Study on temporal and spatial distribution characteristics of Symplectoteuthis oualaniensis in high seas fishing ground of northwest Indian Ocean

Symplectoteuthis oualaniensis is a cephalopod with strong warm water properties in the ocean, with abundant resource reserves that can provide abundant protein for humans. Especially in the high seas of the northwest Indian Ocean, due to the high salinity and water temperature of the Gulf of Aden, as well as unique ocean current conditions, it has formed a superior S. oualaniensis fishing ground and become one of the important deep-sea fishing grounds of China. From 2016 to 2019, both the production and nominal CPUE (nominal catch per unit effort) increased with the increase of fishing boats. However, in recent years, China has continuously increased the number of fishing boats in the northwest Indian Ocean. In 2020, the nominal CPUE has significantly decreased, and in 2021, the decrease in fishing vessel numbers led to an increase in nominal CPUE. Based on the above situation, in order to grasp the spatial and temporal distribution characteristics of the S. oualaniensis fishery in the high seas of the northwest Indian Ocean, especially the changes of the fishery in the high-yield center, we can lay a foundation for the later activities such as fishery situation prediction and fishery prediction. Based on the statistical data of light purse seine fishing production in the Indian Ocean from 2016 to 2021, combined with the statistical analysis of the interannual variation and seasonal fluctuation of the yield and nominal CPUE, this study focused on the impact of time and space factors on the nominal CPUE of S. oualaniensis through the analysis of the yield gravity center, the analysis of the standard deviation ellipse model and cluster analysis, to explore and find out the variation law of the gravity center of the fishing ground and variation characteristics of the central fishing ground in the interannual and seasonal. The results shown that the output was consistent with the increasing trend of the number of fishing vessels, but the excessive number of fishing vessels wound lead to the decline of the nominal CPUE; From 2016 to 2021, the center of fising ground gravity moved to the Northeast along a curve, and the nominal CPUE high value areas in each year were concentrated at 12°-20° N, 58°-68° E; the period of the highest nominal CPUE and output was from October to December, the period of the most widespread distribution of the S. oualaniensis was from October to the following January , and the period of movement from high latitude to low latitude was from February to May; the distribution latitude of the yield gravity center was higher (17°-18° N) from September to December, and lower (15.5°-16.5° N) from January to May; the interannual fishing ground changed direction and the fishing ground changed direction in the production month (September to April of the next year) were northeast southwest, consistent with the Somali ocean current direction. Research had shown that when there were too many fishing vessels, the nominal CPUE would decrease, and the number of fishing vessels should be reasonably controlled. In addition, attention should be paid to the annual and monthly variation patterns of the S.oualaniensis fishing ground, and it should be reasonably applied to fishing activities. This study has better revealed the temporal and spatial changes of the fishery resources and the distribution of the central fishing grounds of the S. oualaniensis in the northwest Indian Ocean, providing a basis for its sustainable utilization of fisheries.