Population Biological Characteristics Research And Health Risk Assessment Of Heavy Metals Of Small Thunnus Albacares In The Central South China Sea

To understand the population biological characteristics and assess the health risks of heavy metals of Thunnus albacares in the South China Sea,this study conducted field sampling surveys of T.albacares with fishing rods in the central South China Sea from May 2021 to August 2022.This study explored the age and growth characteristics,feeding biology characteristics,heavy metal content distribution,and health risks of small T.albacares in the central South China Sea.The main results and conclusions were as follows:1.Opaque/translucent growth zones in the otolith were obvious,so the otolith of T.albacares was used as the identification material for age.62 yellowfin tuna caught in the South China Sea were identified,with 2 age groups（ages 1-2）.Among them,the age 1 individuals accounted for a large proportion of captured individuals,accounting for 77.42%.The dominant fork length group of the obtained samples was45-60 cm,and the dominant body weight group was 2-4 kg.The power function fitted the relationship between the fork length and body weight of all caught samples as W=0.00005 L^2.77（R²=0.98;n=62）.The relationship between the fork length and body weight of female T.albacares fitted by power function was:W=0.00006 L^2.72（R²=0.98;n=21）.The relationship between the male T.albacares fork length and weight fitted by power function was:W=0.00005 L^2.72（R²=0.97;n=41）.Estimated by the ELEFANⅠmethod and the Pauly formula,the von Bertalanffy growth equation of the fork length（FL）and age（t）of small T.albacares in the central South China Sea was as follows:L_t=136.50[1-e^{-0.82（t+0.21）}].2.The results of stable isotope and stomach content analysis showed that there were mainly three kinds of dominant bait for small T.albacares in the central South China Sea,namely Symplectoteuthis oualaniensis,Decapterus maruadsi,and Cypselurus arcticeps.The contribution rate of C.arcticeps to South China Sea T.albacares with fork lengths from 30 to 45 cm was as high as 53.80%,but as the T.albacares grew and developed,its contribution rate gradually decreased,and the contribution rate of South China Sea T.albacares with fork length greater than 75 cm dropped to 18.80%.The contribution rate of D.maruadsi to South China Sea T.albacares with fork lengths from 30 to 45 cm was 33.60%.However,as T.albacares grew and developed,its contribution rate gradually increased,and the contribution rate of South China Sea yellowfin tuna with a fork length greater than 75 cm increased to58.50%.Compared with the above two baits,the contribution rate of S.oualaniensis to T.albacares in the South China Sea was always low.As the growth and size of the central South China Sea small T.albacares continue to increase,the food composition of the central South China Sea small T.albacares changes accordingly.This performance was specifically that with the increase in growth and development,the South China Sea T.albacares preferred to capture food with a higher trophic level.3.The contents of 5 heavy metals（Cd,Cu,Cr,Se,Zn）in the muscles and livers of South China Sea T.albacares with three sizes of large,medium,and small were determined.The results showed that with the change in body size,the accumulation characteristics of heavy metal elements were different.The content of Se and Cd in muscle increased with the increase in body size;the opposite was true for Zn and Cu;and the content of Cr had no significant difference in the three body size groups.In liver tissue,the content of Se,Cu,and Cd increased with the increase in body size;the accumulation trend of Cr was the opposite;the content of Zn had no significant change in the three body size groups.The contents of Cd,Cu,Se,and Zn in the liver of the three body types were significantly higher than those of the corresponding metal elements in the muscle,while the Cr content had no significant difference between the muscle and the liver.In terms of heavy metal food safety,according to the"National Food Safety Standard"（GB 2762-2017）,this study conducted a health risk assessment of Cr and Cd in T.albacares muscle.The results showed that the HQ and CR of the above two elements were all less than the limit standard,so this study considered that there was no food heavy metal health risk in South China Sea T.albacares.