Stock assessment of roughscale sole Clidoderma asperrimum off the Pacific coast of northern Honshu, Japan

An age-structured production model of stock dynamics was applied to data of the stock of roughscale sole Clidoderma asperrimum off the Pacific coast of northern Honshu, Japan, in order to estimate stock sizes from 1971 to 2020. The data sets used for estimations were series of catch over the years, that of standardized abundance index over 1972-2020, average body weight over 2008-2020 and several biological and operational parameters. Parameters of number of recruits at saturation are divided into those in three periods (1971-1976, 1977-1998, 1999-2020) to incorporate the effect of regime shift into the models of stock dynamics. Unknown parameters were estimated minimizing the residual sum of squares of natural logarithms of the data of abundance index and mean body weight, and the parametric bootstrap method was used to estimate the variance of parameter estimates. The results of estimations showed that point estimates of spawning stock biomass in 1971, 2000 and 2020 were 72.3 (CL 56.9-87.7), 2.8 (2.0-3.6), and 9.3 (6.2-12.4) thousand tons, and the fishing coefficient peaked in 1978 [0.46 (0.35-0.57) per year] and gradually decreased to 0.03 (0.02-0.04) in 2020. Interpretation of the results, modifications of the models, and the data to be collected in the near future are discussed.