Variations in the instantaneous mortality rate between larval patches of Pacific bluefin tuna Thunnus orientalis in the northwestern Pacific Ocean

Abstract

We followed three patches of larval Pacific bluefin tuna ( Thunnus orientalis) using Lagrangian drifters in 2004 and 2005 from May to June and determined the age-specific larval instantaneous mortality rates, which were estimated to be between 0.06 and 2.75 day −1. The mortality rates differed significantly between patches and between ages within the same patch, which were higher than most of the estimations for other marine fish larvae. The results indicate that for the purpose of understanding the process of the larval survival, the use of larval patches is appropriate as a unit of observation. Evidence of the occurrence for net avoidance is presented and also it is concluded that net extrusion was observed for the larvae. Length-specific catch probability of the larvae was estimated to correct for length distribution biases derived from the net avoidance and net extrusion.