Volume 39 Issue 8
Aug.  2020
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Jianguo Du, Meiling Xie, Yuyu Wang, Zehao Chen, Wenhua Liu, Jianji Liao, Bin Chen. Connectivity of fish assemblages along the mangrove-seagrass-coral reef continuum in Wenchang, China[J]. Acta Oceanologica Sinica, 2020, 39(8): 43-52. doi: 10.1007/s13131-019-1490-7
Citation: Jianguo Du, Meiling Xie, Yuyu Wang, Zehao Chen, Wenhua Liu, Jianji Liao, Bin Chen. Connectivity of fish assemblages along the mangrove-seagrass-coral reef continuum in Wenchang, China[J]. Acta Oceanologica Sinica, 2020, 39(8): 43-52. doi: 10.1007/s13131-019-1490-7

Connectivity of fish assemblages along the mangrove-seagrass-coral reef continuum in Wenchang, China

doi: 10.1007/s13131-019-1490-7
Funds:  The National Natural Science Foundation of China under contract No. 41676096; the Natural Science Foundation of Fujian Province of China under contract No. 2017J01075; the Technology Foundation for Selected Overseas Chinese Scholar Project “Impacts of Climate Change on Biology and Economy in the East China Sea”; the National Key Research and Development Program of China under contract No. 2018YFC1406503; the China-ASEAN Maritime Cooperation Fund Project “Monitoring and Conservation of The Coastal Ecosystem in The South China Sea”.
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  • Understanding the connectivity of fish among different typical habitats is important for conducting ecosystem-based management, particularly when designing marine protected areas (MPA) or setting MPA networks. To clarify of connectivity among mangrove, seagrass beds, and coral reef habitats in Wenchang, Hainan Province, China, the fish community structure was studied in wet and dry seasons of 2018. Gill nets were placed across the three habitat types, and the number of species, individuals, and body size of individual fish were recorded. In total, 3 815 individuals belonging to 154 species of 57 families were collected. The highest number of individuals and species was documented in mangroves (117 species, 2 623 individuals), followed by coral reefs (61 species, 438 individuals) and seagrass beds (46 species, 754 individuals). The similarity tests revealed highly significant differences among the three habitats. Approximately 23.4% species used two habitats and 11.0% species used three habitats. A significant difference (p<0.05) in habitat use among eight species (Mugil cephalus, Gerres oblongus, Siganus fuscescens, Terapon jarbua, Sillago maculata, Upeneus tragula, Lutjanus russellii, and Monacanthus chinensis) was detected, with a clear ontogenetic shift in habitat use from mangrove or seagrass beds to coral reefs. The similarity indices suggested that fish assemblages can be divided into three large groups namely coral, seagrass, and mangrove habitat types. This study demonstrated that connectivity exists between mangrove–seagrass–coral reef continuum in Wenchang area; therefore, we recommend that fish connectivity should be considered when designing MPAs or MPA network where possible.
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Connectivity of fish assemblages along the mangrove-seagrass-coral reef continuum in Wenchang, China

doi: 10.1007/s13131-019-1490-7
Funds:  The National Natural Science Foundation of China under contract No. 41676096; the Natural Science Foundation of Fujian Province of China under contract No. 2017J01075; the Technology Foundation for Selected Overseas Chinese Scholar Project “Impacts of Climate Change on Biology and Economy in the East China Sea”; the National Key Research and Development Program of China under contract No. 2018YFC1406503; the China-ASEAN Maritime Cooperation Fund Project “Monitoring and Conservation of The Coastal Ecosystem in The South China Sea”.

