Estimation of the length and age compositions of Pacific saury Cololabis saira in the Northwest Pacific Ocean during the fishing season in 2018

We collected 462 Pacific sauries from an onboard sampling program by three stick-held dip net fishing vessels of Chinese Taipei operated in the Northwest Pacific Ocean (40° - 50°N, 149° - 166°E) from July to November in 2018. A total of 455 otoliths classified by the otolith types according to the formation of the hyaline zone were used for the age determination. Our results indicated that the Pacific saury exhibits spatial and temporal changes in the size (and age) structure during the fishing season. The smaller fish (< 26 KnL cm) were found in July-August while the larger fish (> 32 KnL cm) in September-November. The average length of fish caught in the area of 46 °N and 157 °E (mainly from July and August) were generally smaller than those caught in the other areas. The relationship between the fraction of age 1 and body length can be described by a logistic curve with KnL50 and KnL95 of 26.85 (95% C.I of 26.48 and 27.22) and 28.85 (95% C.I of 28.47 and 29.23) cm, respectively. We found that the fish with small ROAs (< 540 μm) were mainly located in the eastern area (164 - 165 oE); and with the large ROAs (> 540 μm) occurred in the western area (150 - 155 oE). We suggest that the length composition data developed in this study could be used as input data of a particular fleet in the integrated stock assessment model for the Pacific saury in the Northwest Pacific Ocean.