan Island represents an ideal model system to investigate these mechanisms. It is characterized by an intricate, sinuous coastline extending 1855.27 km, separated from the Leizhou Peninsula to the north by the Qiongzhou Strait and bordered by the Beibu Gulf to the west [6]. Its seafloor topography and complex hydrological conditions have fostered diverse fish communities, establishing these coastal waters as one of the most critical fishing grounds in the South China Sea [7]. Hainan Island is also widely acknowledged as a biodiversity hotspot, characterized by its exceptionally rich plant and animal resources [6,7,8]. Consequently, the fishery industry constitutes a cornerstone of the regional economy. However, the depletion of fishery resources has emerged as a critical crisis. Evidence shows that among China’s 52 traditional offshore fishing grounds, approximately 77% have experienced the disappearance of seasonal “fish floods” (peak fishing periods), and over 80% no longer support the formation of concentrated shoals [9]. Furthermore, human activities [10,11], climate change [12,13] have posed significant threats to local fish migration patterns and habitats.

A schematic map of ocean currents around Hainan Island during winter and spring was drawn based on Su [14] (Figure 1). Specifically, the Guangdong Coastal Current (GDCC) (solid arrows) carried cold coastal water southward, South China Sea West Current (SCSWC) mainly affects the western and southern regions, while the peripheral circulation (dashed arrows) mediated the expansion of mixed water in the Qiongzhou Strait and the western Beibu Gulf.

Based on bottom trawl surveys conducted in Winter 2023 (December) and Spring 2024 (April) in the coastal waters of Hainan Island, this study examined how water masses influence fish community composition, dominant species, and biodiversity. Two hypotheses were tested: (1) Different water masses harbor distinct fish communities due to variations in temperature and salinity; (2) Fish community structure shifts in response to seasonal water mass dynamics. The goal is to elucidate the environmental mechanisms driving the spatio-temporal distribution of fish communities around Hainan Island, thereby providing scientific support for the conservation and sustainable management of local fish resources.

2. Materials and Methods

2.1. Study Area

The study area was located between 108.35 and 111.55° E and 17.80–20.20° N. 50 stations were established along the coastal waters of Hainan Island at intervals of 0.25° longitude and 0.25° latitude (Figure 2) after conducting a preliminary analysis of the marine geography and location of Hainan Island’s nearshore waters. They effectively cover areas ranging from river-influenced estuarine zones and coastal current-influenced strait regions to the open ocean. Sampling stations provide sufficient spatial coverage to capture the heterogeneity of fish community structures across different water masses.

2.2. Water Mass Division

Following the methodology of Tian et al. [4], the sampling stations were clustered into three categories using the K-means algorithm based on four parameters: sea surface temperature (SST), bottom sea temperature (BST), sea surface salinity (SSS), and bottom sea salinity (BSS). The goal of K-Means is to minimize the sum of squared distances between each data point and the centroid (mean) of its assigned cluster. This is captured by the formula:

$$J={\displaystyle \sum }_{k=1}^C\sum _{r=1}^{m_k}{\left(x_r-{\overline{\chi }}_k\right)}^2$$

(1)

In this formula, J represents the objective function to be minimized to ensure cluster cohesion. C denotes the total number of clusters, m_k represents the number of samples contained in the k-th cluster, r is the re-indexed sequence number of a sample within the k-th cluster, x_r is the observation vector of the r-th station, ${\overline{x}}_k$ is the mean of the samples in the k-th cluster.

2.3. Sample Collection and Identification

Field surveys were conducted across two seasons: winter (28 November 2023–12 December 2023) and spring (3–13 April 2024). Bottom trawlings were performed using a single-vessel trawl (net height: 3.0 m; width: 21.0 m; minimum cod-end mesh size: 2.5 cm). Each haul lasted approximately 0.5 h, with an average trawling speed of 3.3 knots. The fishery resource survey was conducted using the Research Vessel “Qionglinyu 12368#”. Captured fish were identified to the species level based on Wu and Zhong [15], and both abundance and biomass densities were calculated for each haul. Ecological types for each species were determined with reference to FishBase [16] and Chen [17]. Warm-water (WW) fishes include those distributed in tropical and subtropical zones, while warm-temperate (WT) fishes refer to those distributed in temperate zones. The systematic arrangement of taxa followed the classification systems of Betancur [18] and Nelson [19]. Physicochemical parameters, including temperature, salinity, pH, and dissolved oxygen (DO), were measured synchronously in the surface and bottom water at each site using YSI ProDSS Multiparameter Digital Water Quality Meter (YSI Inc., Yellow Springs, OH, USA). Depth data were concurrently collected using an onboard echo sounder.

2.4. Estimates of Fish Resource Density

The resource density at each sampling station was estimated using the swept area method [20]. The calculation formula is as follows:

$$p_i = {\displaystyle \frac{C_i}{\left(a_{i}q\right)}}$$

(2)

p_i denotes the fish biomass or abundance, measured in kg/km² or ind./km²; C_i represents the catch per hour at the i-th station (kg/h or ind./h); a_i is the swept area per hour of the fishing gear at the i-th station (km²/h); q is the catchability coefficient. Following standard procedures, the catchability coefficient (q) for demersal fish was set to 0.5 [21].

2.5. The Index of Relative Importance (IRI)

The ecological dominance of fish communities during different seasons was evaluated using the index of relative importance (IRI) [22]:

$$\mathit{IRI} = (N + W) \times F \times \mathrm{10,000}$$

(3)

The IRI is computed by considering three factors: N represents the percentage of individuals of a particular fish species relative to the total catch number; W represents the percentage of the weight of that fish species relative to the total catch weight; and F represents the percentage of occurrences of that fish species relative to the total number of sampling stations. Species with an IRI ≥ 1000 are categorized as dominant species, species with IRI between 100 and 1000 are classified as important species, species with IRI between 10 and 100 are considered common species, and species with IRI < 10 are regarded as rare species [21].

2.6. Linear Regression Analysis

Simple linear regression analysis was performed to determine the relationship between the abundance/biomass density of species and the total density of the fish community [23]. The standardized regression coefficient (β) was used to evaluate the contribution rate of each species to the variation in total stock density. Data were analyzed using IBM SPSS Statistics 26.

2.7. Alpha Diversity Used Indices

The Shannon-Wiener diversity index (H′), Margalef species richness index (D), and Pielou evenness index (J′) were employed to evaluate the abundance-based diversity of the fish community [21].

