Otolith Morphology of Neritic Tuna of the Genus Auxis (Teleostei: Scombridae) Reveals Mixed Stocks in the Celebes Sea

Neritic tunas of the genus Auxis have been commercially exploited as early as the 1970s in the Philippines. However, basic biological information on local stocks remains sparse despite being threatened by population declines. This study investigated the otolith shape morphology and phenotypic stocks of bullet tuna (BT, Auxis rochei) and frigate tuna (FT, A. thazard) within the Celebes Sea (western Celebes Sea, Davao Gulf, Moro Gulf, and Sarangani Bay) using otolith shape analysis. A total of 265 BT and 315 FT otoliths were successfully extracted, and results revealed that as fish grow in body length, otoliths develop more circular and elliptical shapes. All shape morphometric size parameters (otolith length, OL; otolith width, OW; otolith area, OA; and otolith perimeter, OP), alongside shape indices (circularity, CR; ellipticity, EL; rectangularity, RE; form factor, FF; and roundness, RO) showed significant correlations to species’ fork length (FL). Applying the “scaling effect” using linear regression minimizes the allometric growth effects on otoliths. While PERMANOVA successfully delineated phenotypic variations within BT (F = 13.2; p > 0.001) and FT (F = 9.24; p > 0.001) among sites, the Principal Component Analyses also support that all shape indices (CR, EL, FF, RE, RO) of BT and FT contributed most to the stock differentiation. These results imply that both species have distinct spatial populations, which could be attributed to their life-history traits and long-term habitat exposure. This study provides insights into these species’ complex population structures within the Celebes Sea for strengthened site-specific conservation and management measures.