If we can predict the hydrographic condition of a sea area, we may well be said to have a ground for inferring the locations of fishing grounds and also the state of fishes. With our present knowledge of oceanography and fish behavior, however, it is often very difficult to predict the sea conditon of a sea area, much more the locations of fishing grounds therein. This is partly because a sea area can be called a fishing ground only when a certain amount of fish catch is expected from it by a commercial fishing method, and partly because distribution and movement of fish schools depend upon both the environmental condition and the fish's habit. In this connection, the writer stresses the importance of studying hydrographic and ecological factors intensively in fishing grounds from the viewpoint that fishing grounds are the environment for the life of fish.
The writer describes a few conspicuous hydrographic features found in the Japanese coastal waters and in the neighboring Oyashio and Kuroshio systems. In Table I are summarized the typical seasonal north- and southward movements of the monthly isotherms for the surface tmeperatures of 5°, 10° and 20°C. The 5°C isotherm can be regarded as representing the boundary of the Oyashio water mass, and the 20"C isotherm, that of the Kuroshio water mass. By examining the year-to-year variations in the
position and movement of these isotherms, one can deduce a fundamental hydrographic pattern of this sea region. In Figs. 2, 3 and 4 are shown some typical distributions of the current and water mass systems in the offshore waters around Japan.
The writer is of opinion that the following are prerequisite to making reliable forecast concerning fishing grounds.
1. Various environmental factors in fishing grounds are measured concurrently with fishing operations, and the data of such meaurement are accumulated.
2. The biology, particularly the habit and the migratory range, of each economically important fish species is studied thoroughly.
3. In forecasting a fishing ground of a migratory fish, attention is paid to the movement of that isotherm which represents the lowest temperature which the species normally inhabits.
4. In infering the distribution of the egg and larva of a fish, attention is given to the current sytsem which washes the spawning area, and to those current rips which are formed along the border of that current system.
5. The efficiency of various fishing gears is studied from the viewpoint that it varies according to the sea condition and the behavior of fish school.
6. The possibility that fish, through learning, may acquire the ability to avoid the fishing gears of usual types is takne into account.