Migration Model and Dynamics Parameters of Large-Sized Yellowtails in the Pacific along the Japanese Coast Inferred from Tag Recaptures after the Year of Release

Abstract

1. A system model on fishing, migration and survival of the tagged yellowtails after the year of release was developed. The data of tag recaptures after the year of releases of the large-sized fish tagged in 1926 to 1938 period were applied to the model and the rate of disappearance from fishing ground, proportion of fish reappearing next year first to each fishing ground, and natural mortality rate were estimated. Some of these values were compared with the estimates from the same year recaptures.
2. The rate of disapearance during the fishing season was high in Kumano-nada and Tosa Bay likewise the results from the same year recaptures. Discontinuity in the distribution of fish at these points was reaffirmed. Majority of the fish released in Sagami Bay and Kumano-nada reap-peared first to Kumano-nada next year, while the fish released in Kyushu area returned to the same area. Estimates of the natural mortality rate. between fishing seasons ranged from 57% to 70 (M=0.84-1.20) and the weighted mean was 61% (M=0.94). If such factors as tag shedding and tag martality were affecting, these values would be overestimation. For the fish released in Sagami Bay and Kumano-nada, no southward shift of areas of recapture was observed according to the number of years after release. Estimates from the recaptures in second year and in and after third year showed similar values.
3. The fishing rates for the tagged fish after the year of release and some of the rates of disappearance were assumed to be the same as that in the year of release. Even if this was not the case, effect on estimation error of the rate of disappearance would be small. It is possible that the bias of overestimation of the survival rate is as large as 50% but actually such a large bias seems to be unlikely.