Characteristics Of Carbon And Nitrogen Stable Isotope Of Pacific Saury In The Northwest Pacific Ocean

Cololabis saira（Cololabis saira）is widely distributed in the northwest Pacific Ocean and is one of the important oceangoing objects in China.In order to study the characteristics of the feeding and trophic niche of pacific saury,based on the samples collected from July to November 2018 in the sea of the Northwest Pacific Ocean,the stable isotope characteristics of carbon and nitrogen in muscle samples of pacific saury were analyzed.The results show that:（1）Stable isotopes of carbon and nitrogen were significantly different before and after lipid removal,δ¹³C andδ¹⁵N values increased by 1.08‰and 0.51‰,respectively,and C:N decreased by 0.68 after lipid removal.According to Akaike information criteria（AIC）,the best fitting model for lipid removal wasΔδ¹³C=（6.92×C:N--19.89）/（C:N+1.58）.（2）In 2018 samples of stable carbon isotope value range between 24.60‰～20.69‰,nitrogen stable isotope values range between 6.49‰～11.11‰,copepods as baseline,trophic level range is between 2.44～3.71;2019 samples of carbon stable isotope value range between 22.91‰～19.70‰,nitrogen stable isotope values range between 5.25‰～11.04‰,copepods as baseline,trophic levels range from 2.08 to 3.69.（3）There were no significant differences inδ¹³C value,δ¹⁵N value and trophic level among male and female samples（P>0.05）;The trophic niche（SEAc）and total niche（TA）of male samples were higher than those of female samples.（4）Theδ¹³C values of 2018 samples showed significant differences between months（P<0.05）,and theδ¹⁵N values of 2019 samples showed significant differences between months（P<0.05）.The maximum of 2018 sample niche was in November,and the maximum of 2019 sample niche width was in September.The sample niche overlap rate was high in 2018,and the sample niche was dispersed in 2019.There was a significant positive correlation betweenδ¹³C and sea surface temperature（P<0.05）,but no correlation betweenδ¹³C and chlorophyll-a concentration.（5）There were significant differences inδ¹³C andδ¹⁵N values between the samples from the Oyashio extension and the northwest subarctic grey area in 2018（P<0.05）,and theδ¹³C,δ¹⁵N and trophic level values in the Oyashio extension area were higher than those in the northwest subarctic grey area.The SEAc and TA in the Oyashio extension area are lower than that in the northwest subarctic grey area.There were significant differences inδ¹³C andδ¹⁵N values between the western and eastern Oyashio extension areas in 2019（P<0.05）,andδ¹³C,δ¹⁵N and nutrient class values in the western Oyashio extension area were higher than those in the eastern Oyashio extension area.The SEAc and TA in the western part of the Oyashio extension area are lower than those in the eastern part of the Oyashio extension area.（6）The Generalized Additive Model（GAM）showed thatδ¹³C,δ¹⁵N and trophic level values increased with the increase of body length（P<0.05）.In 2018,the SEAc of251～260 mm body length group was higher than that of other body length groups.In2019,the SEAc of 281～290 mm body length group was the highest,and the SEAc of<240mm body length group was the lowest,and the overlap rate of<240mm body length group and>280mm body length group was the lowest.The cluster analysis results showed that the samples in 2018 could be divided into three groups with body length less than280mm and greater than 280mm,and the samples in 2019 could be divided into three groups with body length less than 240mm,251mm-280mm,241-250 and 281-290mm,and>290mm.In conclusion,month,habitat area,and life history stages can affect stable isotope values and feeding of Pacific saury.The stable isotope values of carbon and nitrogen of Pacific saury samples have significant differences in different seasons,and Pacific saury has higher bait diversity,larger niche area,and stronger utilization ability of bait.The protide-extending area and the northwest subarctic circulation affect the stable isotope values and feeding of the Pacific saury,leading to the differences in nutrient levels and food sources of the species in different sea areas.In addition,Pacific saury can feed on plankton with higher nutrient levels as they grow.The results can provide reference for the study of feeding ecology of Pacific saury in the Northwest Pacific Ocean.