West Atlantic Skipjack MSE

History

The western Atlantic skipjack tuna (SKJ-W) stock has been identified as a priority for Management Strategy Evaluation (MSE) due to its importance in tropical tuna fisheries. An initial demonstration of the SKJ-W MSE framework (Huynh et al., 2020) was presented to the SCRS, and the MSE process has been formally started with the initial guidance in Rec. 21-01, which emphasized the need to improve the management of tropical tunas through the use of harvest control rules and MSE.

Rec. 21-01, along with Rec. 15-07 and Res. 19-14, established a general framework for the development of MSE across priority stocks, including skipjack. Furthermore, the Commission provided management objectives in Res. 22-02 and Rec. 24-04 for the SKJ-W MSE. In line with the guidance from the Commission, the SCRS has developed Operating Models (OMs) and Candidate Management Procedures (CMPs) for SKJ-W through an iterative conditioning process which began in 2022.

Operating models (OMs)

The Operating Models (OMs) for West Atlantic skipjack (SKJ-W) have been developed based on the 2022 stock assessment (Anon., 2022). These models included alternative assumptions about selectivity, growth, natural mortality, and steepness. The conditioning of operating models, robustness testing, and development of Candidate Management Procedures (CMPs) were built on openMSE. Further refinement of OMs and testing of CMPs have continued from 2022 to 2025 (Sant’Ana et al., 2023; Sant'Ana and Mourato, 2024). In 2025, the newly updated OMs results were provided by updating all data (catch, length data, and relative abundance indices) through 2024 (Sant’Ana et al., 2025).

Materials
More materials can be found in References
Summary	MSE Codes	Shiny App

Management Objectives

Rec. 24-04

Stock Status	Safety	Yield	Stability
Prob. >= 60% in green quadrant	Prob. < 10% of stock falling below BLIM	Maximize overall catch	TAC changes between periods +25%/-25%

Candidate Management Procedure (CMPs)

Various empirical index-based and assessment model-based CMPs were initially developed since 2023 (Sant’Ana et al., 2023; Sant’Ana and Mourato 2024a and Sant’Ana and Mourato 2024b),and the following four CMPs were sent to the Commission for their final selection in 2025 (Sant’Ana et al., 2025).

• CE: Constant Exploitation (CE) CMP is an empirical approach that adjusts the TAC based on the ratio between historical and current exploitation rates, derived from catches and relative abundance indices.
• IR: Index Ratio (IR) CMP is an empirical approach that structures a management measure based on the TAC scaling derived from recent catches weighted by a factor that aims to reflect the relative change in the relative abundance index.
• SP: A model based CMP is the form of the harvest control rule based on a linear penalty function to be implemented from a B_targ to a B_LIM.
• SPAH: A model based CMP is the form of the harvest control rule based on a nonlinear function (cubic function) to address the response of the effort penalty in relation to the stock's biomass status.

Management Procedure (MP)

The Commission adopted the Index Ratio (IR) MP in 2025 (Rec. 25-02, Annex 1).

The IR MP is an empirical MP that uses a single standardized relative abundance index to structure its harvest control rule. The core principle of the IR MP is to adjust the TAC in proportion to systematic changes in the index (for example, catch levels increase when the index shows an increasing tendency and decrease when the index signals a decline). This is implemented by comparing the mean value of the index over a recent window (i.e. past 3 years) to a historical reference level, producing a ratio that reflects the relative condition of the stock.

Rec. 25-02 Annex 1

Exceptional Circumstances

The Commission plans to adopt the Exceptional Circumstances Protocol (ECP) in 2026
(Rec. 25-02).

References of West Atlantic skipjack