Assessing billfish stocks: A review of current methods and some future directions

• Assessments of billfishes are based on catch-only methods, biomass dynamics models and statistical catch-at-age analysis.
• Assessments of most billfish stocks should be based on sex-structured population dynamics models.
• Effective quantification of uncertainty requires a broader consideration of model scenarios than is commonly the case at present.

Billfishes, excluding swordfish, are typically not the primary targets of large-scale fisheries, which has historically led to a lack of targeted monitoring efforts. The lack of data on age-composition, missing catch data and species identification problems for some species, lack of fishery-independent index data, as well as environmental influences on population dynamics is compounded by the international nature of how fisheries for billfishes are assessed and managed. This paper overviews the most recent assessments conducted for 20 stocks of billfishes in the Indian, Atlantic and Pacific Oceans, and how management advice is provided for these stocks. Assessments for billfishes are conducted using a wide range of techniques, ranging from catch-only methods which infer stock status based primarily on the prior distributions assumed for the parameters of a population dynamics model, to statistical catch-at-age analyses that integrate a wide range of data types. Key recommendations arising from this review include that age-structured stock assessments should be based on models that allow sex-structure to be represented, a full accounting for uncertainty requires adequately representing uncertainty regarding growth rates, natural mortality, the form and parameters of the stock–recruitment relationship, and how data are weighted, and that if biomass dynamics models are to be applied, they should be based on Bayesian state-space formulations rather than observation or process error estimators because such formulations are better able to represent uncertainty.