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In this document, different scenarios of uncertainties in a multispecies fishery are presented. Uncertainties have been considered in the estimation of the species composition of the tropical tuna catch as well as in the total catch. Although three species (yellofin, skipjack and bigeye) are involved in the tropical tuna fishery, only uncertainties in the bigeye vs skipjack have been taken into account, in order to simplify the analysis. Regarding the total catch, bigeye has been considered as the species with higher uncertainty due to the IUU catch component. Analyses have been focused in evaluating the sensitivity of the current assessment methods to these uncertainties. Taking into account that the two sources of uncertainty affect the age structure of the catch as well as the total catch, surplus production methods have been applied in the assessment.

Atlantic bigeye is a highly migratory species widely distributed through the whole Atlantic. Recent assessments conducted by the ICCAT SCRS are highly uncertain due to the large uncertainties in biological parameters, such as M, as well as in fishery statistics (e.g., IUU fleets, juvenile catch) or abundance indexes. Similar uncertainties can be found in the Indian or Pacific stocks. The development of an operational model has been recommended by different regional fishery organizations as a good approach to investigate the sensitivity of the assessment results to the uncertainties in the different inputs. This document presents an operational model for the Atlantic bigeye as part of a simulation framework that will allow the performance of management strategies based on current and improved biological knowledge and management procedures to be evaluated.