Current Situation Of Fishery Resources And Microplastic Pollution In Marine Ranching Of Wuzhizhou Island,Sanya,Hainan Province

In this paper,the status of fishery resources and micro plastic pollution in marine ranch of Wuzhizhou Island in Sanya,Hainan Province were evaluated.Three cruises were carried out in April,August and December 2019.The fishery resources,species composition and spatial distribution characteristics of marine ranch of Wuzhizhou Island were evaluated by using the methods of fishery resources acoustics and swimming animal trawl survey;Carbon and nitrogen stable isotope technology was used to analyze the trophic level of fishery resources obtained by trawl;The quantity and types of microplastics in gut and gill tissues of catches were analyzed,the pollution degree and source of microplastics in marine ranches were evaluated,and the relationship between microplastics pollution and trophic level of fishery resources was analyzed.The results are as follows:（1）Current situation of fishery resources of Marine Ranch on Wuzhizhou IslandThe average density of fish resources in April,August and December were 63.71t/km²（near the island core）,8.29 t/km²and 7.21 t/km²respectively.From the perspective of spatial distribution,the average quantity density and resource density in the east of the island are the highest,which are 12.3 times and 7.7 times of that in the north and west of the island respectively in April.In August and December,the fishery resource intensive area also appeared in the southwest of Wuzhizhou Island,and the resource density of fish center was more than 600 t/km².This area is a natural current channel between the coastal area of Wuzhizhou Island and Hainan Island.There are abundant coral reefs in the east of Wuzhizhou Island,and seagrass and seaweed beds in the West,Therefore,it may be an important channel for fish migration.In this study,Decapterus maruadsi,Trichiurus nanhaiensis,Trachurus japonicus and other important economic fish in the adjacent waters outside the marine ranch generally accounted for a small proportion of the total resources,which may be related to the overfishing of economic fish.（2）Current status of fish community nutritional structure in Marine Ranch on Wuzhizhou IslandThe trophic level of fish community in April and August was close,ranging from2.43 to 3.53,and the trophic level of fish captured in December was greater than 3.4.On the whole,the lower trophic level of fish in marine ranch and surrounding waters of Wuzhizhou island may be due to overfishing.Based on fish biologyδ¹³C、δ¹⁵N value,the evaluation results of fishery biological community nutritional structure were obtained.TA（Total area of convex hull）and CD value（Mean distance to centroid）represent the total and average degree of trophic diversity of food web,respectively.TA of fish community in three surveys are 4.73,3.62 and 6.55,respectively;The CD values were 0.94,0.88 and 0.83,respectively,which indicated that the total and average trophic diversity of fish community food web was low;CR value(δ¹³C Range)the habitat destruction caused by human impacts in Haitang Bay reduced the diversity of the initial food sources of the food web,resulting in a low CR value;NR value(δ¹⁵N Range)indicates the nutritional level occupied by consumers in the food web.Overfishing leads to the decrease of nutritional level of fish,resulting in the decrease of NR value;The NND（Mean nearest neighbor distance）and SDNND（Standard deviationof nearest neighbor distance）reflect the nutritional redundancy of the food web.The NND and SDNND values of the fish community in the three survey were much lower than those of the bottom fish community in the southwest shelf area of the Spratly Islands.（3）Assessment of microplastics pollution in Marine ranches of Wuzhizhou Island and its relationship with nutrient levelsAccording to the data analysis of microplastics in fish digestive tract and gills,fibrous microplastics,transparent microplastics and microplastics with particle size of0-1000μm accounted for the highest proportion;Among the three parts of gill,the proportion of micro plastic on gill arch is the highest,followed by Gill rake,which indicates that gill arch and gill rake can effectively reduce the harm of micro plastic ingested by fish.According to the abundance of microplastics in the digestive tract and gills of fish in August and December,the pollution degree of microplastics has no correlation with the trophic level,but only with the living water layer of fish.The abundance of microplastics in the bottom layer fish is higher than that in the upper and middle layer fish,for example,the abundance of microplastics in the digestive tract of bottom layer fish is 4.88～46.51/g,and that in the upper and middle layer fish is 2.14～16.63/g in August;In December,the abundance of microplastics in digestive tract of bottom layer fish was 7.44～11.9/g,and that of pelagic fish was 0.65～20.24/g,The results showed that there were five kinds of microplastics in fish:Cellophane,PP,PC,PPE and PET.The sources may include plastic waste caused by tourism activities around Haitang Bay,as well as woven bags,strapping ropes,fishing nets and other wastes used by fishermen.These plastic products gradually degrade into micro plastics.In the future,we should strengthen the standardization of the use of plastic products in the tourism activities of tourists around the marine ranch and the daily production of fishermen,and properly handle the plastic waste.