Abstract: Understanding the connectivity of fish among different typical habitats is important for conducting ecosystem-based management, particularly when designing marine protected areas (MPA) or setting MPA networks. To clarify of connectivity among mangrove, seagrass beds, and coral reef habitats in Wenchang, Hainan Province, China, the fish community structure was studied in wet and dry seasons of 2018. Gill nets were placed across the three habitat types, and the number of species, individuals, and body size of individual fish were recorded. In total, 3 815 individuals belonging to 154 species of 57 families were collected. The highest number of individuals and species was documented in mangroves (117 species, 2 623 individuals), followed by coral reefs (61 species, 438 individuals) and seagrass beds (46 species, 754 individuals). The similarity tests revealed highly significant differences among the three habitats. Approximately 23.4% species used two habitats and 11.0% species used three habitats. A significant difference (p<0.05) in habitat use among eight species (Mugil cephalus, Gerres oblongus, Siganus fuscescens, Terapon jarbua, Sillago maculata, Upeneus tragula, Lutjanus russellii, and Monacanthus chinensis) was detected, with a clear ontogenetic shift in habitat use from mangrove or seagrass beds to coral reefs. The similarity indices suggested that fish assemblages can be divided into three large groups namely coral, seagrass, and mangrove habitat types. This study demonstrated that connectivity exists between mangrove–seagrass–coral reef continuum in Wenchang area; therefore, we recommend that fish connectivity should be considered when designing MPAs or MPA network where possible.

Jianguo Du, Meiling Xie, Yuyu Wang, Zehao Chen, Wenhua Liu, Jianji Liao, Bin Chen. Connectivity of fish assemblages along the mangrove-seagrass-coral reef continuum in Wenchang, China[J]. Acta Oceanologica Sinica, 2020, 39(8): 43-52. doi: 10.1007/s13131-019-1490-7
Citation: Jianguo Du, Meiling Xie, Yuyu Wang, Zehao Chen, Wenhua Liu, Jianji Liao, Bin Chen. Connectivity of fish assemblages along the mangrove-seagrass-coral reef continuum in Wenchang, China[J]. Acta Oceanologica Sinica, 2020, 39(8): 43-52. doi: 10.1007/s13131-019-1490-7
    • Mangroves, seagrass beds, and coral reefs are three typical marine ecosystems, which are often adjacent to one another, forming connections through chemical, biological, and physical interactions (Ogden, 1997). Examples of connections that exist across these habitats include the ontogeny of larval, juvenile, and adult fauna (Lowe and Falter, 2015) and exchange of carbon, nitrogen, and phosphorus among different habitats (Nagelkerken, 2009), leading to connectivity. Several studies have shown that the connectivity of different habitats provides abundant food resources and suitable habitats for the biodiversity of adjacent ecosystems, playing an important role in maintaining population structure and regulating ecological processes (Kindlmann and Burel, 2008; Bauer and Hoye, 2014). However, the above ecosystems are negatively affected by various anthropogenic activities and climate change in recent decades, resulting in habitat loss and fragmentation (Krosby et al., 2010; Brodie et al., 2012). Consequently, the connectivity among these ecosystems is severely reduced (Hughes, 2003), leading to a decrease in the diversity of species and changes in community structures (Pandit et al., 2017). Therefore, it is necessary to increase our understanding about the connectivity across mangroves, seagrass beds, and coral reefs.

      Studies on the mechanism of connectivity have mainly focused on the transfer of organisms and nutrients, especially on the fauna migration between ecosystems (Du et al., 2015). Commonly, juvenile and adult fish occupy different habitats to meet their own needs, and undeniably mobile organisms move between and within habitat patches. Furthermore, some species often migrate to nearshore habitats to avoid being preyed (Shulman, 1985; Nakamura et al., 2003) or for food (Nagelkerken et al., 2000a) and as a nursery (Nagelkerken, 2009; Weinstein and Heck, 1979; Dorenbosch et al., 2004; de la Morinière et al., 2002; Nagelkerken et al., 2000b), which increased connectivity among different habitats. On the contrary, the biological connectivity enhances the productivity and biodiversity (Costanza et al., 1997). Therefore, it is important to maintain connectivity across different habitats to safeguard the structure and diversity of fish assemblages. However, the ontogenetic shifts and utilization patterns of fish from the typical marine ecosystems remain largely unknown, especially in China.

