Estimating and monitoring the stock size of sandfish Arctoscopus japonicus in the northern Sea of Japan

The stock size of sandfish in the northern Sea of Japan was estimated by a virtual population analysis (VPA) and sensitivity analyses were attempted on the VPA estimate. The stock size estimates were approximately 600–900 million until 1975, but since 1976 they have rapidly decreased. In the sensitivity analyses, the estimates of absolute stock size were not sensitive against the changes in the fishing mortality coefficient for terminal age and the measurement error in catch-at-age. This suggested that the relative stock size remains almost unaffected by the error in the data used in the VPA, if the degree of catch-at-age error and the natural mortality coefficient is correct. The relationships between the biomass estimated by the VPA and the density index from Danish seine fisheries, and between the biomass and the catch per unit effort (CPUE) from the experimental survey using Danish seine nets, were also examined. The density index and the CPUE indicated significant relations with the biomass. Consequently, the CPUE is useful to monitor the relative stock size in a timely manner, and the VPA estimate and the CPUE should be utilized for adjusting the total allowable catch in the multiseasons.