1Hokkaido Abashiri Fisheries Experimental Station, Hokkaido 099-3119, 2Department of Aquatic Biosciences, Tokyo University of Fisheries, Tokyo 108-8477, 3Hokkaido Central Fisheries Experimental Station, Hokkaido 046-8555, 4Hokkaido National Fisheries Research Institute, Hokkaido 085-0802, Japan
We tagged and released 3,025 Kichiji rockfish caught in the southern Okhotsk Sea. A total of 490 recaptures were reported for seven years after release. Assuming the constant fishing and natural mortality rates, we estimated a rapid population decrease from 24 million in 1993 to 8 million in 1999. We examined effects of the rates of tag shedding and tagging mortality by the model selection procedure using AIC, and selected the model with the constant fishing mortality and changing natural mortality rates at two years after release. The exploitation rate after the change point was estimated 0.1421±0.0106, and the above population size was found to be overestimated about 30%. The tag reporting rate was very high at 94.3% estimated by the census of the fishermen. However, 40 tags reported from fish brokers strongly suggested large detection error of tags on fishing boats. The sensitive analysis considering the detection error showed that the population size could be overestimated by a maximum of 4.3 times.