Commercial harvest of hatchery-reared masu salmon Oncorhynchus masou estimated by a coast-wide sampling program in Hokkaido, northern Japan, and the two-stage sampling schemes of landings

This paper evaluates the stocking effectiveness of masu salmon Oncorhynchus masou in Hokkaido, northern Japan, through a coast-wide two-stage sampling survey of commercial landings. From January to June 1994-1996, commercial landings of masu salmon at 33-36 fish markets were sampled at 7-10 days intervals, and 60 866-72 124 fish were examined for marks indicated by fin clips. Based on the survey data, numbers of total and hatchery-reared masu salmon landed were estimated. To examine the structure of the errors, stratification of fish markets was implemented on the basis of geography and magnitudes of landings, and the stratification improved accuracy and precision of the estimates. Accuracy of the estimated numbers of total fish was evaluated by being compared to the true numbers of masu salmon landings reported by fishermen's cooperative associations. Estimates of total masu salmon landings were within +/- 10% of the true numbers. The estimated recovery rates (+/- SE) for hatchery-reared masu salmon smelts were variable ranging from 0.18 (+/- 0.06) to 3.50 (+/- 0.41)% among the stocked groups. An optimal sampling strategy was examined to obtain precise estimates for future studies.