Population dynamics and sustainable use of Japanese eel (Anguilla japonica) in Kao-Ping River: application of YPR and SPR models
Date Issued
2009
Date
2009
Author(s)
Lin, Yu-Jia
Abstract
Japanese eel (Anguilla japonica) is a highly-priced commercial fish in Asia. Due to heavy exploitation, the Japanese eel population declined drastically and thus conservation and management of the eel population are urgently needed. In this study the population dynamics of the eel population in the lower reaches of Kao-Ping River in the southern Taiwan were examined by estimating its growth, mortality rate and maturation parameters. The sustainable use of the eel population was evaluated by assessing the risks of growth and recruitment overfishing using yield per recruit (YPR) and spawner per recruit (SPR) models. Uncertainties in parameter estimation were incorporated into the models by Monte Carlo simulation. n order to obtain accurate age estimates, otolith annuli were validated by known-age cultured eels and otolith marginal increment ratio of the wild eels. Then the back-calculated lengths-at-age from otolith were used to estimate the growth parameters in von Bertalanffy growth model, which best fitted the lengths-at-age data according to information theory. Mortality rates estimated from mark-recapture data were found different between eel origins and marks. The maturation curves, described by the silvering of the eels, were found to be different between females and males, indicating a sexual dimorphism in the silvering of the eels. nalysis of YPR model indicated that the risks of growth overfishing (GOF), defined as probabilities that current fishing level were larger than Fmax at which the YPR was at its maximum, ranged 1 to 5 % for both sexes. A more conservative risks of GOF, defined as the probabilities that current fishing mortality exceed F0.1 at which the increase in YPR was 10 % of that when the eels were not exploited (F = 0), were from 30 to 40 %. Two risks of recruitment overfishing (ROF), defined as the probabilities that the SPR under current fishing mortality was less than 40 and 50 % of that when F = 0, were relatively higher, 23.9 to 84.8 % and 67.0 to 97.6 %. When the minimum legal sizes increased, the risks of GOF decreased considerably but the decreases in risk of ROF were less apparent. When the maximum legal sizes decreased, the risks of GOF decreased a bit but the risks ROF were effectively reduced due to better protection of larger spawners. The risks of GOF for the eel fisheries in the study area were low to considerable, while the risks of ROF were substantially higher. To sustain the eel resources, especially for supplying sufficient amount of glass eels for eel culture, control of fishing mortality rate, and exploitation rate, e.g. a 40 % reduction, and the enforcement of minimum legal sizes of not smaller than 500 mm or a maximum legal sizes of not larger than 850 mm were recommended for the eel fisheries in the lower reaches of Kao-Ping River.
Subjects
Japanese eel
Estimation of vital parameters
YPR and SPR models
Growth and recruitment overfishing
Sustainable use
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