Evaluating alternative management strategies for bigeye tuna, &lt;i&gt;Thunnus obesus&lt;/i&gt;, in the Indian Ocean

Xinjun Chen,Yong Chen,Yuhe Tong

doi:10.3989/scimar.03823.11a

Abstract

Bigeye tuna ( Thunnus obesus ) support a large commercial fishery in the Indian Ocean. However, explicit management strategies and harvest control rules are yet to be developed for the management of this fishery. We used a stochastic age-structured production model as an operating model to evaluate several potential management strategies under different assumptions of stock productivity. Five management strategies—constant fishing mortality, constant catch, quasi-constant catch, constant escapement, and status-dependent strategies—were evaluated and compared using the performance indicators including average catch, average spawning stock biomass, variation in catch, average fishing mortality and lowest biomass during the time period considered in the simulation. This study shows that (1) for the constant catch strategy, an annual catch of 90000 t would result in a low risk of stock being overfished while obtaining a stable catch; (2) for the constant fishing mortality strategy fishing mortality of 0.3 per year could yield a higher catch, but might have a high probability (64%) of stock dropping below the spawning stock biomass (SSB) that could achieve maximum sustainable yield ( SSB msy ); and (3) for the quasi-constant catch strategy an annual catch of 110000 t was sustainable if the current SSB was higher than SSB msy . Constant escapement and status-dependent strategies were robust with respect to different levels of virgin recruitment and steepness. This study suggests that it is important to incorporate uncertainties associated with key life history, fisheries and management processes in evaluating management strategies.

Highlights

A management strategy, referred to as a “management procedure” (Bentley and Langley 2012), a “harvest control rule” (Kell et al 1999, Breen 2009), and a “decision rule” (Bentley et al 2005), is a group of regulations defined with key management parameters to address certain management objectives (Deroba and Bence 2008)
Five management strategies—constant fishing mortality, constant catch, quasi-constant catch, constant escapement, and status-dependent strategies—were evaluated and compared using the performance indicators including average catch, average spawning stock biomass, variation in catch, average fishing mortality and lowest biomass during the time period considered in the simulation
This study shows that (1) for the constant catch strategy, an annual catch of 90000 t would result in a low risk of stock being overfished while obtaining a stable catch; (2) for the constant fishing mortality strategy fishing mortality of 0.3 per year could yield a higher catch, but might have a high probability (64%) of stock dropping below the spawning stock biomass (SSB) that could achieve maximum sustainable yield (SSBmsy); and (3) for the quasi-constant catch strategy an annual catch of 110000 t was sustainable if the current SSB was higher than SSBmsy

Summary

Introduction

Two types of management strategy are often considered: constant rules or one-parameter control rules (Thompson 1999); and status-dependent rules or multiparameter control rules ( known as “precautionary control rules”; Restrepo and Powers 1999). Constant rules keep a single management parameter constant, often including constant fishing mortality rate (CF) and constant catch (CC). They are easy to use, but CF and CC rules may have a high risk of resulting in overfishing or even overfished stocks because management regulations usually do not change with the status of populations. Status-dependent rules, which enable management regulations to be adjusted according to the status of populations, are comparatively precautionary in reducing the likelihood of overfishing while achieving optimum yields (Zhang et al 2011)

Objectives

Methods

Results

Discussion

Conclusion