Shannon-Wiener diversity index (H′):

$$H^{\prime }=-\sum _{i=1}^s\left(P_i\mathrm{l}\mathrm{n}{P}_i\right)$$

(4)

Margalef species richness index (D):

$$D={\displaystyle \frac{(S-1)}{\ln N}}$$

(5)

Pielou evenness index (J′):

$$J^{\prime }={\displaystyle \frac{H^{\prime }}{\ln S}}$$

(6)

In the formula, S represents the total number of species of captured fish in the study area, P_i denotes the weight of the i-th fish species, N represents the total number of individuals of captured fish.

2.8. ANOSIM and SIMPER Analysis

We utilized cluster analysis to investigate the community structure based on the Bray–Curtis dissimilarity. One-way analysis of similarities (ANOSIM) [24], was employed to evaluate the significance of variations in community structure among different water masses, based on abundance and biomass data. Prior to analysis, both abundance and biomass data underwent square-root transformation to mitigate the influence of highly dominant species. Additionally, similarity percentage (SIMPER) analysis was conducted to determine the contribution of specific species to intra-group similarity and inter-group dissimilarity within the community structure [25]. All computational analyses were performed using Primer 6.0.

3. Results

3.1. Classification and Properties of Water Masses

The waters across both seasons were classified into three water masses: Coastal Water (CW), Mixed Water (MW), and Offshore Water (OW) (

Table 1. Ranges and average values of sea surface temperature (SST), bottom sea temperature (BST), sea surface salinity (SSS), and bottom sea salinity (BSS), in Coastal Water (CW), Mixed Water (MW), and Offshore Water (OW) winter and spring.

Season	Water Mass		SST (°C)	BST (°C)	SSS	BSS
Winter	CW	Range	23.70–25.00	23.60–24.90	31.60–34.40	31.70–34.20
		Average	24.37 ± 0.09	24.39 ± 0.10	33.07 ± 0.20	32.99 ± 0.20
	MW	Range	24.90–26.00	25.00–25.80	30.60–34.00	32.30–34.20
		Average	25.47 ± 0.07	25.40 ± 0.06	33.07 ± 0.19	33.25 ± 0.13
	OW	Range	25.90–27.20	25.90–27.20	32.90–34.20	31.60–34.20
		Average	26.64 ± 0.11	26.61 ± 0.12	33.6 ± 0.10	33.21 ± 0.15
Spring	CW	Range	22.00–35.70	22.20–35.50	31.80–34.90	33.40–35.20
		Average	24.06 ± 0.85	23.95 ± 0.84	33.93 ± 0.22	34.29 ± 0.17
	MW	Range	25.20–27.30	23.60–25.90	33.90–34.80	33.50–34.90
		Average	26.03 ± 0.23	25.04 ± 0.23	34.48 ± 0.13	34.46 ± 0.19
	OW	Range	25.60–28.90	25.90–29.00	33.70–34.80	33.70–34.80
		Average	27.30 ± 0.13	27.23 ± 0.13	34.29 ± 0.05	34.34 ± 0.05

, Figure 3). The identified water masses exhibited distinct hydrographic signatures. The CW, characterized by relatively low temperature and salinity, was primarily distributed in the northeastern waters of Hainan Island, spanning from the Wanquan River estuary to the Qiongzhou Strait. Conversely, the OW featured high-temperature and high-salinity profiles, predominantly localized in the southern and western sectors from the vicinity of Sanya to the Changhua River estuary. The MW, representing the transitional zones between high-salinity offshore water and coastal water, showed intermediate values with pronounced salinity fluctuations, mainly occupying the southeastern and northwestern regions (Figure 3b).

Overall, both temperature and salinity were higher in spring than in winter. OW exhibited the highest thermal stability, characterized by the smallest standard error of the mean in spring. In the CW, the average SSS rose from 33.07 in winter to 33.93 in spring. The temperature and salinity of OW remained uniform throughout the water column in both seasons, maintaining its characteristic high-temperature and high-salinity profile. The spatial distribution of water masses also varied between the two seasons.

Winter featured 20 MW stations and 15 OW stations, whereas spring had 9 MW stations and 26 OW stations. This indicates a strengthened influence of OW in spring, which occupied more transitional stations.

3.2. Specific Composition

The total catch comprised 56,960 individuals, distributed across 396 species, 228 genera, 91 families, and 27 orders (

Table A1. Complete list of collected fish species and their IUCN Red List status.