      Wenchang, located on the east coast of Hainan Province, has the largest area of seagrass in China, and it supports one of the few mangrove–seagrass–reef continuums in China. However, the coverage of seagrass and hermatypic corals in this area is sharply declining (Chen et al., 2015; Wu et al., 2017). In this study, we aimed to analyze the fish assemblages and connectivity along mangroves, seagrass beds, and coral reefs in Wenchang. The results of this study are expected to improve our understanding of the connectivity of fish assemblages among habitats in Hainan Province, which could help improve the habitats protection strategies and fisheries management.

    • Wenchang has tropical oceanic monsoon climate, sufficient light, high temperature, and abundant rainfall (Yang et al., 2017; Chen et al., 2015; Wu et al., 2011). The fish assemblages in the mangroves, seagrass beds, and coral reefs in Wenchang were studied, during the wet (March) and dry seasons (August) in 2018 (Fig. 1). Three sites passing through mangroves, seagrass beds, and coral reefs were designed. The site in mangrove is situated in the mouth of a lagoon at a depth of 0.5 m; the sediment is mainly silt and sand and the dominant species in mangrove are Bruguiera gymnorrhiza, Kandelia candel, and Aegiceras corniculatum. The site in seagrass is approximately 1 km from the coastline at the depth of 1 m, the most dominant species in seagrass beds is Enhalus acoroides, and the sediment is mainly composed of coral chips, shell chips and gravels. In the recent years, the seagrass distribution in this area has become patchy or scattered (average cover 33.55%) (Chen et al., 2015). The site in coral reef fringe is approximately 3 km from the coastline at the depth of 3 m. The dominant species are Platygyra daedalea and Lobophyllia corymbosa (approximately 65% coverage, but with only 0.33% live coral remaining) (Wu et al., 2011).

      Figure 1.  Three study sites in the mangrove–seagrass–reef continuum in Wenchang, Hainan Province, China. The mangrove area was the closest to land (approximately 360 m) and the seagrass beds were in an intermediate location of mangroves and coral reefs, whereas the coral reefs were located along the edge of the continental shelf.

    • Gill nets (height 1 m, width 150 m, and mesh size 0.5 cm) were used in all the three habitats, and were collected at 2–3 h during the day time (05:00 to 17:00), one net per day for five consecutive days. Gill nets provide a non-destructive method for sampling, and they are an effective method for sampling diurnal near-shore fish assemblages in seagrass and other typical habitats (Unsworth et al., 2007).

      All fish were identified to the species level (Chen and Yang, 2013; Liu et al., 2013; http://www.fishbase.org), while it was not possible to identify six species beyond the genus level. The total length (TL) of each specimen was measured to 0.1 cm accuracy. The collection, treatment, and analysis of samples were in accordance with the relevant provisions of the Marine Survey Specifications (General Administration of Quality Supervision, Inspection and Quarantine of the People's Republic of China and Standardization Administration of the People's Republic of China, 2007).

      To determine how the fish utilized the habitat, the species were divided into the following seven habitat groups: (1) mangrove species, which were only present in mangrove; (2) seagrass species, which were only present in seagrass beds; (3) reef species, which were only present in coral reefs; (4) mangrove–seagrass species, which were present in both mangrove and seagrass beds; (5) seagrass–reef species, which were present in both seagrass beds and coral reefs; (6) mangrove–reef species, which were present in both mangrove and coral reefs; and (7) mangrove–seagrass–reef species, which occurred in all three habitats (Nakamura and Sano, 2004).

    • All specimens were collected from the three sites. The family composition of species and the number of individuals in each habitat were analyzed. A nonparametric Kruskal–Wallis H-test was used to determine the difference in individual numbers and species in each habitat, because assumptions of homogeneity of variance could not be met by some data, even after transformation (Shibuno et al., 2008). Differences in body length among mangroves, seagrass beds, and coral reefs were analyzed. The numbers of species and individuals were also compared using Mann–Whitney U test in different seasons among the three habitats.

