Temporal And Spatial Distribution Patterns Of Fish Diversity In Salt Marsh Creeks Of The Yangtze River Estuary

One salt marsh function in estuarine and coastal ecosystems is to be an important nursery habitat for fishes and thus support estuarine and coastal fisheries. Due to high productivity and habitat heterogeneity, salt marshes provide abundant food sources and shelters for fish to escape predators. Therefore, fish densities, growth rates and survival in salt marshes are generally higher than in other estuarine and coastal habitats. Most dominant fishes in salt marshes constitute important fishery populations. Findings from previous studies have revealed that estuarine or near shore fishery yields were positively correlated with the areas of salt marsh. In addition, salt marshes export net productivity and are considered to be a significant source of secondary production in food webs of estuarine and coastal ecosystems. Part of the primary productivity is delivered to estuarine and coastal ecosystems by fishes. However, salt marshes are commonly located in regions with high human population density and developed economies. Salt marsh areas have rapidly decreased in the world in recent decades. Under unavoidable human activities, it is important to understand the conservation value of salt marshes for fishes, and to identify essential fish habitats in salt marshes. Therefore, fish knowledge is greatly needed for the conservation, restoration and management of salt marsh ecosystems.In China, salt marshes are widely distributed in the Yangtze River estuary, Hangzhou Bay, Yellow Sea and Gulf of Bohai. In the past 60 years, the area of salt marshes has greatly diminished owing to reclamation. The extant salt marshes have also multiple additional threats such as alien species invasions. It is necessary to study fish diversity patterns in Chinese salt marshes to understanding the salt marsh's conservation value for offshore fish diversity and to improve the knowledge of salt marsh ecosystem functions. However, baseline research on salt marsh fishes is very limited in China.In this study, intertidal salt marsh creeks in the Chongming Island and Jiuduansha Islands of the Yangtze River estuary are selected as study sites. The aim of this study is to reveal the temporal and spatial distribution patterns of fish diversity in these intertidal salt marsh creeks. Dial, lunar and seasonal changes of salt marsh fishes are delimited. Along a creek-order gradient, the spatial distribution patterns of salt marsh fishes are also described. Effects of salinity and geomorphological features on fish spatial distributions of intertidal salt marsh creeks are explored.From July 2004 to December 2008, fyke nets, seine nets and channel nets were used to sample fishes in intertidal salt marsh creeks of Chongming Island and Jiuduansha Islands. The major results are listed as follows:I. Composition of fish communities in intertidal salt mash creeksA total of 136,425 individuals weighing 157.417 kg were caught from intertidal salt marsh creeks in Chongming Island and Jiuduansha Islands in July and November 2004, February to May 2005, March to July and September to December 2007, January to March, May to July and December 2008. Fishes belonged to 13 orders,25 families,63 genera and 70 species (for details see Appendix I).The most species-rich orders were Perciformes (10 families,30 genera and 34 species) and Cypriniformes (2 families,17 genera and 17 species), which comprised 48.6%and 24.3% of the total species. The most species-rich families were Gobiidae (8 genera and 22 species) and Cyprinidae (14 genera and 14 species), together comprising 31.4% and 20.0% of the total species. The most numerically abundant families were Gobiidae (39.0%), Cyprinidae (24.4%), Mugilidae (20.2%), Engraulidae (12.1%) and Lateolabracidae (2.7%).The dominant species (percentage contribution of individuals equal to or greater than 1%) were:Pseudolaubuca sinensis (19.5%), Synechogobius ommaturus (15.0%), Liza affinis (14.5%), Mugilogobius abei (9.5%), Tridentiger trigonocephalus (7.1%), Coilia mystus (6.3%), Coilia nasus (5.8%), Chelon haematocheilus (5.7%), Hemiculter bleekeri (4.5%), Lateolabrax maculates (2.7%), Periophthalmus magnuspinnatus (2.3%), Acanthogobius luridus (2.2%), Periophthalmus modestus (1.0%). These 13 fish species comprised 96% of the total catch. Among these, S. ommaturus, L. affinis, C. mystus, C. nasus, C. haematocheilus and L.maculatus were important fishery species in the Yangtze River Estuary. These findings indicate that salt marshes are important for sustaining fisheries in the Yangtze River Estuary.According to the division of fish ecological guilds defined by Elliott et al. (2007), eight such guilds using intertidal salt marsh creeks were recognized:marine migrants (21 species), freshwater stragglers (16 species), estuarine species (15 species), freshwater migrants (8 species), marine stragglers (6 species), anadromous (2 species), catadromous (1 species) and semi-anadromous (1 species). The fish ecological guilds ranking by the individuals percentage were:marine migrants (33.8%), estuarine species (29.4%), freshwater migrants (19.9%), semi-anadromous (6.3%), anadromous (5.8%), freshwater stragglers (4.8%), catadromous (0.04%) and marine stragglers (0.04%). These