A Comparative Study Of The Tropical Tunas’ Otolith Typical Trace Element In The Western Central Pacific Ocean

Tunas are the typical oceanic highly migratory fishes,which are the main target species of the distant-water fishing country and entity.The study on the fish otolith is beneficial to understanding the characteristics of tuna otolith micro structures and trace elements.It is helpful to understanding population structure,life history and migration history of the fish species by studying the otolith micro structures and trace elements.In this paper the main trace elements of bigeye tuna(Thunnus obesus),yellowfin tuna(Thunnus albacares)and skipjack(Katsuwonus pelamis)otoliths,which were collected on board of Zhongtai No.3 pures seiner of Zhong Yu Global Seafood Corp.in three areas of the Western Central Pacific Ocean(3°49′N,174°01′W;7°21′S,149 ° 55 ′ W;2 ° 30 ′ S,179 ° 18 ′ W)during June 2016,were measured and compared.The biological data were collected including 79 bigeye tuna,yellowfin tuna and skjpjack’s length and weight.And finally,61 otoliths were obtained.The typical trace elements of the selected 25 otoliths were measured by ICP-MS internal standard method and compaired.The results were as follows:(1)The otolith of tuna fish has obvious three-dimensional structure.It has non-flat otolith surface.Three kinds of tuna fish otolith have obvious gully in the obverse side,bigeye tuna and yellowfin tuna otolith are similar,bigeye tuna otolith has slightly narrower gully than that of the yellowfin tuna.Skipjack otolith is smaller,dorsum zone and wing zone has certain similarities with other two kinds of otolith.Rostrum zone and lateral zone is in smooth diamond shape extremely different from the other two kinds of fish tending to sharp triangular shape.Bigeye tuna and yellowfin tuna otolith are still similar in reverse side,showing gully hollow area of the cone like and connecting dorsum zone,wing zone,rostrum zone and lateral zone together.The skipjack otolith is divided into dorsum zone,wing zone,rostrum zone and lateral zoneby the groove.Regarding the side view,bigeye tuna and yellowfin tuna have similar curvature,can be easily erected in the plane,but the curvature of the skipjack otolith is not obvious,and it is difficult to stand on the plane.(2)There are 47 trace elements in all the otolith samples when Ca is used as an internal standard to correct for variation in ablation yield among samples.The experimental results showed that the bigeye tuna,skipjack,yellowfin tuna otolith have nine typical trace elements,e.g.Ca,Na,Sr,Fe,Mg,Zn,Ba,Cu,Li and Co.(3)There were significant differences in Na,Sr,Li,and Mg content among different age of the bigeye tuna,but Fe,Zn,Ba,Co,or Cu content were not significantly different.(4)Among different age of skipjack,Na,Mg,Zn,Ba,Co,Cu,Fe,Li and Sr content were not significantly different.(5)There were significant differences in Na,Mg,Zn,Sr,and Ba content among different age of yellowfin tuna,Co,Cu,Fe,and Li content were not significantly different.(6)Sr element content was suitable for analysis of bigeye tuna and yellowfin tuna migratory history,Mg element content was suitable for analysis of the life history of bigeye tuna and yellowfin tuna.(7)The similarity of trace elements among bigeye tuna,skipjack and yellowfin tuna can reflect the similarity life history of the fish.In the experiment,the Sr/Ca ratio of bigeye tuna,skipjack and yellowfin tuna were 3.5~5.5,3.5~5.5,3.3~5.6,respectively.Sr/Ca ratio of three species was very close.Comparing Sr element content of bigeye,skipjack,and yellofin,Sr element content of bigeye tuna and yellowfin tuna otolith from the same area existed significant differences during different age stages.This suggests that bigeye tuna and yellowfin tuna samples were existed fluctuation during different age stages and they were tropical migratory population.Sr element content of skipjack tuna had no significant differences.This suggests that skipjack samples were not existed fluctuation during different age stages and it’s likely to be the local population near the sampling area.(8)The life history mode of bigeye tuna,skipjack and yellowfin tuna are very similar.(9)There were multiple skipjack spawning grounds and the skipjack is the local population nearby the sampling sites in the Western Central Pacific Ocean.