Takeru Kitahara

A preliminary study on quantitative relations among growth, reproduction and mortality in fishes

As a quantitative approach to the life histories of fishes, the present paper attempted to predict a relation among reproduction, growth and mortality numerically with a technique of control theory, the discrete maximum principle. A method for predicting the relation was derived on the postulate that natural selection maximized the net reproductive rate subject to a few constraints. The derived method was applied to Atlantic cod and Atlantic herring populations in the Southern Gulf of St. Lawrence as numerical examples. The examples demonstrated that the theoretical reproductive effort and body weight were well consistent with the observed ones every age but the theoretical survival rates were slightly different from the observed ones. For the reasons mentioned below, however, it should be interpreted that the examples rather support the adopted postulate to a certain degree. First, in general, it is very difficult to obtain good estimates of the rates with traditional methods. Second, intense fishing pressure possibly changes the life history parameters to some extent in fish populations. Moreover, the examples also suggested that, to examine the postulate in further detail, similar analyses had to be made with the data of many fish populations on which intense fishing pressure had not been exerted.

NUMERICAL PREDICTION OF A RELATION AMONG GROWTH, REPRODUCTION AND MORTALITY IN ITEROPAROUS FISH POPULATIONS

As a quantitative approach to the life history, the present paper numerically analyzed a relation among growth, reproduction and mortality for nine fish populations with a technique of optimal control theory, the discrete maximum principle. The analytical method used was derived on the postulate that natural selection maximized the net reproductive rate subject to a few constraints. A comparison between the theoretical and observed survival rate showed that the former was discernibly lower than the latter in all the populations except the three species. For the reasons mentioned below, however, the analyzed life history data should not be interpreted as a refutation of the adopted postulate and the method. First, it is generally very difficult to obtain a good estimate of the rate with traditional methods. Moreover, it is probable in most fish populations that the rate considerably changes with age even in the adult stage though it is usually estimated on the assumption that it is constant in a certain age range. Second, an intense fishing pressure possibly alters the life history characteristics of fish populations to some extent.

Demersal fish populations in the Japan Sea from the viewpoint of benthic fish management

First, this paper reviews the main studies so far performed on fishing ground environments and the faunistic peculiarities of demersal animals in the Japan Sea. Next, an examination is made of changes in the catch of demersal populations over a long period. Of the exploited demersal fish populations in the Japan Sea, some have decreased in catch, while others have been relatively stable for a few decades. A comparison of the life history characteristics of flatfishes is carried out on the basis of the above finding. The comparison shows that populations with strong resistance to intensive fishing slow their somatic growth rate and contribute vigorously to reproduction at young ages relative to life span, and vice versa. In addition, a theoretical analysis suggests that the effect of fishing pressure on a population probably depends on the extent of variations in its life history characteristics, which have a genetic basis, in the period when it was little exploited.

An attempt to estimate the size of an exploited migrant population.

This paper attempts to derive a reasonable method for simply estimating the size of an exploited population which migrates in a certain direction, under the assumption that emigrant number in an area is equal to the immigrant number in its adjacent areas. In deriving the method, it was assumed that the emigration rate was the same in the considered areas and that the natural mortality rate was negligibly small as compared with the emigration rate. An application of the derived method gave about 0.1 during 5 days an an estimate of the emigration rate of the swimming crab Portunus trituberculatus population along the Buzen District, Fukuoka Pref., in 1978 and 1979. The present method can not be used for the populations that immigrate continuously.