Feeding Characteristics And Behavioral Responses Of Fish To Microplastics

Microplastics typically refer to plastic fibers,pellets,or films smaller than 5millimeters in size,and have become one of the most important pollutants in the world recently.Previous studies have shown that microplastics are ubiquitous in various environmental media and can enter organisms to cause adverse effects.However,there is still a lack of effective methods and sufficient evidence to accurately assess the ecological risk of microplastics.One important reason is that the solid-state characteristics of microplastics lead to great differences from common chemical pollutants in their exposure pathways and toxic dynamic processes.The current ecotoxicological research methods on microplastics are mainly based on exposure of chemical pollutants and detection of toxicological endpoints.Ingestion is the most important way for microplastics to enter the body of organisms,and feeding behavior may affect the "bioavailability" of microplastics.However,feeding behavior has been largely overlooked in current research on the ecological effects of microplastics.Therefore,there is an urgent need to strengthen the in-depth study of the feeding process and mechanism of organisms on microplastics as well as their behavioral response.To this end,this thesis selects fish,which have important ecological and economic status in the food chain,as the research objective,and combines field surveys and laboratory experiments to explore the process and characteristics of fish feeding on microplastics,as well as the behavioral response of fish to exposure to microplastics.The main research content and conclusions of the study are as follows:(1)A typical marine fishery(Wentai fishery)and a plastic production area(Cixi,Zhejiang)were selected to study the characteristics of microplastic pollution in fish.The results showed that in 612 fish samples from 22 species in Wentai fishery,the detection rate of microplastics was 33%,and the average abundance was 0.8 items/ind.,indicating a relatively low pollution level.The coefficient of variation of microplastic abundance in the gastrointestinal tract of the same species was as high as 141%,indicating that the microplastic intake of individual fish varied greatly.In the 126 fish samples of 4 species from the plastic processing area,the abundance of microplastics in the gastrointestinal tract of Hemiculter leucisculus ranged from 2 to 6 items/ind.,and no microplastic fragments or pellets were found in the gastrointestinal tract of Hypophthalmichthys molitrix,Carassius auratus,and Cyprinus carpio,although these types of microplastics were detected in their habitats.The abundance of microplastics in the water ranged from 0.4 to 20.5 items/L,and in the sediment from 44.4 to 124.7items/kg,indicating a higher pollution level in the plastic production area.In addition,the organ index of liver histopathology in H.leucisculus from the plastic production area(8.07 ± 1.85)was significantly higher than that from the reference site(1.63 ±0.89).(2)To explore the feeding characteristics of different fish species on microplastics,an observation platform for fish behavior was established,and four fish species with different feeding types were selected.The results showed that filtering-feeding fish(H.molitrix)did not show the capturing behaviors on microplastic pellets,while two swallowing-feeding fish(Micropterus salamides and Pelteobagrus fulvidraco)and one sucking-feeding fish(C.auratus)showed the capturing behaviors.The swallowingfeeding fish had a 20 to 40 percent chance of swallowing pellets.All species did not actively capture microfibers;instead,they passively sucked in microfibers while breathing.Fish showed a rejective behavior which was spontaneously coughing up microfibers mixed with mucus.Moreover,the abundance of ingested microfibers in the fish bodies was increased in the presence of food.Therefore,this study proposes that fish ingest microplastics inadvertently rather than intentionally(3)To further investigate the individual differences in microplastic ingestion by fish,the visible implant fluorescent tags were used to recognize individual Chindongo demasoni for the study of microplastic ingestion characteristics and swimming behavior.The results showed that even within the same exposure group,there was significant variation in microplastic ingestion among different individuals,with a coefficient of variation of 67.5% ± 18.3%.The number of fibers in an individual’s body was positively correlated with its mean speed(r = 0.512),acceleration(r = 0.402),and swimmingcovered area(r = 0.301).The number of pellets ingested by the individual was positively correlated with its acceleration(r = 0.370)and the time spent at the front position(r = 0.399).These findings suggest that personality plays an important role in microplastic ingestion by fish,with active fish having more possibility to ingest microplastics.4)To investigate the responses of fish to exposure to microplastics and the effects of microplastics on fish movement and schooling behavior,two-dimensional and threedimensional platforms were established to observe individual and group fish,and idtrackers.ai and other methods were used to identify and track fish.The results showed that different fish species have different behavioral responses to pellets.The swimming speed of C.auratus increased by 72.6% to 119.2% due to multiple capturing and rejecting behaviors,while that of Micropterus salamides and Pelteobagrus fulvidraco decreased by 9.2% to 38.5%.H.molitrix was not affected by microplastics.In twodimensional platform system,microplastics increased the nearest neighbor distance of C.demasoni by 19.5%-25.6% and enhanced their spatial exploration ability.The effect of microplastics on schooling behavior was reduced in three-dimensional platform systems with water flow,but still affected the spatial distribution of Paracheirodon axelrodi.5)Based on the comprehensive analysis of the results from the field investigation and laboratory experiments,it can be concluded that the differences in feeding types result in differences in microplastic ingestion among fish species,and the differences in swimming behavior are the main reasons for the differences in individual microplastic ingestion.The feeding response of fish to microplastic can lead to changes in their swimming and schooling behaviors.On this basis,this thesis further summarizes the factors that lead to differences in microplastic abundance in the fish bodies,including biological characteristics,microplastic characteristics,and experimental methods.It is suggested that whether in the field investigation or microplastic exposure in the laboratory,these factors should be considered in future studies.This thesis uses an interdisciplinary approach to explore the interaction between fish and microplastics from a new perspective.By establishing the method of fish behavior response to microplastics,we found the feeding and rejecting processes of fish on microplastics,revealing the mechanism of fish ingesting microplastics and the reasons for the differences of microplastic in fish.This thesis provides a new understanding of the interaction between organisms and pollutants and provides the scientific basis for the establishment of ecotoxicological research methods suitable for microplastics.