findings indicate that the salt marshes of the Yangtze River Estuary are important habitats for different fish ecological guilds with different ecological functions.Fish communities were dominated by juvenile fishes, which comprised 85% of the total catches. Juvenile individuals in each of freshwater migrants, freshwater stragglers, anadromous, catadromous and semi-anadromous fishes, respectively comprised more than 90%of the total catch of each ecological guild. Juveniles of estuarine species and marine migrants comprised 80.4% and 75.2%. Juveniles of marine stragglers comprised 50.8% of its total catch. Thus, the salt marshes in the Yangtze River Estuary are likely to be important nursery habitats for juvenile fishes.II. Temporal distribution patterns of fishes in intertidal salt marsh creeks1) Seasonal changesSeasonal changes of fish diversity in intertidal salt marsh creeks were investigated in July and November,2004, and February and May,2005 in Jiuduansha. Fishes were collected by consecutive day and night samplings using fyke nets during the ebbing spring tides. A total of 25,010 individuals were caught during the study.17 families, 31 genera and 34 species were documented. The dominant species (Index of Relative Importance, IRI> 100) were:S. ommaturus, L. affinis, C. haematocheilus, Ophichthus apicalis, L. maculatus and Odontamblyopus lacepedii. There were significantly seasonal changes in fish use of salt marshes. No significant effects of diel periodicity on the fish community were found except for fishes sampled in summer. There were 19,20,13 and 9 fish species respectively recorded in Spring, Summer, Autumn and Winter. The highest abundance of fishes was documented in Spring, and the highest biomass of fishes in Summer. Non-metric multidimensional scaling analysis showed a clear separation of fish communities for different seasons. The seasonal pattern in fish species was probably due to fish spawning seasons adapted to hydrological characteristics (temperature and salinity).2) Tidal patternsThe changes in fish diversity between spring-neap tides were investigated from March 2007 to February 2008 in Dongtan, except for August 2007 owing to poor weather. Fishes were collected using fyke nets. A total of 74,062 individuals, belonging to 18 families,45 genera and 50 species, were captured. The dominant species (IRI> 100) were S. ommaturus, L. affinis, C. haematocheilus, P. sinensis, A. luridus, C. nasus, T. trigonocephalus, O. lacepedii and H. bleekeri. The fish community was dominated by juvenile individuals, comprised 97% of the total catches. ANOVA analysis revealed that fish species richness and total biomass were significantly affected by spring-neap tides, but the total abundance was not significantly different between spring and neap tides. In Spring, Autumn and Winter, there was higher richness of species during spring tides than during neap tides. In Summer, no significant difference in species richness between spring and neap tides was observed. Among dominant species, only C. nasus and O. lacepedii showed significant differences in abundance between spring and neap tides. C. nasus only appeared in Summer. More individuals of C. nasus were captured during neap tides than spring tides. O. lacepedii showed a reversed trend. The spring-neap changes of fish diversity in salt marsh creeks may be associated with water depth.3) Lunar patternsThe lunar changes of fish diversity were investigated in eight lunar phases in May through June 2008 in intertidal salt marsh creeks of the Jiuduansha wetland. A total of 17,935 individuals, belonging to 9 families,21 genera and 23 species, were captured. ANOVA analysis revealed that fish species richness, abundance, and biomass were significantly affected by lunar phases. There were higher species richness, abundance and biomass in the full moon, waning crescent moon, new moon and waxing crescent moon than in other lunar phases. Fish abundance and species richness peaked in the new moon and full moon. Abundance of M. abei and C. lucidus reached maxima during the new moon and waxing gibbous moon. There was higher abundance of L. affinis, B. pectinirostris and L. maculatus in the full moon or waning gibbous moon than in other lunar phases. Two-way ANOSIM analyses revealed significant lunar changes in fish communities (Global test R=0.542, P=0.001). Redundancy analysis revealed that water depth and temperature are the main environmental variables in shaping lunar changes in fish communities. Thus, hydrological characteristics of salt marsh creeks may determine lunar changes in fish diversity. 4) Diel patternsEffects of diel periodicity on fish communities were all involved in above-mentioned three studies. Generally, there were inconspicuous differences between day and night for species richness, total abundance, total biomass and abundance of dominant species. Significant differences in fish abundance and biomass between day and night were only observed in special season, month, spring-neap tides or lunar phases.