Class	Order	Family	Genus	Species	IUCN Redlist
Elasmobranchii	Orectolobiformes	Hemiscylliidae	Chiloscyllium	Chiloscyllium plagiosum	NT
	Carcharhiniformes	Carcharhinidae	Scoliodon	Scoliodon macrorhynchos	NT
	Torpediniformes	Platyrhinidae	Platyrhina	Platyrhina sinensis	EN
				Platyrhina tangi	VU
		Narcinidae	Narcine	Narcine lingula	VU
				Narcine prodorsalis	EN
				Narcine timlei	VU
	Rajiformes	Rajidae	Okamejei	Okamejei acutispina	VU
				Okamejei boesemani	VU
				Okamejei hollandi	VU
	Myliobatiformes	Dasyatidae	Telatrygon	Telatrygon zugei	VU
			Hemitrygon	Hemitrygon akajei	NT
				Hemitrygon bennetti	VU
			Neotrygon	Neotrygon kuhlii	DD
		Gymnuridae	Gymnura	Gymnura japonica	VU
Actinopteri	Anguilliformes	Synaphobranchidae	Dysomma	Dysomma anguillare	LC
		Muraenidae	Gymnothorax	Gymnothorax chilospilus	LC
				Gymnothorax minor	LC
				Gymnothorax pindae	LC
				Gymnothorax reevesii	LC
				Gymnothorax richardsonii	LC
			Strophidon	Strophidon dorsalis	LC
				Strophidon sathete	LC
		Ophichthidae	Scolecenchelys	Scolecenchelys macroptera	LC
			Neenchelys	Neenchelys parvipectoralis	LC
			Muraenichthys	Muraenichthys hattae	DD
			Ophichthus	Ophichthus apicalis	LC
			Pisodonophis	Pisodonophis boro	LC
				Pisodonophis cancrivorus	LC
			Brachysomophis	Brachysomophis crocodilinus	LC
			Caecula	Caecula pterygera	DD
		Muraenesocidae	Muraenesox	Muraenesox bagio	LC
				Muraenesox cinereus	LC
			Congresox	Congresox talabonoides	LC
		Congridae	Uroconger	Uroconger lepturus	LC
			Gnathophis	Gnathophis heterognathos	LC
			Rhynchoconger	Rhynchoconger ectenurus	LC
			Ariosoma	Ariosoma majus	NE
		Moringuidae	Moringua	Moringua macrocephalus	DD
		Nettastomatidae	Saurenchelys	Saurenchelys fierasfer	LC
	Clupeiformes	Engraulidae	Setipinna	Setipinna tenuifilis	DD
			Stolephorus	Stolephorus balinensis	NE
				Stolephorus chinensis	LC
				Stolephorus indicus	LC
				Stolephorus waitei	DD
			Thryssa	Thryssa adelae	DD
				Thryssa chefuensis	DD
				Thryssa dussumieri	LC
				Thryssa hamiltonii	LC
				Thryssa setirostris	LC
			Encrasicholina	Encrasicholina punctifer	LC
		Pristigasteridae	Ilisha	Ilisha elongata	LC
				Ilisha melastoma	LC
		Clupeidae	Sardinella	Sardinella fimbriata	LC
				Sardinella hualiensis	LC
			Nematalosa	Nematalosa nasus	LC
			Anodontostoma	Anodontostoma chacunda	LC
	Siluriformes	Ariidae	Arius	Arius arius	LC
			Plicofollis	Plicofollis nella	NE
		Plotosidae	Plotosus	Plotosus lineatus	LC
	Aulopiformes	Synodontidae	Synodus	Synodus fuscus	LC
				Synodus hoshinonis	LC
				Synodus jaculum	LC
				Synodus macrops	LC
				Synodus rubromarmoratus	LC
				Synodus tectus	LC
				Synodus variegatus	LC
			Trachinocephalus	Trachinocephalus myops	LC
			Harpadon	Harpadon nehereus	NT
			Saurida	Saurida elongata	LC
				Saurida filamentosa	LC
				Saurida gracilis	LC
				Saurida tumbil	LC
				Saurida undosquamis	LC
	Myctophiformes	Myctophidae	Benthosema	Benthosema fibulatum	LC
				Benthosema pterotum	LC
	Gadiformes	Bregmacerotidae	Bregmaceros	Bregmaceros lanceolatus	NE
				Bregmaceros mcclellandi	NE
				Bregmaceros pescadorus	NE
				Bregmaceros pseudolanceolatus	NE
	Holocentriformes	Holocentridae	Sargocentron	Sargocentron ittodai	LC
				Sargocentron rubrum	LC
			Ostichthys	Ostichthys sheni	NE
	Ophidiiformes	Ophidiidae	Brotula	Brotula multibarbata	LC
			Sirembo	Sirembo imberbis	LC
	Scombriformes	Centrolophidae	Psenopsis	Psenopsis anomala	NE
		Stromateidae	Pampus	Pampus chinensis	NE
				Pampus cinereus	NE
				Pampus echinogaster	NE
				Pampus minor	NE
				Pampus punctatissimus	NE
		Ariommatidae	Ariomma	Ariomma indica	NE
		Scombridae	Rastrelliger	Rastrelliger kanagurta	LC
		Trichiuridae	Lepturacanthus	Lepturacanthus savala	NE
			Trichiurus	Trichiurus brevis	NE
				Trichiurus japonicus	LC
				Trichiurus nanhaiensis	NE
			Tentoriceps	Tentoriceps cristatus	NE
	Syngnathiformes	Dactylopteridae	Dactyloptena	Dactyloptena gilberti	LC
				Dactyloptena orientalis	LC
		Pegasidae	Pegasus	Pegasus laternarius	DD
		Mullidae	Upeneus	Upeneus japonicus	NE
				Upeneus spottocaudalis	NE
				Upeneus sulphureus	LC
				Upeneus tragula	LC
			Parupeneus	Parupeneus heptacanthus	LC
			Mulloidichthys	Mulloidichthys vanicolensis	LC
		Callionymidae	Callionymus	Callionymus belcheri	NE
				Callionymus curvicornis	LC
				Callionymus hindsii	NE
			Repomucenus	Repomucenus beniteguri	LC
				Repomucenus huguenini	NE
				Repomucenus planus	NE
				Repomucenus virgis	NE
			Calliurichthys	Calliurichthys japonicus	NE
			Dactylopus	Dactylopus dactylopus	LC
		Syngnathidae	Halicampus	Halicampus grayi	LC
				Halicampus spinirostris	LC
			Hippocampus	Hippocampus mohnikei	VU
				Hippocampus trimaculatus	VU
			Trachyrhamphus	Trachyrhamphus serratus	DD
		Fistulariidae	Fistularia	Fistularia commersonii	LC
				Fistularia petimba	LC
		Solenostomidae	Solenostomus	Solenostomus armatus	LC
	Kurtiformes	Apogonidae	Ostorhinchus	Ostorhinchus doederleini	LC
				Ostorhinchus fasciatus	LC
				Ostorhinchus kiensis	LC