      The similarity of fish assemblages in the three habitats was calculated using data obtained from five consecutive days of sampling within each habitat for two seasons. The samples were ordinated based on Bray–Curtis dissimilarity matrices. The fish abundance results were visualized by nonmetric multidimensional scaling (NMDS). Fish community composition difference among habitat types were conducted using a nonparametric multivariate analysis of variance (NPMANOVA; a=0.05). The analysis was performed using the “vegan” package of R ver. 3.5.2 (R development Core Team).

    • A total of 3 815 individuals were collected from the three habitats, comprising 154 species belonging to 57 families (Table A1). In the mangrove area, 2 623 individuals of 117 species belonging to 48 families were collected. In contrast, fewer fish were recorded in the seagrass beds (754 individuals belonging to 46 species of 28 families) and coral reefs (438 individuals belonging to 61 species of 35 families). More number of species and individuals were recorded in the mangroves than that in the seagrass beds and coral reef habitats (Table 1). The mean number of individuals in the samples collected for five consecutive days in mangrove area was significantly higher than that in seagrass beds and coral reefs in wet season (p<0.05) (Fig. 2a). The mean number of species in the samples collected for five consecutive days in mangrove was significantly higher than that in coral reefs in wet season and in seagrass in dry season (p<0.05) (Fig. 2b). In addition, there was no significant difference in the mean number of individuals and species between seagrass beds and coral reefs.

      SpeciesCommon nameFamilyNumber of individuals
      MangroveSeagrass bedCoral reef
      Cynoglossus macrolepidotusTonguesoleCynoglossidae001
      Cynoglossus joyneriRed tonguesoleCynoglossidae200
      Brachirus orientalisOriental soleSoleidae910
      Pardachirus pavoninusPeacock soleSoleidae100
      Sardinella zunasiJapanese sardinellaClupeidae591
      Konosirus punctatusDotted gizzard shadClupeidae040
      Thrissa setirostrisLongjaw thryssaEngraulidae010
      Dasyatis akajeiWhip stingrayDasyatidae101
      Siganus fuscescensMottled spinefootSiganidae260 48 104
      Siganus guttatusOrange-spotted spinefootSiganidae14 01
      Leiognathus brevirostrisShortnose ponyfishLeiognathidae514
      Leiognathus equulusCommon ponyfishLeiognathidae202 11 1
      Leiognathus berbisBerber ponyfishLeiognathidae100
      Nuchequula nuchalisSpotnape ponyfishLeiognathidae14 13
      Gazza minutaToothponyLeiognathidae15 00
      Gerres oblongusSlender silver-biddyGerreidae200 53 1
      Gerres filamentosusWhipfin silver-biddyGerreidae48 34 0
      Gerres erythrourusDeep-bodied mojarraGerreidae050
      Gerres limbatusSaddleback silver-biddyGerreidae010
      Gerres macracanthusLongspine silverbiddyGerreidae15 00
      Upeneus tragulaFreckled goatfishMullidae74 23
      Upeneus sulphureusSulphur goatfishMullidae100
      Parupeneus ciliatusWhitesaddle goatfishMullidae201
      Parupeneus indicusIndian goatfishMullidae012
      Parupeneus multifasciatusManybar goatfishMullidae001
      Terapon jarbuaJarbua teraponTerapontidae15 66 39
      Therapon oxyrhynchusSharpbeak teraponTerapontidae200
      Terapon therapsLargescaled teraponTerapontidae200
      Pelates quadrilineatusFourlined teraponTerapontidae218 35 0
      Scarus frenatusBridled parrotfishScaridae004
      Scarus ghobbanBlue-barred parrotfishScaridae605
      Leptoscarus vaigiensisMarbled parrotfishScaridae100
      Sillago maculataTrumpeter whitingSillaginidae98 12 30
      Sillago japonicaJapanese sillagoSillaginidae38 10
      Lutjanus malabaricusMalabar blood snapperLutjanidae001
      Lutjanus russelliiRussell's snapperLutjanidae12 36
      Lutjanus fulviflammaDory snapperLutjanidae2117
      Lutjanus argentimaculatusMangrove red snapperLutjanidae101
      Lethrinus haematopterusChinese emperorLethrinidae018
      Lethrinus ornatusOrnate emperorLethrinidae301
      Lethrinus nebulosusSpangled emperorLethrinidae10 11
      Lethrinus harakThumbprint emperorLethrinidae001
      Scolopsis monogrammaMonogrammed monocle breamNemipteridae17 010
      Scolopsis vosmeriWhitecheek monocle breamNemipteridae204
      Scolopsis lineataStriped monocle breamNemipteridae003
      Scolopsis taeniopteraLattice monocle breamNemipteridae300
      Pentapodus setosusButterfly whiptailSparidae100
      Acanthopagrus schlegeliiBlackhead seabreamSparidae007
      Acanthopagrus chinshiraOkinawan yellow-fin seabreamSparidae100
      Rhabdosargus sarbaGoldlined seabreamSparidae010
      Labracinus cyclophthalmusFire-tail devilPseudochromidae001
      to be continued