Ⅲ. Spatial distribution patterns of fishes in intertidal salt marsh creeks1) Spatial distribution along a creek order gradientFish spatial distribution patterns along a creek order gradient (1-4 orders) were investigated in a low salinity intertidal salt marsh creek network. Fish were sampled using seine nets during daytime high slack water of spring tides for two or three days each in May through July 2008. A total of 1169 individuals, belonging to 8 families, 14 genera and 16 species, were captured. The dominant species were P. magnuspinnatus, L. affinis, M. abei, L. maculatus, P. modestus, C. haematocheilus and S. ommaturus. Rank abundance curves indicated a higher evenness of nekton assemblages in lower-order creeks compared to higher-order creeks. Fish abundance tended to increase with increasing creek order. Dominant fish species displayed different variation trends in abundance or in size-frequency distribution along the creek order gradient. The spatial separation of fish assemblages between the first-third orders and the fourth order could be attributed to geomorphological factors (distance to mouth and cross-section area). These findings indicate that both lower-and higher-order creek edges play important yet different roles for nekton species and life history stages in salt marshes.2) Effects of salinity on spatial distributions of fishesEffects of salinity on spatial distributions of fishes were investigated in three oligohaline creeks (salinity 0.3～4.5, mean 2.0) and six mesohaline creeks (salinity 5.8～13, mean 8.8) in the Chongming Dongtan wetland. Fishes were sampled with fyke nets during spring tides between May and December 2008. A total of 7818 individuals, belonging to 12 families,31 genera and 33 species, were captured. The dominant species were M. abei, L. affinis, L. maculatus, P. magnuspinnatus, C. haematocheilus, B. pectinirostris, S. ommaturus, O. lacepedii and T. trigonocephalus. These dominant species responded to salinity differently. M. abei, P. magnuspinnatus, B. pectinirostris and O. lacepedii were mainly distributed in the mesohaline creeks; L. maculates and L. affinis were mainly distributed in the oligohaline creeks; T. trigonocephalus was distributed in both mesohaline and oligohaline creeks. A higher abundance of L. affinis was found in oligohaline creeks during May and in mesohaline creeks in December. S. ommaturus showed the reversed trend. Estuary residents dominated the fish assemblage in mesohaline creeks, and juveniles of marine transients dominated fish assemblage in oligohaline creeks. Redundancy analysis revealed a clear spatial separation of fish assemblages between oligohaline and mesohaline marsh creeks, which could be attributed to salinity.3) Effects of geomorphological features on spatial distributions of fishesEffects of geomorphological features on spatial distributions of fishes were investigated in seven small creeks within one large creek at the Chongming Dongtan wetland. Fishes were collected using channel nets in March through July, September, November 2007 and February 2008. A total of 10432 individuals, belonging to 8 family,23 genera and 25 species, were captured. The dominant species were L. affinis, C. haematocheilus, C. auratus, S. ommaturus, A. luridus and L. maculatus. Canonical correlation analyses showed that species richness, total abundance and biomass were strongly related to the creek geomorphological variables. Intertidal creeks with shallow water depth, gentle slope and steepness supported higher fish abundance. Species richness was positively correlated with depth, cross sectional area, volume and steepness. L. affinis and C. haematocheilus favored small size creeks with high elevations, and L. maculatus preferred deep creeks with large cross section, slope, and steepness. Geomorphological features of creeks play important roles for fishes in selecting habitats.In sum, salt marshes in the Yangtze River estuary show strong ecosystem functions for supporting fish diversity, and provide important habitats for multiple fish ecological guilds and as nursery grounds of economically important fishery species. The temporal and spatial distribution patterns of the fish community in the intertidal salt marsh creeks can be summarized as follows:a) The fish community was dominated by a few species; b) Juveniles comprised the major part of the fish community; c) Marine migrants and estuarine species were the dominant fish ecological guilds; d) The fish communities showed strong patterns of seasonal changes or lunar changes, and related weak patterns of spring-neap tide changes or diel changes; e) Along a vertical direction to the coastline, fish community structures showed significant differences between lower-order creeks and higher-order creeks; f) Parallel to the coastline, fish community structures showed significant differences between oligohaline and mesohaline creeks; g) Hydrological characteristics, geomorphological features and fish life history are the major forces shaping temporal and spatial distribution patterns offish diversity in these habitats.These findings have important implications for the conservation, restoration and management of the Yangtze River Estuary salt marshes:a) Salt marshes are important for sustainable utilization of fisheries in the Yangtze River estuary; b) Spring and Summer were the critical periods for fish use of salt marshes; c) Creek geomorphological features should be taken into account for salt marsh restorations; d) Well-developed intertidal creek systems in salt marsh zones with different salinity should be protected and reserved.