				Ostorhinchus semilineatus	NE
			Jaydia	Jaydia arafurae	NE
				Jaydia carinatus	NE
				Jaydia lineata	LC
				Jaydia novaeguineae	LC
				Jaydia sp.	NE
				Jaydia striata	LC
				Jaydia striatodes	LC
				Jaydia truncata	LC
			Verulux	Verulux cypselurus	LC
		Trichonotidae	Trichonotus	Trichonotus setiger	LC
	Gobiiformes	Gobiidae	Oxyurichthys	Oxyurichthys auchenolepis	LC
				Oxyurichthys ophthalmonema	LC
			Trypauchen	Trypauchen taenia	NE
				Trypauchen vagina	LC
			Paratrypauchen	Paratrypauchen microcephalus	LC
			Amblyotrypauchen	Amblyotrypauchen arctocephalus	LC
			Parachaeturichthys	Parachaeturichthys polynema	LC
			Valenciennea	Valenciennea wardii	LC
			Egglestonichthys	Egglestonichthys bombylios	LC
				Egglestonichthys melanoptera	LC
			Gobiodon	Gobiodon okinawae	LC
			Cryptocentrus	Cryptocentrus russus	NE
			Myersina	Myersina filifer	LC
		Eleotridae	Butis	Butis koilomatodon	LC
	Carangiformes	Sphyraenidae	Sphyraena	Sphyraena flavicauda	LC
				Sphyraena pinguis	LC
				Sphyraena putnamae	LC
		Lactariidae	Lactarius	Lactarius lactarius	NE
		Polynemidae	Polydactylus	Polydactylus sextarius	NE
		Citharidae	Brachypleura	Brachypleura novaezeelandiae	LC
			Citharoides	Citharoides macrolepidotus	LC
		Bothidae	Laeops	Laeops kitaharae	LC
				Laeops parviceps	LC
			Arnoglossus	Arnoglossus aspilos	LC
				Arnoglossus macrolophus	LC
				Arnoglossus polyspilus	LC
				Arnoglossus tapeinosoma	DD
				Arnoglossus tenuis	LC
			Engyprosopon	Engyprosopon grandisquama	LC
				Engyprosopon latifrons	DD
				Engyprosopon maldivense	NE
				Engyprosopon multisquama	LC
			Crossorhombus	Crossorhombus azureus	LC
				Crossorhombus kanekonis	NE
			Psettina	Psettina gigantea	LC
				Psettina iijimae	LC
			Parabothus	Parabothus taiwanensis	LC
			Grammatobothus	Grammatobothus krempfi	DD
			Japonolaeops	Japonolaeops dentatus	NE
			Bothus	Bothus myriaster	LC
			Asterorhombus	Asterorhombus intermedius	LC
		Paralichthyidae	Pseudorhombus	Pseudorhombus malayanus	LC
				Pseudorhombus oligodon	LC
				Pseudorhombus triocellatus	LC
			Tarphops	Tarphops oligolepis	LC
		Samaridae	Samariscus	Samariscus japonicus	DD
				Samariscus latus	LC
			Samaris	Samaris cristatus	LC
		Soleidae	Zebrias	Zebrias quagga	LC
				Zebrias zebrinus	DD
			Solea	Solea ovata	LC
			Aesopia	Aesopia cornuta	LC
			Aseraggodes	Aseraggodes kobensis	LC
			Pardachirus	Pardachirus pavoninus	LC
		Cynoglossidae	Symphurus	Symphurus sp.	NE
			Cynoglossus	Cynoglossus arel	DD
				Cynoglossus kopsii	LC
				Cynoglossus nanhaiensis	DD
				Cynoglossus nigropinnatus	LC
				Cynoglossus ochiaii	LC
				Cynoglossus oligolepis	DD
				Cynoglossus puncticeps	LC
				Cynoglossus quadrilineatus	LC
				Cynoglossus roulei	NE
				Cynoglossus semilaevis	DD
		Carangidae	Alepes	Alepes kleinii	LC
				Alepes melanoptera	LC
			Decapterus	Decapterus maruadsi	LC
				Decapterus tabl	LC
			Megalaspis	Megalaspis cordyla	LC
			Carangoides	Carangoides armatus	LC
				Carangoides chrysophrys	LC
				Carangoides malabaricus	LC
			Selar	Selar crumenophthalmus	LC
			Alectis	Alectis indica	LC
			Parastromateus	Parastromateus niger	LC
			Atropus	Atropus atropos	LC
			Kaiwarinus	Kaiwarinus equula	NE
			Trachurus	Trachurus japonicus	NT
			Caranx	Caranx ignobilis	LC
				Caranx sexfasciatus	LC
	Cichliformes	Opistognathidae	Opistognathus	Opistognathus evermanni	LC
		Pomacentridae	Chromis	Chromis fumeus	LC
			Teixeirichthys	Teixeirichthys jordani	LC
	Mugiliformes	Mugilidae	Moolgarda	Moolgarda cunnesius	NE
				Moolgarda pedaraki	NE
				Moolgarda perusii	LC
			Planiliza	Planiliza macrolepis	LC
	Blenniiformes	Blenniidae	Xiphasia	Xiphasia setifer	LC
			Plagiotremus	Plagiotremus spilistius	LC
	Perciformes	Serranidae	Diploprion	Diploprion bifasciatum	LC
			Tosana	Tosana niwae	NE
			Epinephelus	Epinephelus areolatus	LC
				Epinephelus awoara	DD
				Epinephelus bleekeri	DD
				Epinephelus coioides	LC
				Epinephelus craigi	NE
				Epinephelus latifasciatus	LC
				Epinephelus quoyanus	LC
				Epinephelus sexfasciatus	LC
			Cephalopholis	Cephalopholis boenak	LC
			Triso	Triso dermopterus	NE
			Chelidoperca	Chelidoperca hirundinacea	NE
				Chelidoperca margaritifera	NE
		Labridae	Suezichthys	Suezichthys gracilis	LC
			Leptojulis	Leptojulis lambdastigma	DD
			Xiphocheilus	Xiphocheilus typus	LC
		Uranoscopidae	Uranoscopus	Uranoscopus japonicus	LC
				Uranoscopus oligolepis	LC
				Uranoscopus tosae	NE
		Pinguipedidae	Parapercis	Parapercis cylindrica	LC
				Parapercis lutevittata	NE
				Parapercis ommatura	NE
				Parapercis pulchella	LC
				Parapercis sexfasciata	NE
				Parapercis snyderi	LC
		Ammodytidae	Bleekeria	Bleekeria viridianguilla	NE
		Platycephalidae	Inegocia	Inegocia japonica	LC
			Grammoplites	Grammoplites knappi	NE
			Sunagocia	Sunagocia arenicola	LC
			Suggrundus	Suggrundus meerdervoortii	NE
			Platycephalus	Platycephalus indicus	DD
			Cociella	Cociella punctata	LC
			Elates	Elates ransonnettii	NE
			Onigocia	Onigocia spinosa	LC
