Habitat Characteristics Of Dominant Species Of Fish Larvae In The Yangtze Estuary Based On Species Distribution Model

Fish larvae are important component of estuarine systems,and their survival rate directly affects the early replenishment process of fish.The Yangtze estuary is a nursery habitat for many economically important fish species and occupies an important position in the process of fishery resource replenishment in the Yangtze estuary and adjacent waters.Based on the biological characteristics of fish larvae and the way they use the estuary,fish larvae in the Yangtze estuary can be divided into four ecological guilds:diadromous category,freshwater category,marine category and estuarine category.In this paper,based on the fishery survey data of eggs and fish larvae of important fishery resources and environmental survey in May(spring)and August(summer)of 2020 in the waters of the Yangtze estuary,we selected diadromous category C.mystus,freshwater category P.sinensis,marine category E.japonicus and estuarine category R.giurinus were used as representatives of four different ecological guilds of fish larvae to analyze the distribution characteristics of their resources and the spatial and temporal distribution characteristics of environmental factors.Random forest(RF)model was used to analyze the relationship between the distribution of the four species and the habitat factors,to investigate the importance of the habitat factors,and to select the most suitable RF model to predict the distribution of the four species.Combining the importance of habitat factors on the distribution of fish larvae in the RF model,the suitability indices of single habitat factors were assigned weights,and the Weighted Arithmetic Mean Model(WAMM)and Weighted Geometric Mean Model(WGMM)were used to predict the distribution of fish larvae.The Habitat Suitability Index(HSI)model was constructed and the optimal model was selected by cross-validation to investigate the relationship between habitat conditions and habitat factors of the four fish larvae species,analyze the degree of influence of habitat factors on the habitats of fish larvae of different ecological guilds,and determine the optimal habitats of the four fish larvae species.(1)The mean sea surface salinity,sea bottom salinity,sea surface chlorophyll a concentration and sea bottom chlorophyll a concentration in the Yangtze estuary were higher in spring than in summer,but the sea surface temperature as well as the mean sea bottom temperature were lower than in summer.The temperature and depth of the south branch of the Yangtze estuary were higher than those of the north branch,while the salinity of the south branch was lower than that of the north branch,and there was no obvious pattern of change in chlorophyll a concentration.The total of 82,891 fish larvae were collected in the spring cruise,and 47 species and 5 undetermined species were identified;6,475 fish larvae were collected in the summer cruise,and 43 species were identified.Among them,four species of fish larvae,namely,C.mystus,P.sinensis,E.japonicus and R.giurinus,accounted for 66%and 43%of the total catches in the spring and summer voyages,respectively.The highest average density of C.mystus in spring was 2.52 ind/m~3,and the highest average density of P.sinensis in summer was 0.08 ind/m~3.(2)The main environmental factors affecting the distribution of the four species and their importance were:C.mystus:sea bottom chlorophyll a concentration(25.33%),sea surface temperature(24.66%)and sea surface salinity(20.79%);P.sinensis:sea surface salinity(18.80%),latitude(16.67%),longitude(11.81%)and sea surface temperature(9.15%);E.japonicus:longitude(27.24%)and sea surface salinity(22.06%);R.giurinus:sea surface temperature(16.79%),latitude(13.37%),sea bottom chlorophyll a concentration(10.20%)and sea surface salinity(9.24%).Chlorophyll a concentration was the most important for the distribution of C.mystus habitat,because diadromous species are more sensitive to environmental changes and can effectively select bait-rich nursery grounds.Salinity is of greater importance for both larvae distribution and is an important reason for the spatial ecological niche differentiation of P.sinensis and E.japonicus.R.giurinus is affected by several environmental factors such as temperature,because estuarine species are relatively insensitive to changes in environmental factors,can adapt to the variable environment of the estuary,and widely distributed.(3)The spatial ecological niche width of R.giurinus was the highest in May(3.04)and August(2.98),and the spatial ecological niche of all four species of fish larvae did not overlap significantly,with the highest overlap between R.giurinus and C.mystus in May(0.55)and between R.giurinus and P.sinensis in August(0.58)competition.(4)The habitat suitability estimation of P.sinensis was optimal for WAMM in spring and WGMM for the remaining three species,and the habitat suitability estimation of R.giurinus was optimal for WGMM in summer and WAMM for the remaining three species.There were some seasonal variations in the optimum habitats of the four species of fish larvae,and the distribution of the optimum habitats varied among the four species of fish larvae due to their different use of estuarine habitats.In spring,the optimum habitat of C.mystus is mainly located outside the north branch of the Yangtze estuary at 122°E,the HSI value is low in the Yangtze estuary,especially in the south branch;P.sinensis is mainly located in the south branch of the Yangtze estuary and near Chongming Island,the points with high resource density are basically within their optimal habitat;E.japonicus is mainly located outside the Yangtze estuary,and R.giurinus is distributed outside the mouth of the south branch of the Yangtze estuary,outside the mouth of the north branch and in the offshore area.The optimal habitat for all four species of fish larvae in summer is further south than in spring.