Identifying The Potential Spawning Grounds Of Phytophilic Fish In Poyang Lake Based On Remote Sensing And Hydrodynamic Model

Poyang Lake is the largest freshwater lake in China,and its excellent natural environmental conditions provide a suitable habitat for freshwater fish,sustaining a rich freshwater fish resource.In particular,the remarkable seasonal differences in the hydrological conditions of Poyang Lake lead to changes in the wetland landscape during the year,providing a high-quality spawning ground for fish that lay sticky eggs.In the context of the ten-year fishing ban in the Yangtze River basin,identifying potential spawning grounds can provide a baseline distribution for monitoring aquatic resources and thus test the effectiveness of the fishing ban policy.Traditional methods of spawning ground identification are based on random field surveys and sampling,which are difficult to reveal the spatial and temporal variability of spawning grounds at large scales.In recent years,studies on identifying fish spawning grounds based on remote sensing technology have begun to emerge.However,most have focused on marine ecosystems,neglecting freshwater ecosystems where hydrological conditions change rapidly,and human activities are frequent.Carp and crucian carp are fish species that prefer to spawn in shallow inundated grassland areas.This study combines remote sensing technology and a hydrodynamic model to classify Poyang Lake’s wetlands,extract the grassland distribution range by remote sensing data,and simulate the daily water depth data of Poyang Lake by the hydrodynamic model to obtain the shallow water area suitable for fish spawning.The aim was to identify the potential spawning grounds of grass-continent fish in Poyang Lake from 2000 to 2020,analyze their spatial and temporal variability,and investigate the efficiency of the existing Poyang Lake Carp Spawning Ground Provincial Nature Reserve in protecting the potential spawning grounds,to serve future systematic spawning ground surveys and provide a scientific basis for the management and protection of critical aquatic habitats.The main findings of the study are as follows:(1)During the dry period from 1990 to 2020,the grassland was mainly distributed in the southwestern,southern,and eastern parts of Poyang Lake.After 2000,the grassland gradually spread to the lake’s center,and the average distribution elevation dropped from 11.82 m to 11.35 m.After 2005,the spread was more drastic.In the last30 years,the grassland area of Poyang Lake during the dry season showed a significant increasing trend,and the average grassland area was 1414.21 km~2.During the spawning period,the grassland area increased slowly,with an average area of 551.59 km~2.The average grassland area after 2000 was significantly larger than before 2000 in both the dry and spawning seasons.During the dry season of Poyang Lake,grassland and other types of wetlands have different degrees of conversion from 1990 to 2020.The conversion between grassland and mudflat is the most intense,followed by the conversion between grassland and water.(2)From 2000 to 2020,the shallow water area suitable for fish spawning tended to migrate to the lake’s center.The spatial distribution pattern changed from 2004,especially the shallow water area in the center and north of the lake increased.After2005,the shallow water area in the southwest of Poyang Lake shifted more obviously to the center and north of the lake.In the recent 20 years,the shallow water area has not shown a significant increase or decrease trend,with an extensive fluctuation range,which may be related to the significant change of the hydrological regime of Poyang Lake.(3)From 2000 to 2020,the potential spawning ground area of phytophilic fish in Poyang Lake showed a slow increase trend,with an average area of 671.47 km~2.The potential spawning ground area around the lake shore gradually decreased and expanded to the lake’s center.Before 2005,the average area of potential spawning ground was 519.97 km~2,mainly distributed in the west,south,and east of Poyang Lake,with only sporadic distribution in the north.After 2005,the average area of potential spawning ground was 694.42 km~2,and its spatial distribution began to spread from around the lake shore to the center and north of the lake.The area of the potential spawning ground expanded since 2005 was 571.93 km~2,the size of the potential spawning ground decreased by 262.01 km~2,and the area of the potential spawning ground remained unchanged at 223.81 km~2.Under typical flood conditions,the potential spawning ground area was 38.12 km~2,mainly distributed in the south and east of Poyang Lake.The potential spawning ground area under typical dry conditions was356.48 km~2,concentrated in central,southern,and eastern regions.(4)From 2000 to 2020,the center of gravity of potential spawns always lies within the administrative division of the newly built area.The movement of the center of gravity shows a trend of "westward and northward" on the whole,with an apparent upward movement range in the south and north but a relatively small change range in the east-west direction.In recent 20 years,the center of gravity of the potential spawning field moved 10.83 km to the northwest direction at a speed of 0.52 km/ year,indicating that the southeast direction is strongly affected,leading to the overall movement of the potential spawning field to the northwest direction.Because the viscous eggs do not drift,the shift of the center of gravity of the potential laying ground is related to the spatial change of the grassland and shallow water area.In the recent 20 years,the standard elliptic of the potential spawning ground has moved from south to north,similar to that of the center of gravity.(5)There was a weak correlation between the grassland area and the potential spawning ground area in Poyang Lake(R=0.32,P=0.15).At the same time,there was a significant correlation between the shallow water area and the potential spawning ground area(R=0.79,P<0.01),indicating that the dynamic hydrological conditions of Poyang Lake were the main reason for the change of the potential spawning ground area.From 2000 to 2020,17.75% of potential spawning grounds were distributed in the Poyang Lake carp spawning grounds nature reserve,indicating that the protection efficiency of potential spawning grounds should be further improved.(6)In order to maintain fish biodiversity in Poyang Lake aquatic ecosystem,it is suggested to systematically investigate the distribution of potential spawning grounds in Poyang Lake to obtain an accurate distribution range of actual spawning grounds and then rationally optimize the scope and functional zoning of protected areas.The method used in this study can effectively quantify the long-term changes in the potential spawning ground of phytophilic fish in Poyang Lake,serve the conservation of aquatic biodiversity in Poyang Lake and the Yangtze River Basin,and provide a reference for biodiversity research in other freshwater ecosystems around the world.