			Thysanophrys	Thysanophrys chiltonae	LC
			Rogadius	Rogadius asper	LC
			Kumococius	Kumococius rodericensis	LC
		Synanceiidae	Apistus	Apistus carinatus	NE
			Erisphex	Erisphex pottii	LC
				Erisphex simplex	NE
			Paracentropogon	Paracentropogon rubripinnis	LC
			Ablabys	Ablabys macracanthus	LC
			Richardsonichthys	Richardsonichthys leucogaster	NE
			Choridactylus	Choridactylus multibarbus	LC
			Inimicus	Inimicus cuvieri	NE
			Trachicephalus	Trachicephalus uranoscopus	LC
			Minous	Minous monodactylus	LC
				Minous pictus	NE
				Minous pusillus	NE
		Triglidae	Lepidotrigla	Lepidotrigla abyssalis	NE
				Lepidotrigla alata	NE
				Lepidotrigla japonica	NE
			Chelidonichthys	Chelidonichthys spinosus	LC
		Scorpaenidae	Neomerinthe	Neomerinthe megalepis	LC
			Scorpaenopsis	Scorpaenopsis neglecta	LC
				Scorpaenopsis ramaraoi	LC
			Pterois	Pterois lunulata	LC
				Pterois russelii	LC
			Parapterois	Parapterois heterurus	LC
			Dendrochirus	Dendrochirus brachypterus	LC
			Brachypterois	Brachypterois serrulata	LC
			Setarches	Setarches longimanus	NE
			Sebastiscus	Sebastiscus marmoratus	NE
	Centrarchiformes	Terapontidae	Terapon	Terapon jarbua	LC
				Terapon theraps	LC
			Pelates	Pelates quadrilineatus	NE
		Cirrhitidae	Cirrhitichthys	Cirrhitichthys aureus	LC
	Acropomatiformes	Champsodontidae	Champsodon	Champsodon longipinnis	NE
				Champsodon snyderi	NE
		Acropomatidae	Acropoma	Acropoma japonicum	NE
	Acanthuriformes	Chaetodontidae	Roa	Roa modesta	NE
			Heniochus	Heniochus diphreutes	LC
		Leiognathidae	Leiognathus	Leiognathus equula	NE
			Photopectoralis	Photopectoralis bindus	LC
			Eubleekeria	Eubleekeria jonesi	LC
			Deveximentum	Deveximentum ruconius	NE
			Photolateralis	Photolateralis stercorarius	LC
			Equulites	Equulites berbis	LC
			Gazza	Gazza minuta	LC
				Gazza rhombea	LC
			Nuchequula	Nuchequula mannusella	LC
				Nuchequula nuchalis	NE
			Karalla	Karalla daura	LC
		Sciaenidae	Johnius	Johnius amblycephalus	LC
				Johnius belangerii	LC
				Johnius borneensis	LC
				Johnius carouna	LC
				Johnius fasciatus	LC
				Johnius grypotus	LC
				Johnius taiwanensis	LC
				Johnius trewavasae	LC
			Pennahia	Pennahia anea	LC
				Pennahia argentata	LC
				Pennahia macrocephalus	LC
				Pennahia pawak	LC
			Larimichthys	Larimichthys crocea	CR
			Nibea	Nibea semifasciata	DD
			Dendrophysa	Dendrophysa russelii	LC
			Chrysochir	Chrysochir aureus	LC
			Otolithes	Otolithes ruber	LC
		Sparidae	Evynnis	Evynnis cardinalis	EN
			Acanthopagrus	Acanthopagrus latus	DD
				Acanthopagrus pacificus	LC
				Acanthopagrus schlegelii	LC
				Acanthopagrus taiwanensis	DD
		Nemipteridae	Nemipterus	Nemipterus bathybius	LC
				Nemipterus furcosus	LC
				Nemipterus japonicus	LC
				Nemipterus marginatus	LC
				Nemipterus nemurus	LC
				Nemipterus peronii	LC
				Nemipterus virgatus	VU
				Nemipterus zysron	LC
			Scolopsis	Scolopsis vosmeri	LC
			Pentapodus	Pentapodus setosus	LC
		Siganidae	Siganus	Siganus canaliculatus	LC
				Siganus fuscescens	LC
		Sillaginidae	Sillago	Sillago aeolus	NE
				Sillago asiatica	NE
				Sillago ingenuua	NE
				Sillago japonica	LC
				Sillago sihama	LC
		Ephippidae	Ephippus	Ephippus orbis	NE
		Haemulidae	Pomadasys	Pomadasys grunniens	NE
				Pomadasys maculatus	LC
			Parapristipoma	Parapristipoma trilineatum	LC
		Lutjanidae	Pristipomoides	Pristipomoides filamentosus	LC
				Pristipomoides multidens	LC
			Lutjanus	Lutjanus johnii	LC
				Lutjanus lutjanus	LC
				Lutjanus malabaricus	LC
				Lutjanus monostigma	LC
				Lutjanus russellii	LC
			Pterocaesio	Pterocaesio chrysozona	LC
		Cepolidae	Acanthocepola	Acanthocepola krusensternii	NE
				Acanthocepola limbata	NE
		Malacanthidae	Branchiostegus	Branchiostegus albus	NT
				Branchiostegus argentatus	NT
		Priacanthidae	Priacanthus	Priacanthus macracanthus	LC
		Lobotidae	Hapalogenys	Hapalogenys analis	NE
		Gerreidae	Gerres	Gerres filamentosus	LC
				Gerres oblongus	LC
				Gerres oyena	LC
				Gerres septemfasciatus	NE
		Lethrinidae	Lethrinus	Lethrinus lentjan	LC
		Scatophagidae	Scatophagus	Scatophagus argus	LC
		Drepaneidae	Drepane	Drepane punctata	LC
	Lophiiformes	Lophiidae	Lophiomus	Lophiomus setigerus	LC
		Ogcocephalidae	Halieutaea	Halieutaea indica	LC
		Antennariidae	Antennarius	Antennarius hispidus	LC
				Antennarius nummifer	LC
				Antennarius striatus	LC
	Tetraodontiformes	Tetraodontidae	Lagocephalus	Lagocephalus gloveri	DD
				Lagocephalus inermis	LC
				Lagocephalus spadiceus	LC
				Lagocephalus suezensis	LC
			Takifugu	Takifugu oblongus	LC
				Takifugu poecilonotus	LC
			Torquigener	Torquigener brevipinnis	LC
			Canthigaster	Canthigaster rivulata	LC
		Diodontidae	Diodon	Diodon holocanthus	LC
		Monacanthidae	Paramonacanthus	Paramonacanthus pusillus	LC
			Monacanthus	Monacanthus chinensis	LC
			Thamnaconus	Thamnaconus hypargyreus	LC
			Aluterus	Aluterus monoceros	LC
			Anacanthus	Anacanthus barbatus	LC
			Stephanolepis	Stephanolepis cirrhifer	LC

