In order to achieve the milestone “1.5 Update CASAL model” of the ongoing research plan submitted by Japan and France (WG-SAM19/08), the CASAL models for Dissostichus eleginoides at Division 58.4.4b were revised.
Following WG-SAM-2019 suggestions (para 6.76 in the WG-SAM2019 report), we conducted an exploration of the impact of Age Length Keys (ALK), CPUE standardization, and different scenarios of IUU on the CASAL assessment, especially B0 estimation. Furthermore, we estimated the harvest rate (CAY and MAY) as defined in CASAL manual (Bull et. al. 2012) using each CASAL runs according to WG-SAM-2019 suggestions (para 6.76 in the WG-SAM2019 report).
A single-sex age structured CASAL model was built to assess the stock of D. eleginoides at the research block 1 of Division 58.4.4b. The base model (M1) is an update the M4 model described by Okuda and Massiot-Granier (2019). It includes updated catch at age data based on annual ALKs and data sources were weighted using an automatic procedure (“DataWeighting” package, Francis 2014). In addition of this first model, three other models were tested (M2, M3 and M4), exploring both the influence of CPUE standardization and IUU scenarios on the assessment. Compared to the CASAL results reported in WG-SAM-2019/30, the MPD profiles show improvements under all scenarios to better fit the model.
Comparing the MPD profile between M1 (base model), M2 (no 2007/08 CPUE), and M3 (no season effect on CPUE), unnaturally high standardized CPUE in 2007/08 reduce 25 t B0 and difference of standardized CPUE (with/without “Spawning season” effect) cause 10% difference in B0 estimation at the block 1 of Division 58.4.4b (Table 1). Changing IUU level (26.5 t to 40 t) in 2009/10 fishing season cause increasing of B0
In all scenarios, the estimated MCYs for D. eleginoides were higher than current catch limit 19 tonnes in block 1 at Division 58.4.4b. Furthermore, harvest rates achieving the CCAMLR management target (50% B0) were estimated to be c.a. 6%. This value is higher than current precautionary harvest rate for “data-limited” explanatory/research fisheries (4%) where there is no estimate of B0.