Title: Comparison of two stock assessment models of Namibian monkfish (lophiiformes)

Author(s): Suama Niinkoti
Final project
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Supervisors: Pamela Woods , Magnus Thorlacius
stock assessment; model; comparison; Namibia; monkfish;


Fisheries management relies on stock assessment models to provide estimates of population abundance and to shed light on the underlying dynamics of the resources being managed. It is necessary to quantify and understand the uncertainty about model parameters and reference points to evaluate the consequences of alternative management actions. This report presents a comparison of two different assessment methods and the state of the Namibian monkfish stock (Lophius vomerinus and L. vaillanti) in Namibian waters. The objective of this study is to compare the currently used ASPM model implemented in Automatic Differentiation Model Builder (ADMB) on monkfish assessment with an a4a model to see if they return similar management advice. Two age-based alternative assessment methods are used: Age Structure Production Model (ASPM) and the Assessment for All Initiative (a4a) model implemented in the R package (The Fisheries Library in RFLR). Most of the data used in this study are from the National Marine Research Information Centre (NatMRIC). Age-disaggregated observations are used as input data and a Beverton-Holt (B-H) stock-recruitment relationship was used to estimate recruitment for both models. Recruitment was high for 2017 and it is around 15 billion and 30 billion according to the ASPM and a4a models, respectively, but the ASPM model estimated the spawning stock biomass as higher than the a4a models. The different models gave similar trends but were dissimilar in fishing mortality rates over the period studied (2000-2016). The fishing mortality estimated by the two models is between 0,2 to 0,35 with the highest estimate given by the a4a model in 2016. Short-term predictions from the a4a model suggested that recruitment and spawning stock biomass will decrease by 30 % over the next three years (2017-2019) while the fishing mortality is predicted to be higher than the current level (2016), if fishing effort remains the same.

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