Notes: CR, Critically Endangered; EN, Endangered; VU, Vulnerable; NT, Near Threatened; LC, Least Concern; DD, Data Deficient; NE, Not Evaluated.

). A substantial overlap of 196 shared species was observed between the two seasons, whereas the number of species occurring exclusively in winter and spring was 98 and 102, respectively (Figure 4). At the Order level, Acanthuriformes was the most speciose order with 77 species, accounting for 19.44% of the total number of species, followed by Carangiformes (67 species), Perciformes (64 species), and Syngnathiformes (26 species) (Figure 5). Seasonally, the winter survey yielded 26,253 individuals across a total number of 294 species, while spring saw an increase in both numbers (30,707 individuals) and the number of species (298).

In winter, WT species totaled 31, comprising 29 marine (Ma) species, 1 brackish (Br) species, and 1 euryhaline (Eu) species. Simultaneously, WW species reached 263, including 181 Ma species, 61 Br species, and 21 Eu species. During the spring survey, the WT species numbered 30, comprising 27 Ma species, 2 Br species, and 1 Eu species. Simultaneously, WW species reached a total of 268, which included 182 Ma species, 61 Br species, and 25 Eu species. Across both seasons, WT species and Ma species remained the dominant components within all water masses, consistently accounting for over 50.00% of the total fish community composition (Figure 6). The CW region experienced the most dramatic seasonal shifts; in winter, the proportion of Eu species reached 25.48%. As salinity increased in spring, the Eu proportion plummeted to 2.34%. Conversely, the ratio of WT-Ma species surged from 4.60% in winter to 17.94% in spring. In winter, the MW was co-dominated by WW-Ma (56.58%) and WW-Br (24.20%) species. In spring, the dominance of WW-Ma species in the MW further strengthened to 68.38%, while Eu species nearly disappeared; the species composition in this transitional zone showed a clear successional trend toward OW characteristics. The OW remained the most environmentally stable with the strongest marine affinity; WW-Ma groups accounted for over 70% in both seasons, and proportions of Eu and Br groups were minimal in the OW, with even lower values recorded in spring than in winter.

3.3. Seasonal Variations in Fish Resource Density

The average fish biomass density in both seasons was 335.16 kg/km². Biomass density consistently peaking in the OW and remaining lowest in the MW during winter. A Rise in biomass density was observed in spring, particularly in the OW region (Figure 7). The maximum density at a single station was 854.63 kg/km², located in the northern CW region, with Johnius taiwanensis (418.94 kg/km²) as the most abundant species. In spring, a high-density area emerged in the transition zone between southeastern OW and MW, where the maximum station density was 2437.43 kg/km², mainly by the presence of Thamnaconus hypargyreus (1371.37 kg/km²).

The average abundance density of fish across both seasons was 20,235.75 ind./km². Fish abundance showed clear seasonal changes. Across both seasons, OW consistently supported the highest densities, while CW maintained the lowest. Although average abundance increased in spring, the spatial contrast between the high-density OW and low-density CW remained, with OW still hosting the most fish and CW the least. (Figure 7). During winter, a high-density area was identified in the southern OW region, where the maximum station was 103,918.22 ind./km², by the presence of Alepes kleinii (3869.08 ind./km²). In spring, high-density patches emerged in the transition zone between southeastern OW and MW, with the maximum density reaching 128,134.11 ind./km², by the presence of Saurida undosquamis (12,125.35 ind./km²).

3.4. Seasonal Variation of Dominant and Key Contributing Species

Although Acropoma japonicum exhibited the highest IRI values (919.13), no single species met the strict threshold for dominance (IRI ≥ 1000). However, 14 important species were recorded, with 1 Eu species, 5 Br species, and the remainder as Ma species. In spring, S. undosquamis emerged as the only dominant species. Additionally, 12 important species were identified, with only 1 Br species and the rest being Ma species. Common dominant or important species across both seasons included S. undosquamis, Champsodon snyderi, Brachypleura novaezeelandiae, A. japonicum, Decapterus kurroides, Upeneus japonicus, Ostorhinchus kiensis, Terapon theraps, and Pennahia macrocephalus (

Table 2. Community composition, abundance (N), biomass (W), frequency of occurrence (F), and index of relative importance (IRI) of dominant and important fish species in different seasons.

Seasons	Species	W (%)	N (%)	F (%)	IRI	Temperature Adaptation	Salinity Adaptation
Winter	Acropoma japonicum	1.53	18.45	0.46	919.13	WW	Ma
	Johnius taiwanensis	8.11	4.62	0.42	534.59	WW	Eu
	Decapterus maruadsi	7.11	3.57	0.5	534.2	WW	Ma
	Terapon theraps	6.60	3.07	0.46	444.57	WW	Br
	Photopectoralis bindus	0.94	4.61	0.72	399.87	WW	Br
	Champsodon snyderi	2.02	3.20	0.76	396.45	WW	Ma
	Pennahia macrocephalus	4.18	5.32	0.34	322.98	WW	Ma
	Brachypleura novaezeelandiae	2.97	3.94	0.44	304.16	WW	Ma
	Cynoglossus arel	3.31	2.77	0.46	279.7	WW	Br
	Ostorhinchus kiensis	0.31	4.53	0.54	261.54	WW	Br
	Upeneus japonicus	1.75	3.10	0.4	193.99	WW	Ma
	Saurida undosquamis	1.49	1.78	0.46	150.41	WW	Ma
	Deveximentum ruconius	0.26	2.04	0.56	129.14	WW	Br
	Trachinocephalus myops	4.56	1.59	0.18	110.66	WW	Ma
Spring	Saurida undosquamis	18.43	8.16	0.56	1488.95	WW	Ma
	Champsodon Snyderi	1.52	11.83	0.46	614.01	WW	Ma
	Saurida tumbil	5.79	2.11	0.7	552.87	WW	Ma
	Acropoma japonicum	1.64	10.87	0.38	475.21	WW	Ma
	Paramonacanthus pusillus	2.70	4.41	0.56	397.99	WW	Ma
	Ostorhinchus kiensis	0.47	6.28	0.58	391.63	WW	Ma
	Thamnaconus hypargyreus	8.21	8.20	0.2	328.11	WW	Ma
	Upeneus japonicus	2.75	3.13	0.52	305.76	WW	Ma
	Decapterus maraudsi	3.13	2.65	0.42	242.74	WW	Ma
	Terapon theraps	4.13	2.08	0.32	198.67	WW	Br
	Brachypleura novaezeelandiae	1.18	2.57	0.5	187.5	WW	Ma
	Pennahia macrocephalus	3.10	3.57	0.24	159.92	WW	Ma
	Priacanthus macracanthus	1.41	1.03	0.42	102.1	WW	Ma

).

