Relative abundance of yellowfin tuna Thunnus albacares (Bonnaterre, 1788) in the tropical area of the north-eastern Indian Ocean

Abstract

 The tropical area of the north-eastern Indian Ocean has historically been a prominent fishing ground for yellowfin tuna
Thunnus albacares (Bonnaterre, 1788). However, low effort from either Japanese or Taiwanese fleet in the last two decades
has driven some uncertainties in relative abundance estimates of yellowfin tuna, which affects the robustness of the current
stock analysis for an already declined population. Hence this study was carried out to bridge the research gap by utilising the
Indonesian scientific observer data from 2006-2021, which covers the north-eastern Indian Ocean. A delta-lognormal model
was adopted to fit the data, with the catch per unit effort as the response variable and year, quarter, moon phase, latitude
and longitude as known covariates. The optimal model was chosen using a backward method based on Akaike Information
Criterion (AIC) (Akaike, 1974), Bayesian Information Criterion (BIC) (Schwarz, 1978) and the values of the coefficient
of determination (R2). In general, the relative abundance of yellowfin tuna remained at a very low level, with an apparent
declining trend in the first decade, followed by a slight sign of recovery in the past three years. However, the increasing trend
was overshadowed by uncertainties, likely caused due to natural variation and low coverage in some years. Although the
series was considered short, it managed to picture the dynamics of the abundance of yellowfin tuna in the tropical area of the
north-eastern Indian Ocean, which is essential for conducting a more robust harvest strategy evaluation.

Keywords: Declining stock, Fisheries-independent data, Highly migratory, Relative abundance, Scientific observer