A comparison between fingerling release and fishing controlbased on Reproductive Value

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In recent years, stocking of natural populations with artificially produced fry are performed in order to enhance fishing productivity. Two kinds of stock enhancement effects are expected to result from such artificial stockings. One is the direct effect from the recruitment of stocked individuals themselves. The other is the indirect effect from the increase in the recruitment caused by the reproduction of stocked individuals.

We have three plans for spawning stock enhancement : fingerling release, fishing control and improvement of environment and habitat. The purpose of this study is comparison between fingerling release and fishing control. But these plans are hard to compare reproductive effect, because the fishing control is mainly indicated by estimating the fishing effect of adult fishes. On the other hand, the effect of fingerling release is mainly indicated by degree of young fishes added to natural resources.hancement : fingerling release, fishing control and improvement of environment and habitat.

To compare reproductive effect between these plans, we introduced the concept of "reproductive value" as a measure of the relative spawning ability to which individuals of different ages. Reproductive value is explained as the expected relative contribution of an individual to its population by reproduction, now and in the future. The basic idea in this study is comparison of reproductive value between the fingerling soon after released and the natural fish which we shall catch. We call the ratio of 'one fingerling reproductive value' to 'loss of reproductive value by one fish catch at age t' as Relative Index of age t. By calculation of this index, the number of fingerling release that can restock for loss of spawning by one fish caught at each age is known.

The relative index is applied to data of Japanese red sea bream in coastal area of Pacific Ocean and Seto Inland Sea. In Seto Inland Sea, for example, 61 fingerlings are required to restock in return for catch of one fish at age 6 before spawning season and protection of one natural fish at age 4 is proportional to 38 fingerling release.

In addition, sensitive analysis which vary one of some parameters is occurred. The result is that such species have relatively high reproductive effect of stocking : survival rate is large, growth and mature are fast, and have high fecundity. Finally, we discuss future view. If number of fingerling release and stock number at each age are known, we can calculate Reproductive Potential (Katsukawa and Matsumiya 1998), which the index of total population's ability to spawn. It is important to estimate this ratio in order to consider genetics problem and influence of fingerling release to fishing control.

Thank you.