3.5. Key Contributing Species in Water Masses

Water masses had a significant influence on the spatial distribution patterns of fish communities in both seasons (ANOSIM, p < 0.05) (Figure 8). SIMPER analysis revealed distinct variations in community composition across different water masses (Figure 9).

In winter, the primary contributors to abundance in MW (contribution > 10%) were B. novaezeelandiae (11.07%) and C. snyderi (10.01%), with B. novaezeelandiae (12.24%) also being the main biomass contributor. In OW, A. japonicum (10.44%) was the main abundance contributor, while T. theraps (12.79%) dominated the biomass contribution. In CW, J. taiwanensis was the main contributor to both abundance (14.80%) and biomass (14.40%). In spring, the main abundance contributors in MW were S. undosquamis (11.82%) and C. snyderi (10.60%). Biomass was primarily contributed by S. undosquamis (19.84%), D. maruadsi (14.98%), and S. tumbil (13.70%). In OW, A. japonicum (10.40%) remained the dominant abundance contributor, whereas S. undosquamis (14.40%) and S. tumbil (11.24%) were the main biomass contributors. In CW, J. taiwanensis (14.40%) led the abundance contribution, while S. tumbil (17.91%) became the primary contributor to biomass.

3.6. Drivers of Community Succession

To further identify the primary drivers of community succession, this study compared the contributions of the top five dominant species and the aforementioned key water mass contributors to community biomass (W) and abundance (N). Multiple linear regression analysis revealed a significant seasonal turnover in the species driving variations in fish community composition.

In winter, the total W was jointly driven by multiple species. Specifically, J. taiwanensis, T. theraps, and D. maruadsi exhibited highly significant positive contributions (p < 0.01), serving as the core factors determining winter biomass distribution patterns. In contrast, variations in winter total N were significantly correlated only with A. japonicum (p < 0.05). In spring, the driving force of the community shifted markedly toward marine species; S. undosquamis became the primary factor influencing abundance (p < 0.01), with a standardized coefficient (β) of 0.39. These results indicate an overlap between the top five dominant species and the major water mass contributors: J. taiwanensis and S. undosquamis, which collectively drive the spatial heterogeneity of the community. However, certain dominant species, such as A. japonicum in winter, primarily exerted influence at the numerical level, with relatively limited impact on the biomass distribution patterns (

Table 3. Summary of multiple linear regression analysis of key fish species contributing to the variation in total biomass density (W) and total abundance (N) across seasons.

Season	Species	W			N
		β	t	p	β	t	p
Winter	Acropoma japonicum a,b	0.18	1.14	-	0.38	2.24	*
	Johnius taiwanensis a,b	0.52	4.43	**	0.13	0.85	-
	Decapterus maruadsi a	0.43	3.34	**	0.24	1.36	-
	Terapon theraps a	0.45	3.88	**	0.03	0.19	-
	Photopectoralis bindus a	−0.02	−0.22	-	0.15	0.87	-
	Brachypleura novaezeelandiae b	−0.03	−0.19	-	0.01	0.04	-
	Champsodon snyderi b				−0.11	−0.54	-
Spring	Saurida undosquamis a,b	0.43	1.70	-	0.39	1.77	*
	Champsodon snyderi a,b	0.15	0.61	-	0.27	1.23	-
	Saurida tumbil a	−0.34	−1.36	-	0.02	0.10	-
	Acropoma japonicum a,b	−0.19	−1.08	-	−0.17	−1.00	-
	Paramonacanthus pusillus a	0.32	1.22	-
	Decapterus maruadsi b	−0.14	−0.87	-
	Ostorhinchus kiensis b				0.01	0.06	-
	Paramonacanthus pusillus b				−0.05	−0.16	-
	Johnius taiwanensis b				−0.05	−0.39	-

Notes: β represents the standardized regression coefficient. t represents test statistic. p represents significance. ** denotes p < 0.01, * denotes p < 0.05. Superscript a (^a) indicates the species is among the top five dominant species according to the IRI. Superscript b (^b) indicates the species is a major contributor to the similarity/dissimilarity of water masses identified by SIMPER analysis (contribution > 10%). Winter W: R² = 0.486, Adj. R² = 0.414, F = 6.772, p < 0.01, SE = 170.77 kg/km². Winter N: R² = 0.260, Adj. R² = 0.137, F = 2.110, p = 0.063, SE = 18,173.18 ind./km². Spring W: R² = 0.278, Adj. R² = 0.178, F = 2.763, p < 0.05, SE = 391.88 ind./km². Spring N: R² = 0.316, Adj. R² = 0.202, F = 2.776, p < 0.05, SE = 20,536.27 ind./km². R² = coefficient of determination; Adj. R² = adjusted R²; SE = standard error of the estimate.

).

3.7. Seasonal Variations in Diversity

Fish community diversity exhibited distinct seasonal variations. In winter, high-value zones for the Margalef richness index (D) were primarily concentrated in the high-salinity OW region. However, high-value zones for the Shannon–Wiener diversity index (H′) and Pielou’s evenness index (J′) were mainly distributed in the confluence area between CW and MW. The Pielou’s evenness index (J′) in the MW was significantly higher than in the OW (p < 0.05). This pattern may be attributed to the dominance of a single abundance contributor, A. japonicum, in the OW region, which resulted in relatively lower evenness. In spring, the centers of community diversity displayed a distinct northward and shoreward shift. High-value zones for the Margalef’s richness index (D) transitioned to the CW and MW regions in eastern Hainan Island, with the highest number of species in the CW. Simultaneously, the Shannon–Wiener diversity index (H′) and Pielou’s evenness index (J′) peaked in the CW region near northeastern Hainan Island and the Qiongzhou Strait. Both indices were significantly higher than those in the OW (p < 0.05) (Figure 10).

4. Discussion

4.1. Fish Composition

The fish community around Hainan Island is characterized by high species richness and a clear dominance of warm-water marine (WW-Ma) groups. The identification of 396 species across both seasons aligns with previous records from the South China Sea shelf [21]. However, the community structure undergoes a significant reorganization driven by seasonal water mass shifts. The key species driving total biomass and abundance distributions are highly congruent with the core taxa defining specific water masses.

