Using Generalized Depletion Model to Access the Elver Fishery in a River of Northeastern Taiwan

We applied generalized depletion model to assess a small-scale elver fishery of the Japanese eel, Anguilla japonica, in a river of northeastern Taiwan during the fishing season from 19 Dec, 1980 to 21 Feb, 1981 with complete effort and catch data. Results showed that approximately 160,000 individuals of elvers recruited to the river during this season. We identified seven recruitment waves with population sizes ranging from 781 to 66,185 individuals were identified. The overall exploitation rate was 1.63 % with a range from 0.2 % to 60 %, which reflected low fishing pressure of this river, averagely 2 fishermen per day. The effort response parameter was 1.26, indicating the effort synergy that every additional fish hour yielded a disproportionate increase in catch. The population response parameter was 0.76, suggesting that the elver population exhibited hyperstability that catch per unit effort changed with less magnitude than that of fishing effort. The natural mortality was high, 0.12 per day, which may mix with the emigration rate. Applying generalized depletion model can extract more information about the population and fishery from the historical catch and effort data and therefore lead to better understanding of the elver recruitment trend.