Feeding Of Bigeye Tuna(Thunnus Obesus)in The Tropical Eastern Pacific Ocean Base On Biomarker Techniques

Bigeye tuna（Thunnus obesus）is the top opportunistic feeders in the ocean,and mainly preying on cephalopods,crustaceans and small and medium-sized fish.It is also preyed by large sailfish、sharks and cetaceans.Bigeye tuna is an important part of energy flow and material circulation in the marine ecosystem.The changes of population dynamics and feeding will affect low-trophic biological resources,it will has an influence on the dynamic balance between the structure and function of the entire marine ecosystem,ecological capacity and consumption of biological resource and replenishment.The researchers have done a lot of studies on the diet composition,feeding intensity and migration of bigeye tuna,but these generally based on the macroscopic analysis of stomach contents,and most of them are concentrated in the Indian Ocean,Atlantic Ocean and Western and Central Pacific Ocean.However,there was no report about using stable isotope and fatty acid to study the feeding ecology of bigeye tuna in the tropical Eastern Pacific Ocean.In the present study,we use a range of biological information and muscle tissue samples of bigeye tuna collected by the resource survey of a tuna longline fishing operation in the Eastern Pacific Ocean from2018 to 2019 to studied the basic biology of bigeye tuna around the Eastern Pacific Ocean,and studied the feeding of bigeye tuna based on the biomarker methods（fatty acid marker and stable isotopic analysis）,which explores the nutritional value,food source,nutritional level,and dietary characteristics of the bigeye tuna during growth.The results can provide basic information for in-depth study of the feeding ecology of bigeye tuna,which has practical significance for the conservation and sustainable use of bigeye tuna.The main conclusions of this study can be found as follows:（1）The range of fork length was from 83cm to 229cm and a mean fork length of 149.68±17.74cm.The range of net weight was from 10kg to 160kg and a mean net weight of 54.79±18.88kg.There was a significant difference in the fork length between males and females.The relationship between fork length and net weight could be described as there was W=10-5FL3.0093（R²=0.9447）.There was no significant difference for the relationship between fork length and net weight for males and females.The ratio of female and male was 0.93:1.The proportion of male increases as the fork length increases.The empty stomach rates of male and female were 5.43%and 2.10%,respectively.There was a significant difference in the feeding intensity of male and female.The feeding intensity increases as the fork length increases.Females reached sexual maturity round 110cm,and meals reached sexual maturity round 90cm.There was a significant difference in the sexual maturity between male and female.（2）35 kinds of fatty acids were detected in muscle tissue of bigeye tuna in the Eastern Pacific Ocean,including 15 kinds of saturated fatty acids（SFA）,9 kinds of monounsaturated fatty acids（MUFA）and 11 kinds of polyunsaturated fatty acids（PUFA）.The content of SFA was significantly higher than that of MUFA and PUFA,and the content of n-3 fatty acids was significantly higher than that of n-6 fatty acids.C14:0、C16:0、C16:1n7、C18:1n9c、C20:3n3、C20:4n6（ARA）and C22:6n3（DHA）were the main fatty acids in muscle of bigeye tuna.In addition,the content of C16:0 is the highest（31.11%）,EPA（C20:5n3）and DHA are 1.475%and 14.77%,respectively.There is no significant relationship between the fatty acid content and gender.（3）Through the analysis of the special fatty acids,it was found that herring and sardines feed more directly macro alga and benthic organism,and they have a high contribution rate to the food of bigeye tuna.The content of EPA and C16:4n1 and C16:1n7 was lower,showing that bigeye tuna and its bait organisms do not feed or seldom feed diatoms.The content of DHA was relatively higher,and EPA/DHA<1,which showing that bigeye tuna feed more directly or indirectly on dinoflagellates.The content of C20:1 and C22:1n9 was higher,showing that bigeye tuna feed more directly or indirectly herbivorous copepods.The content of C18:2n6 and C18:3n3 was lower,showing that the land plants have no directly or indirectly contribution to the food of bigeye tuna.The proportion of C18:1n9 was 15.26%,showing that bigeye tuna is a carnivorous animal.（4）The range ofδ¹³C values in the muscle tissue of bigeye tuna in the Eastern Pacific Ocean were from-19.70‰to-16.60‰,mean value was-17.46±0.41‰,showing that the squid,mackerel,flying fish and shrimp biological were the food items of bigeye tuna.Theδ¹³C values of bigeye tuna were wider,indicating the food sources of bigeye tuna were more complicated than yellowfin tuna,but simpler than bluefin tuna.Theδ¹³C value has no significant correlation with fork length and time.The range ofδ¹⁵N values were from 8.53‰to 15.23‰,mean value was 11.87±1.55‰.Theδ¹⁵N values of bigeye tuna were wider,indicating the trophic range of bigeye tuna were wider than bluefin tuna or yellowfin tuna.The relationship betweenδ¹⁵N and fork length of bigeye tuna showed positively correlated,but the relationship betweenδ¹⁵N and months was no significant.The mean trophic level of bigeye tuna was 4.82±0.65.In this study,the trophic level and possible food sources of bigeye tuna were preliminarily studied by biomarker.Through the analysis of the special fatty acids and stable carbon isotope,it can find the food source and help to better understand the life history of bigeye tuna.Through the analysis of the stable nitrogen isotopes,we can determine the trophic level of bigeye tuna and better understand the status of bigeye tuna in the ocean ecosystem.