潮境に於ける水産資源の定量生態学的研究-I

Abstract

The boundary zone between different water masses in the open sea has been recognized by fishermen as a good fishing ground of various migratory fishes inhabiting in cold or warm waters. Many works have been done by many investigators on the physical aspects of the boundary zone and the ecology of individual fishes involved in it. The most of these works have been rather conformed with qualitative concepts which have been recognized for many years. For the standpoint of fisheries, however, more quantitative approaches will be required. At the first step, we consider the plankton population in the boundary zone as an indispensable source of energy to support the metabolic activity of fishes. Under this assumption, the boundary zone should be the place where the active metabolic exchange of energy is taking place between fishes and their environments, and it must be more fertile in the food materials than other areas. The present work attempts to evaluate the fertility of the boundary zone in precise terms of metabolic energy source by chemical methods. For this purpose, it is desirable to determine the, organic content of the plankton population by direct analyses of the samples collected with plankton nets. However, there are difficulties of dtvising a plankton net suitable enough to meet the demands of this work and at the same time simple enough to be usable for practical application. In addition, we have neither the information nor the available data for the development of such work. Hence, the studies are developed for methods of estimating the approximate organic content of the plankton populations by the increase of the oxygen from a certain oxygen level in a given region. The outline of the estimation and the assumptions involved will be only briefly summarized here. If all the waters from which the production of phytoplankton had been. originated were saturated with oxygen, and the oxygen exchange with the atmosphere during the period of the production had no significant effects upon the distribution of the oxygen, it might be expected that the oxygen supersaturation would be approximately equivalent to the amount of the oxygen liberated by photosyn. thesis of the plant. Under such conditions, the amount of organic material synthesized by the phytoplankton in a unit volume of water can be expressed as carbon corresponding to the oxygen increase, since for every molecule of carbon dioxide utilized by phytoplankton, one molecule of oxygen is liberated. In many cases, however, the organic carbon synthesized by phytoplankton is converted by grazing processes into that of the other forms of organisms such as zooplankton, fishes and other larger animals, and partly returned to carbon dioxide through metabolic processes or by disintegration of the organisms. On the other hand, the oxygen of waters should be reduced remaining the oxygen equivalent to the organic carbon of the plankton population. Hence, assuming that the respiratory quotient is 1, it is possible to estimate the carbon c?ntent of plankton population contained in a unit volume of waters by using the observed supersaturation of the waters. However, it is considered that the oxygen supersaturated by the phytcplankton in the surface water may be partly driven off towards the atmosphere. On the other hand, it has been already known by many workers that in the euphotic layer, for each milligram of phosphorus utilized in photosynthesis, the phytoplankton will take up 76ml. of CO2 and release the same volume of oxygen, and at lower levels, where regeneration is taking place, the consumption of 76ml.