      Table A1.  The number of individuals of fish in mangrove, seagrass bed and coral reef in Wenchang, China

      Habitat/habitat groupAll species/% (n)Individuals/%
      Mangrove
      Mangrove species57.3 (67)31.3
      Mangrove–seagrass species13.7 (16)17.9
      Mangrove–reef species14.5 (17) 2.9
      Mangrove–seagrass–reef species14.5 (17)48.0
      Seagrass bed
      Seagrass species21.7 (10) 2.8
      Mangrove–seagrass species34.8 (16)19.9
      Seagrass–reef species6.5 (3) 0.4
      Mangrove–seagrass–reef species37.0 (17)76.9
      Coral reef
      Reef species39.3 (24)16.1
      Seagrass–reef species4.9 (3) 3.0
      Mangrove–reef species27.9 (17)10.1
      Mangrove–seagrass–reef species27.9 (17)70.8
      Note: Number of fish species is shown in parentheses.

      Table 1.  Percentage contribution by species and individuals for each habitat group

      Figure 2.  Mean number of fish species (a) and individuals (b) during sampling months in each habitat. The error bars are standard deviations. MG, SG, and CR represent mangroves, seagrass, and coral reefs, respectively.

      The most dominant families in the mangrove were Gobiidae (20 species, 17.1%, represented by Oxyurichthys ophthalmonema), followed by Labridae (5 species, 4.3%, represented by Halichoeres nigrescens), and Leiognathidae (5 species, 4.3%, represented by Leiognathus equulus) (Fig. 3a). In the seagrass beds, Gerreidae (4 species, 8.7%), Leiognathidae (3 species, 6.5%), and Lutjanidae (2 species, 4.3%) were the dominant families, and the representative species included Gerres oblongus, Leiognathus equulus, and Lutjanus russellii. In the coral reef, the most dominant families were Labridae (8 species, 13.1%), Lethrinidae (4 species, 6.6%), and Lutjanidae (4 species, 6.6%), with Lethrinus haematopterus being the representative species.

      Figure 3.  Relative family composition of fish species (a) and the number of individuals (b) in the three habitats.

      In terms of individual numbers, Gobiidae was the dominant family in the mangrove area, accounting for 21.3% of all species, followed by Mugilidae and Siganidae, which together accounted for 22.4% of the species (Fig. 3b). Mugilidae was the most dominant species in the seagrass beds, representing approximately 47.7%, followed by Terapontidae (101 individuals, 13.4%) and Gerreidae (94 individuals, 12.5%). Siganidae (105 individuals, 24.0%), Mugilidae (89 individuals, 20.3%), and Terapontidae (39 individuals, 8.9%) were the three dominant species in the coral reef.

      The similarity indices suggested that fish assemblages can be divided into three groups (coral, seagrass, and mangrove habitat type) (Figs 4a, b). The results of similarity tests using NPMANOVA revealed a highly significant difference among habitats (F2, 27=4.01, p=0.003).

      Figure 4.  Nonmetric multidimensional scaling (NMDS) of data from all five days in each habitat in March (a) and August (b).

