Feeding Habits And Trophic Niches Of Coilia Nasus In The Three Gorges Reservoir Area

Coilia nasus,as a freshwater settlement fish,is originally distributed mainly in the middle and lower reaches of the Yangtze River and its affiliated lakes.However,In recent years,it has appeared in large numbers in the Three Gorges Reservoir area and gradually become the dominant species in some river sections.In order to explore the feeding and trophic niche characteristics of Coilia nasus,we sampled Coilia nasus from May to June and October to December in the Three Gorges Reservoir Area,including Wushan,Wanzhou,Fengdu.We examined the changes in food composition,feeding intensity,trophic level,and trophic niche of Coilia nasus with body length,different river sections,and hydrological periods using stomach content analysis and stable isotope analysis.The main research results are as follows:（1）The analysis of stomach contents showed that the feeding habits of Coilia nasus were different in different growth stages,river segments and hydrological periods in the Three Gorges Reservoir area.In the body length range of 130～230mm,Coilia nasus mainly fed on cladicornis and copepods.In the body length range of 230～280mm,prawn feeding began to occur.When the body length was larger than280mm,Coilia nasus mainly fed on fish and shrimp.In the range of 130～230mm body length,the feeding rate and average stomach fullness index of Coilia nasus were lower,the empty stomach rate was higher,the body length was>280mm,the feeding rate and average stomach fullness index were the highest,the empty stomach rate was the lowest,and the feeding intensity of Coilia nasus increased with the increase in body length.The trophic level of Coilia nasus ranged from 2.34 to 4.47,with an average trophic level of 3.50 in the Three Gorges Reservoir area,and the trophic level increased with the increase of body length.The largest niche width（2.27）was in the130-230mm group and the smallest（1.84）when the body length was>280mm in Wushan.while the largest niche width（2.02）was in the 180-230mm group and the smallest（1.69）when the body length was>280mm in Wanzhou.Overall,the niche width of small individuals was larger than that of large individuals.In the Three Gorges Reservoir area,the results of relative importance index（IRI%）showed that the main sources of food for Coilia nasus were cladospilia（38.02%）,copepods（30.78%）and shrimp（18.96%）in Wushan,while the main sources of food were cladospilia（37.45%）and shrimp（24.44%）in Fengdu.The feeding rate（85.19%）and average stomach fullness index（0.3%）of Coilia nasus in the Wushan River section of the Three Gorges Reservoir region were significantly higher than those in the other two river sections.The mean trophic level was 3.28±0.69 in Wushan,3.48±0.53 in Wanzhou,and 3.65±1.84 in Fengdu.Non-parameter analysis showed no significant difference in trophic levels among the three river segments（P>0.05）.The niche width index of Coilia nasus in Wushan was the highest（2.31）,while that in Fengdu was the lowest（1.57）.In the Three Gorges Reservoir area,the food composition of Coilia nasus in the Wanzhou and Fengdu river sections was mainly copepods,cladicans,and shrimp,while in the wet season,it was mainly fish and shrimp.The feeding rate and average stomach fullness index of Coilia nasus in Wanzhou and Fengdu in the dry season were higher than those in wet season,while the empty stomach rate was lower than that in the wet season,the feeding intensity was higher in dry season.The mean trophic level of Coilia nasus in Wanzhou and Fengdu in dry season（3.88,3.39）was lower than that in wet season（4.31,4.10）.The niche width index of Coilia nasus in Wanzhou was higher in dry season（2.11）than in wet season（1.48）.（2）The results of carbon and nitrogen stable isotope analysis showed that there were differences in the feeding capacity and trophic niche of Coilia nasus in different growth stages,river segments and hydrological periods in the Three Gorges Reservoir.δ¹³C values of Coilia nasus in different river segments in the Three Gorges Reservoir area were positively correlated with body length（P<0.01）.There was a negative correlation between the trophic level and body length of Coilia nasus in the Wushan River segment（P<0.01）,a positive correlation between the two in Wanzhou（P<0.01）,and no significant correlation between the two in Fengdu（P>0.05）.The indexes of the trophic niche of Coilia nasus increased first and then decreased with body length in the Three Gorges reservoir area.δ¹³C andδ¹⁵N values of Coilia nasus in the Wushan River segments of the Three Gorges Reservoir area were significantly different from those in Wanzhou and Fengdu（P<0.01）,whileδ¹³C andδ¹⁵N values in Wanzhou and Fengdu were not significantly different（P>0.05）.δ¹³C values of Coilia nasus in Wushan were lower than those in Wanzhou and Fengdu,whileδ¹⁵N values of Coilia nasus were higher than those in Wanzhou and Fengdu.The trophic level of Coilia nasus in the Wushan river segments（3.36±0.34）was significantly higher than that in Wanzhou（2.85±0.19）and Fengdu（2.91±0.16）.The feed contribution rate of Wushan was dominated by shrimp（53.28%）and fish（36.54%）,Wanzhou by shrimp（33.78%）and zooplankton（31.0%）,Fengdu by shrimp（33.25%）and zooplankton（32.96%）.The index of trophic niche Coilia nasus was the highest in Wushan.Theδ¹³C value of Coilia nasus in Wanzhou and Fengdu river segments in the Three Gorges Reservoir area was significantly different in different hydrological periods（P<0.01）,and theδ¹³C value of Coilia nasus in the two river segments in dry season was higher than that in the wet season.The trophic level of Coilia nasus in Wanzhou was less than that of Coilia nasus Fengdu in the dry season（3.85±0.19）than that of the wet season（4.33±0.14）,but there was no significant difference in Fengdu（P>0.05）.The trophic level of Coilia nasus in the dry season（3.85±0.19）was smaller than that in the wet season（4.33±0.14）in Wanzhou,and there was no significant difference in the trophic level of Coilia nasus in Fengdu（P>0.05）.The indexes of trophic niche Coilia nasus in both river segments were higher in the dry season than in the wet season.（3）The results of the studies on the feeding habits and trophic niche of Coilia nasus in the Three Gorges Reservoir area by stomach contents method and stable isotope method are basically consistent.However,there are some differences.For example,the trophic level of Coilia nasus calculated byδ¹⁵N value is different with stomach contents,which are calculated based on the trophic level and proportion of bait.Since only the bait that fish can eat for a short time can be determined,there is an error in the calculation of trophic level.Stable isotopes are calculated by comparing the difference inδ¹⁵N values between the consumer and the baseline organism,and reflect the consumer’s feeding habits over time with greater accuracy.There is also some difference between the feed contribution rate and the food composition of stomach contents calculated using theδ¹³C andδ¹⁵N values of Coilia nasus and potential feed species.For example,the contribution rate of zooplankton was higher than that of small individuals in large individuals in Wushan,while that of fish and shrimp was lower than that of small individuals.This is because theδ¹⁵N value of Coilia nasus was negatively correlated with body length in Wushan.The higher enrichment ofδ¹⁵N value would magnify the contribution ratio of bait.Both stomach contents and stable isotopes have their advantages and disadvantages in determining the feeding habits of fish.Therefore,the two methods should be combined to study the feeding habits and trophic niche of fish so as to make the results more accurate.