The role of uncertainty in fish stock assessment and management: A case study of the Spanish mackerel, Scomberomorus maculatus, in the US Gulf of Mexico

Stock assessments utilize many parameters which are known to contain uncertainty about the point estimate. This uncertainty can arise from many sources including natural variability, estimation procedures, statistical fitting, and lack of knowledge regarding the parameter. The uncertainty in parameter estimates can be incorporated in the stock assessment to produce a variable result under a given management scheme. This variability provides flexibility to managers, but also has implications for the stock depending upon where in the range of possibilities the managers decide. This study uses the Spanish mackerel, Scomberomorus maculatus, fishery in the US Gulf of Mexico as a case study of incorporating uncertainty in stock assessment. Spanish mackerel stock assessments perform calibrated virtual population analysis followed by prediction of recruitment and application of a non-equilibrium F(%SPR) to arrive at a range of allowable biological catches (ABC). A quota called the total allowable catch is then chosen by managers from the range of ABCs. In this paper we present methods to incorporate trends in the tuning indices of virtual population analysis and uncertainty in the shrimp trawl bycatch of Spanish mackerel, growth parameters, and natural mortality. We examine the effects of each source of uncertainty independently and then examine interaction effects among the sources. We use a 3-D graphical presentation of the ABCs for each scenario which allows comparison of uncertainty among and within different sets of parameters. The risk implications of choosing a given total allowable catch from the ABCs are also demonstrated both for the fishing industry and the fish stock.