Stock assessment of Pacific saury in the western North Pacific Ocean using state-space biomass dynamic model which incorporates seasonality

This report provides evaluation of stock status of the Pacific saury (Cololabis saira) in the North Pacific Ocean by a “stochastic Surplus Production model in Continuous Time” (SPiCT). SPiCT is a state-space biomass dynamic model which incorporates seasonality (Pedersen and Berg 2017). The temporal dimension of the continuous-time model is resolved numerically using an Euler scheme (Iacus 2009), which discretizes time into intervals of fixed length, here interval is equal to 2 months.
We successfully fitted SPiCT from 2001 to 2016, when seasonally different CPUE series were available. 
SPiCT can account for process and model errors in addition to observation errors in the biomass indices with priors, but we could follow two of the three base-case scenarios, that differed in survey catchability (q) of the Japanese survey biomass index prior: including all CPUEs and free q (Base-case scenario-3) and fixing the q at 1 (Base-case scenario-2) to consider Japanese survey biomass precise. We couldn’t define q prior from 0 to 1 in SPiCT, thus “Base-case scenario-1” was skipped. 
Two sensitivity analyses were conducted in SPiCT: 1) without the Japanese survey biomass index (excluding survey q) and 2) using only Russian CPUE indices.
We confirm the conclusion of TWG-PSSA first meeting report that Base-case scenario-3 (free q) indicated a value of q >1 in SPiCT. SPiCT estimates were not optimistic with B2016/BMSY = 0.2 and F2016/FMSY =6.7 (free q). The results of 1 base-case scenario and all sensitivity scenarios with free q showed that the stock is depleted. The only one setting where q was fixed in such way that Japanese biomass estimates from surveys were considered precise showed that B-ratio and F-ratio were in the safe zone (green box of Kobe plot). Therefore, we suggest that all other members provide seasonal (quarterly) estimates of CPUE indices and catches to improve inputs for SPiCT. The use of SPiCT can help us to overcome at least one discrepancy in CPUE indices: multidirectional tendencies, which possibly occur due to the differences in time (and consequently in space) for fishing operations conducted by different members.