Testing the sensitivity of the length-converted catch method using the Bigeye Tuna Thunnus obesus (Scombridae) population parameters

The present study investigates how sample size, length interval size, recruitment variation, and mortality over time influence total mortality, which is estimated by applying the length-converted catch method to the bigeye tuna (Thunnus obesus) population. Given the assumption of fish dynamics under an equilibrium state, an increasing sample size and decreasing length interval size can raise the accuracy of total mortality estimates, with a sample size of 3000 individuals and length class interval of 5 cm generally producing the most accurate estimates. When fish dynamics follow a non-equilibrium state, randomly varied recruitment does not affect the estimation of total mortality. However, recruitment that varies with increasing or decreasing trends would affect total mortality remarkably. Therefore, total mortality that varies by time influences estimated total mortality, and in this situation, fish stocks that undergo stable total mortality for four successive years could produce an accurate total mortality figure. Finally, we suggest that the non-equilibrium state of fish dynamics should be considered before applying the length-converted catch method to estimate Z.

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