Using length-based observations in age-based population dynamics models can lead to uncertainty and / or bias due to the conversion from ages to lengths within the model. Thus, the CCAMLR Independent Stock Assessment Review for Toothfish in 2018 recommended that the use of age-length keys be investigated to estimate the age composition of tagged fish and recaptured fish as inputs to toothfish assessments. Here, we develop methods to calculate the numbers at age of tagged fish and of recaptured fish which could be provided to the assessment model of Ross Sea region Antarctic toothfish (Dissostichus mawsoni) instead of numbers at length as has been done in the past.
Results showed that the numbers at age of tagged fish calculated using this age-based approach were close to the numbers at age of tagged fish calculated in the assessment model when using the existing length-based approach, and that the numbers at age of recaptured fish calculated using the age-based approach were consistent with the otolith derived age data for those same recaptured fish where otolith readings were available.
We recommend that the use of the age-based approach for tagged fish and for recaptured fish be explored in future toothfish stock assessments for the Ross Sea region and evaluated as an alternative to the use of the length-based approach.