    • Of the 154 species recorded, 101 species (accounting for approximately 65.6% of all species) occurred in a single habitat, whereas the individuals accounted for 23.7% of all species. However, only 36 species were recorded in two habitats and 17 species were recorded in three habitats; thus, 34.4% of all species were recorded in multiple habitats (Table 1). Specifically, 16 species were recorded in the mangrove–seagrass areas, 17 species were recorded in the mangrove–coral reef areas, 3 species were recorded in the seagrass–coral reef areas, and 17 species were recorded in the mangrove–seagrass–coral reef areas. In the mangrove and coral reef areas, the local species represented more than 35% of the total species. In comparison, in the seagrass area, species that used multiple habitats accounted for 78.3% of all species. In terms of individuals, more than 60% of all individuals used two or three habitats, especially seagrass and coral reef areas. The mangrove–seagrass–coral reef species contributed to approximately 70% of all individuals in seagrass and coral reef areas, although seagrass–coral reef species only represented 0.4% of individuals in the seagrass beds. Minimal differences were found in both fish species and individuals using the mangrove–seagrass–coral reef continuum (Table 1).

      The length of eight species (Mugil cephalus, G. oblongus, Siganus fuscescens, Terapon jarbua, Sillago maculata, Upeneus tragula, L. russellii, and Monacanthus chinensis) was higher in coral reef than that in seagrass and mangrove areas, showing possible ontogenetic habitat shifts from mangrove or seagrass beds to coral reef (Fig. 5). This shift might explain the trend towards changing habitats by individuals and species. On the whole, in their early stages of development, these fish mainly inhabited the mangrove and seagrass beds, which is consistent with the fact that mangrove and seagrass habitats serve as nurseries for fish (Beck et al., 2001).

      Figure 5.  Relative abundance of the eight fish species in the mangrove (blue), seagrass (red), and coral reefs (green) habitats according to the class size using pooled date.

    • This study demonstrated that the structure of fish assemblages across mangroves, seagrass beds, and coral reefs differed with respect to the number of individuals and species (Fig. 3, Table 1). More number of individuals and species were present in mangroves than that in seagrass and coral reef areas. This result is different from that reported in Mindoro and Mindanao Islands in Philippines, where the fish species in coral reef (265) was much higher than that in mangroves (47) and seagrass beds (38) (Honda et al., 2013). This might be because the gill net was used to collect samples in this study, while the diving visual censuses was used in Philippines, which might miss several small fish, such as goby. Moreover, the fish diversity in mangrove is also high in other areas in China; for example, 115 species in Dongzhaigang National Nature Reserve for Mangroves (Shi, 2005) and 127 species in the Leizhou Peninsula (He et al., 2003). In addition, fewer individuals were present in coral reefs than those in seagrass; however, the diversity of species in coral reefs was higher than that in seagrass. The 117 fish species recorded in mangrove accounted for approximately 76.0% of all fish species recorded, whereas only 16 and 17 species exclusively utilized seagrass and coral reef habitats, respectively (Table 1). The fact that 67 species were found only in mangrove habitats emphasizes the need to protect multiple habitats even without considering connectivity.

      In this study, 53 of the 154 species were detected in two to three of the surveyed habitats. The distribution of the eight species based on the body length was significantly different among the three habitats (p<0.05; Fig. 5), which might reflect ontogenetic changes in habitat use among the three habitats. In recent years, several studies have focused on the ontogenetic changes in fish using the gut content and stable isotope (δ13C and δ15N) analyses (de la Morinière et al., 2003; Berkström et al., 2013). Monacanthus chinensis is omnivorous and seagrass is its minor food item (Bell et al., 1975). Most individuals of this species were detected in the mangrove area of this study and were in the early stage of development. Some adult individuals were also recorded in the seagrass beds. However, there was no record of this species in seagrass beds at Had Khanom Mu Ko Thale Tai National Park of Thailand (Sichum et al., 2003).

