Spatial Dietary Niche Of Trichiurus Lepturus And Decapterus Maruadsi In The Northern South China Sea As Revealed By Carbon And Nitrogen Stable Isotope

Food connection is the most important relationship between marine fish.Fish feeding habits and trophic level are important focuses of fishery ecology research.Trichiurus lepturus and Decapterus maruadsi are widely distributed in the coastal ocean of China,and they are also important catches of light luring seine and trawl seine in the South China Sea.In this study,we analyzed the changes of feeding habits and trophic level of Trichiurus lepturus and Decapterus maruadsi under the current environment and the main findings were reported below.Theδ¹⁵N value of Trichiurus lepturus ranges from 10.14 to 15.00,with an average of 11.77 and theδ¹³C value of Trichiurus lepturus ranges from-19.45 to-15.95,with an average of-17.67.The stable isotope characteristics of Trichiurus lepturus at different water depths are different.The stable isotope values of Trichiurus lepturus on the far shore（100～200m）are lower than those of the Trichiurus lepturus with the same sizes on the close shore（20～80m）.According to the water depth,Trichiurus lepturus can be divided into three groups:groups of 20-40m,60-80m and100-200m.Among them,the water depth of 60～80m could be the transitional zone.Theδ¹⁵N value of Trichiurus lepturus at the water depth of 20-40m ranges from10.14 to 15.00,with an average of 12.02,and theδ¹³C value ranges from-18.54 to-15.95,with an average of-17.28.Theδ¹⁵N value of Trichiurus lepturus at the water depth of 60～80m ranges from 10.52 to 13.63,with an average of 11.99,and theδ¹³C value ranges from-18.70 to-16.62,with an average of-17.76.Theδ¹⁵N value of Trichiurus lepturus at the water depth of 100～200m ranges from 10.23 to 12.14,with an average of 10.98,and theδ¹³C value ranges from-19.45 to-17.66,with an average of-18.59.With the development of Trichiurus lepturus at the water depth of 20-40m,its niche experienced a process of initial rising and then decreasing,with the peak value occurring when the anal length reaches approximately 200mm.The niche of Trichiurus lepturus at the water depth of 100～200m rises continuously during the process of growth.In this study,the C/N value was used to indicate the fat content in the muscle.The C/N values of Trichiurus lepturus at the two water depths are also completely different.The C/N value of Trichiurus lepturus at the water depth of20-40m initially decreases and then rises,which is opposite to the changes of its niche.The lowest C/N value occurred when the anal length reaches approximately 200mm.The C/N value of Trichiurus lepturus at 100-200m water depth does not change significantly during the process of development.Since theδ¹⁵N value of Amussium Pleuronectes,which is the baseline organism,is variable between individuals with different sizes or when individuals are located at different depth of water,this study used two strategies（strategy A and strategy B）to calculate the trophic level of Trichiurus lepturus.Strategy A considers that there is no difference（or the difference is negligible）in the stable isotope background values at different water depths in the northern South China Sea,and the averageδ¹⁵N value of the Amussium Pleuronectes can be used as a constant baseline.Strategy B considers that the backgroundδ¹⁵N values have changed,and Amussium Pleuronectes are divided into groups according to their water depth.The averageδ¹⁵N value of Amussium Pleuronectes in each group of water depth is used to calculate the trophic level.The range of trophic level calculated by Strategy A is 3.61-5.04,with an average of 4.09.The trophic level at the water depth of 20-40m ranges from 3.61 to5.04,with an average of 4.16.The trophic level at the water depth of 60-80m ranges from 3.72 to 4.63,with an average of 4.15.The trophic level at the water depth of100-200m ranges from 3.63 to 4.19,with an average of 3.86.The trophic level calculated by Strategy B ranges from 3.21 to 5.07,with an average of 4.01.The range of trophic level at the water depth of 20-40m is 3.64 to 5.07,with an average value of4.20.The trophic level at the water depth of 60-80m ranges from 3.70 to 4.61,with an average of 4.13.The trophic level at the water depth of 100-200m ranges from 3.21 to3.77,with an average of 3.43.Theδ¹⁵N value of Decapterus maruadsi ranges from 7.29 to 12.10,with an average of 10.83 and theδ¹³C value ranges from-19.32 to-16.10,with an average of-17.94.Theδ¹⁵N andδ¹³C values of Decapterus maruadsi increase with body length,reach the peak when the body length is approximately 100mm,and then decrease slightly.The C/N value decreases with growth,and slightly increases after reaching the body length of approximately 100mm.There is no significant difference between the C/N values at different water depths（P>0.05）.Theδ¹⁵N value of female Decapterus maruadsi was significantly higher than that of males（P<0.05）.There is no significant difference inδ¹³C and C/N value between males and females（P>0.05）.The trophic level of Decapterus maruadsi was calculated using the consistent baseline value,and the calculated trophic level ranges from 2.77 to 4.19,with an average of3.81.There is a positive correlation between theδ¹⁵N andδ¹³C values in Trichiurus lepturus and Decapterus maruadsi.We propose that the"abnormal"developmental changes of Trichiurus lepturus niche at the water depth of 20-40m is due to the differences in the adaptability of Trichiurus lepturus at different life stages to the same environment.