Vertical Distribution Structure And Trophic Niche Of Fish Species Captured By Longline Fishery In The North Pacific

Yellowfin tuna(Thunnus albacores),spotted opah(Lampris guttatus),bigeye tuna(Thunnus obesus),wahoo(Acanthocybium solandri),and albacore tuna(Thunnus alalunga)are the main longline-hook species in North Pacific,play an essential role in the ecosystem and are the critical economic species in China pelagic fishery.The study on the mechanism of interspecies coexistence in communities has always been an important part of ecosystem study,which is of great significance to community construction.Furthermore,these species are the essential predators in the North Pacific ecosystem.The changes in their resources will affect the structure of the entire Marine ecosystem,hence,it is necessary to study the relationship between their vertical distribution structure and vegetative niche.Based on the production data obtained during the tuna longline fishing vessels in the North Pacific from October to November 2018,this paper analyzes the longline-hook species yellowfin tuna,spotted opah,bigeye tuna,wahoo,and albacore tuna,using the catenary theory calculation formula to estimate the depth of their vertical distribution,analyze the difference of vertical distribution structure between different fish species,analyses the vertical distribution structure between and within species.The stable carbon and nitrogen isotope ratios and fatty acid profiles in muscle tissues of 5 species were determined,and the differences among species were compared to explore the nutritional niche relationship.The results are shown as below:(1)The theoretical fishing depth range of five species calculated by the catenary formula is 77～318m.The theoretical fishing depth value is corrected using 15%,20%,and 25% shoaling rate,and the difference of vertical depth distribution between different species is compared.It is found that the vertical distribution range of wahoo is the most undersized;yellowfin tuna and bigeye tuna are the largest.Different species congregate in different depths.The maximum catch rate of wahoo is in the depth of 50～100m,yellowfin tuna,albacore,tuna and spotted opah are in the depth of 101～150m,bigeye tuna is in the depth of 200～250m,and there is a significant difference in the depth distribution between bigeye tuna,and spotted opah.To compare the vertical distribution structure of intraspecific individuals,the range of fishing depth between male and female was not the same.However,there is not a significant difference between it,different species the vertical distribution of individual sizes each is different,and the fork length of species tended to increase with depths was examined.(2)The stable carbon and nitrogen isotope ratios of five fish species were determined by stable isotope analysis.The results showed that the various ranges of δ13C and δ15N of five fish species were-20.70‰ –-16.78‰ and 7.76‰ – 13.97‰.There were specific differences among different species.The δ13C values of longfin tuna,bigeye tuna,and spotted opah were similar,indicating that their food sources were similar.Higher δ13C values in wahoo and yellowfin tuna may be related to their feeding habits.The δ15N values of bigeye tuna were significantly different from other species,indicating a high nutritional level.Yellowfin tuna occupied a larger vegetative niche area;the nutritional niche overlap of bigeye tuna and yellowfin tuna was 79.3%,indicating that the food sources between the two species were similar and the intraspecific competition was fierce.(3)The types and contents of fatty acids of five species were determined by fatty acid profiles analysis to understand the nutritional niche relationship between different species.There are 35 kinds of fatty acids detected in the muscle tissue of five species.There are 21 kinds of fatty acids with a percentage greater than 1%,included six kinds of saturated fatty acids and 15 kinds of unsaturated fatty acids.The content range of saturated fatty acids was 29.20% – 39.14%,the content range of monounsaturated fatty acids was 13.97% – 29.92%,and the content range of polyunsaturated fatty acids was32.13% – 47.06%.There were some differences in the contents of some essential fatty acids(C20:5N3,C22:6N3 and C20:4N6)in the muscle tissues of five species,which indicated that the food sources and food contributions were different.There were some differences in the contents of some essential fatty acids(C20:5N3,C22:6N3 and C20:4N6)in the muscle tissues of five species,which indicated that the food sources and food contributions were different.Yellowfin tuna and bigeye tuna occupied a large vegetative niche area,indicating that their feeding habits were relatively generalized,which was also verified by the results of the stable carbon and nitrogen isotope ratios above.The overlap of the vegetative niche area of albacore tuna and spotted opah was as high as 78.43%,indicating an intense resource competition between these two species.Through the above studies on the vertical distribution structure and nutritional niche of the longline-hook species in the North Pacific Ocean,we deepened the understanding of the physiological behavior adaptation mechanisms such as activity and feeding in the coexistence of species in the North Pacific Ocean.We provided some references for further study of the interspecies resource allocation and co-regional coexistence mechanism of oceanic species.