In winter, the spatial heterogeneity of biomass is jointly regulated by a multispecies complex. Notably, J. taiwanensis mainly occurs in shallow tropical and subtropical coastal waters [26]. It serves as a high-fidelity indicator for Coastal Water (CW), with its density peaks strictly confined to the low-temperature, low-salinity boundaries of the Qiongzhou Strait. Conversely, A. japonicum acts as a representative taxon for OW region. Primarily concentrated in the high-salinity waters off Sanya City. This is consistent with its habit of preferring areas of relatively deep water with relatively high salinity [27]. In contrast, the spring transition marks a shift toward a community driven by migratory marine species. The intrusion of warm OW region into transitional zones facilitates the dominance of S. undosquamis, which becomes the primary factor influencing regional abundance. The proliferation of S. undosquamis is linked to spawning migrations [28]. Furthermore, the seasonal strengthening of the SCSWC likely facilitates larval dispersal and connectivity [29]. The synchrony between species turnover and water mass dynamics suggests that these key taxa act as reliable bioindicators of environmental fluctuations. This environmental dependency highlights the role of water masses regulate fish distribution through the selection of species with specific thermohaline adaptations. From a biodiversity perspective, this seasonal habitat partitioning enhances ecosystem resilience by promoting niche differentiation. This temporal segregation allows for greater species coexistence and functional redundancy, buffering the Hainan Island fishery ecosystem against local disturbances.

4.2. The Influence of Water Masses on the Distribution of Fish Species

The waters surrounding Hainan Island are primarily influenced by three types of water masses. Shi et al. [30] identified a year-round westward flow in the Qiongzhou Strait, with a near-bottom average flow rate of 10–15 cm/s during winter and spring. The Guangdong Coastal Current carries a significant volume of supplementary water westward through the strait, while freshwater from the Wanquan and Nandu Rivers discharges into the northeastern region. These combined processes give rise to distinctive CW characteristics in and around the Qiongzhou Strait. The SCSWBC generates coastal currents adjacent to the eastern coast of Hainan Island, flowing southward in winter and northward in summer, with the winter flow being stronger than that in summer [31]. During winter and spring, a clockwise circulation develops around Hainan Island [32], resulting in a stronger influence of high-salinity South China Sea water in the southwestern region. The MW is positioned between these two water masses. During the spring monsoon transition, the influence of high-salinity water strengthens, leading to a contraction of the MW. In winter, the contributing species of each water mass exhibit high specificity: the euryhaline J. taiwanensis is strictly confined to the Qiongzhou Strait dominated by CW, while the marine warm-water species A. japonicum is concentrated in the waters around Sanya City covered by high-salinity water. A weakening of the SCSWBC is observed in spring [33,34], allowing southern OW region to migrate northward and mix with the coastal water in the eastern region. With the arrival of warm OW region, migratory species such as S. undosquamis and S. tumbil complete their overwintering and aggregate in large numbers near the Wanquan River estuary for spawning [29]. These species replace the B. novaezeelandiae and C. snyderi as the main abundance contributors to the MW (Table 3). In spring, the strengthening of high-salinity water reduces the extent of MW along both the eastern and western coasts. Meanwhile, marine species like A. japonicum migrate northwestward with the warm water mass. High abundance and biomass density zones, primarily contributed by marine species such as S. undosquamis, D. maruadsi, and S. tumbil, emerge in the southeastern high-salinity and MW regions, forming a distinct spatial asymmetry. The community exhibits a clear trend of warmer and higher salinity (Figure 6). These findings highlight the need for dynamic fisheries management. Spawning sites identified can be used for seasonal closures to protect critical life stages.

4.3. The Impact of Water Masses on Diversity

The fish community diversity in the waters around Hainan Island showed pronounced seasonal and spatial variations. In winter, high values of the species richness index (D) were concentrated in the high-salinity OW south of the island, indicating that this area serves as an important overwintering ground with considerable species reserves. In contrast, the hotspots of the H′ and J′ were located farther east, mainly within the convergence zone of CW and MW. Thermohaline fronts in the South China Sea may enhance nutrient transport from shallow shelf waters to offshore areas [35], potentially supporting higher evenness in these transitional zones. Statistically, J′ in MW was significantly higher than in OW (p < 0.05), while OW exhibited the lowest H′ among the three water masses. This pattern reflects the strong dominance of A. japonicum in offshore waters during winter [27]. Regression analysis confirmed that A. japonicum was the only significant driver of abundance variation in that season; its locally dense aggregations reduced community evenness and thus constrained overall diversity levels.

In spring, the centers of diversity shifted distinctly northward and shoreward. Higher richness areas moved to the CW and MW regions in eastern Hainan Island, with the highest species richness recorded in CW. Simultaneously, significantly elevated H′ and J′ values were observed in CW near northeastern Hainan Island and the Qiongzhou Strait, both markedly exceeding those in OW during the same period. The seasonal increase in temperature and salinity likely stimulated the inshore migration of Br and Eu species from OW into the lower-salinity MW and CW zones. As conditions change, marine species are colonizing areas that were historically cooler and less saline [36]. Warming water also provides optimal thermal conditions for early life stage development [37]. This influx of species with differentiated ecological niches promoted a more balanced species distribution and ultimately enhanced overall community diversity. The observed diversity shifts reveal coastal ecosystem vulnerability to climate change, underscoring the need to integrate climate adaptation into long-term conservation plans.

5. Conclusions

This study demonstrates that the fish community around Hainan Island is predominantly composed of warm-water marine species, with its spatio-temporal structure significantly modulated by seasonal water mass dynamics. In winter, the euryhaline J. taiwanensis and the marine A. japonicum serve as primary bioindicators for the Coastal Water and Offshore Water, respectively, reflecting strict environmental filtering. The spring monsoonal transition facilitates the intrusion of high-temperature and high-salinity Offshore Water into the Mixed Water region. This hydrographic shift prompts a distinct successional trend where migratory marine species, notably S. undosquamis, replace winter species to become the primary drivers of regional abundance and biomass. Driven by these thermohaline fluctuations, the centers of community diversity undergo a significant northward and shoreward migration, transitioning from the Mixed Water toward the Coastal Water regions in eastern Hainan Island. The identified diversity seasonal migration patterns provide a scientific basis for formulating dynamic resource conservation measures and sustainable utilization plans. Further research should focus on specific characteristics of Hainan Island’s water mass, such as nutrient flux and primary productivity.