      Most individuals of the species L. russellii were detected in the mangroves and coral reefs, and this multiple habitat use reflects a relaxed day–night shift, with individuals feeding in seagrass beds at night and shifting to sheltered areas (mangrove/coral reefs) by daytime (Nagelkerken et al., 2000a; Bond et al., 2018). Upeneus tragula and Sillago maculata utilized almost all habitats. Gerres oblongus is a common species in coastal areas, and it spawns from February to June in the Jaffna Lagoon, Sri Lanka (Sivashanthini and Abeyrami, 2003). Thus, the samples collected from the mangrove and seagrass beds were juveniles, which preferentially use mangroves as a nursery over seagrass beds. Most juveniles and a group of adults of T. jarbua were found occupying the seagrass beds. Other adult fish of this species were found in coral reefs. On the contrary, 77% of Triacanthus biaculeatus individuals were recorded in seagrass beds, and only a few in mangroves. Overall, there was an abundance of fish in the early stages of development in the mangrove and seagrass beds, with more number of individuals in the later stages of development in the coral reefs. This finding was similar to that detected for fish assemblages in the Indo–Pacific (Unsworth et al., 2009) and Caribbean islands (Nagelkerken et al., 2002).

      Studies on connectivity among coastal habitats have mainly focused on the Indo–Pacific (Unsworth et al., 2007, 2008, 2009) and Caribbean (Weinstein and Heck, 1979; Nagelkerken et al., 2002; Kopp et al., 2007), particularly with respect to fish structure among different habitats. It is widely accepted that mangroves and seagrass beds serve as nurseries for reef fish species; however, empirical studies supporting this assumption remain limited. The structure, food resources, and shade provided by mangrove–seagrass beds strongly attract juvenile fish (Verweij et al., 2006). In addition, some studies have emphasized the importance of mangrove and seagrass beds for maintaining fish density in coral reefs (Nagelkerken et al., 2002). Furthermore, overlapping use of seagrass beds with adjacent coral by fish has been documented (Nakamura and Sano, 2004). Consequently, habitat degradation or loss in coastal areas could have significant negative effects on other fauna occupying these habitats.

      The rich fishery resources and numerous fishing gears in the South China Sea have promoted the rapid development of the marine fishery. According to the characteristics of fish and the environment, the choice of suitable gear is an issue worth considering. Trawling is the most important fishing tool in the South China Sea, but it is also the most damaging to fisheries and the marine environment (Yang, 1997). More numbers of young fish can be caught using stow net than by trawling (Zhang, 2014). Diving or snorkeling represent a good approach to study fish assemblages (Honda et al., 2013); however, this method depends on the water visibility is good enough. Gill nets were used in this study considering the complex habitats and low visibility in March and August. In the future, different methods may be used to study the connectivity, such as underwater visual census in appropriate months (Verweij et al., 2006; Hylkema et al., 2015). If possible, combining with the skill of otolith (Lueders-Dumont et al., 2018) would be another choice.

      Coverage of coastal habitats and fish diversity is declining, with multiple ecological habitats requiring protection for the comprehensive management of marine biodiversity. Relevant research is required for rapid recovery of population structures. This study contributes towards advancing our understanding of fish ecology and connectivity among habitats. In particular, the connectivity of coastal habitats should be incorporated into management plans, as these habitats are already severely degraded. For example, there is Eucheuma Nature Reserve of Hainan Province in Wenchang area, however, the key protected objects in this reserve is Eucheuma, but not the habitats (Wu et al., 2017). Moreover, there are also other MPAs, such as Qinglangang Provincial Mangrove Nature Reserve and Tongguling National Nature Reserve in Wenchang area, while the former focuses on mangrove and the latter focuses on coral reef; both do not consider the connectivity between these habitats. Therefore, it is recommended that fish connectivity should be considered when designing MPAs or MPA network where possible.

    • We express our sincere gratitude to the local fishermen in Wenchang, Hainan Province, China for their assistance in collecting the samples. We also extend our gratitude to the two anonymous reviewers for all their constructive comments which helped improve the manuscript.

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