Ovarian development characteristics of North Pacific albacore tuna (Thunnus alalunga )

Based on 364 ovarian samples of North Pacific albacore tuna (Thunnus alalunga) collected in the north Pacific waters (29°08'–41°08' N, 163°50'–144°19' W) from October 2013 to February 2014, using histological analysis, the present study describes oocyte and ovary developmental stage of albacore tuna. The results show that North Pacific albacore is a species of the batch spawning type due to different development stages of oocytes found in the same ovaries. Histologically, the oocyte development of North Pacific albacore can be divided into six phases, and ovarian development into six periods. GSI (gonad somatic index) increases with the mature period during the stages I–V, and decreases in the stage VI. The spawning peak is from the middle December to early January, the GSI increases with the decreasing of latitude but no significant difference can be found. The present study can provide biological information for the stock assessment and sustainable development of north Pacific albacore fisheries, by analyzing the growth characteristics of albacore ovaries.