Relationship between Gill Raker Number and Early Growth of Pacific Bluefin Tuna <i>Thunnus orientalis</i>

This study examined the variation in the gill raker number (GRN: a meristic count) of Pacific bluefin tuna Thunnus orientalis in relation to its early growth using otolith microstructure analysis. A total of 360 age-0 fish (fork length: 102 mm-369 mm) in which the gill raker should have been fully developed were analyzed. The GRN on the upper limb, lower limb, and in total (including one gill raker at the joint of two limbs) ranged from 9-13, 21-26, and 32-40, respectively. GRN was negatively related to early growth, as indicated by the mean otolith increment width. Fish with fewer gill rakers attained better growth during the early life stage than fish with more gill rakers. This relationship was especially evident for growth from 15-19 days after hatching soon before metamorphosis. These results suggest that the early-stage growth of bluefin tuna is an important determinant of morphology. Furthermore, GRN can be an alternative tool for monitoring early growth and may also be related to survival and recruitment success.