Abstract
Frog-flounder, Pleuronichthys cornutus, is one of the commercially important species among flat-fish taken from Tokyo Bay. The present report deals with seasonal and geographical distributions of eggs of the species in relation to the oceanography of the area. (1) The materials on which the present study is based are planktonic samples taken by the Tokai Regional Fisheries Research Laboratory and by Kanagawa Prefectural Fisheries Experimental Station during the period from 1957 to 1962 inclusive through the cooperative investigations on important neritic-pelagic fisheries resources. The samples were collected by vertical hauls of conical plankton-nets of 45cm or 60cm in mouth diameter. The nets were usually hauled from a depth of 150m or the ocean bottom when it was shallower than that up to the sea surface. (2) The eggs occur during the period from the late October till the early part of the following January. The peak appears during the period from the mid-November till the mid-December. The positive areas are the shallow zones on both east and west sides of the entrance of Tokyo Bay. None of eggs are found from the inner part of Tokyo Bay and Sagami Bay. (3) The ranges of temperature for occurrence are 16-21°C and those of chlorinity, 18-19‰. Thus, the positive area corresponds to the neritic zones where the water from the open sea does not strongly affect. The eggs are distributed neither in the low chlorinity region (inner part of Tokyo Bay) nor in the Sagami Bay which accepts the influence of open sea water directly. (4) The eggs of this species are spawned at around the entrance of Tokyo Bay. And this species grows in that area or after enterde into the bay, No commercial catch of this species is taken from the innermost part of the bay. There are two modes in the length composition of the summer catch from Tokyo bay, i. e. at around 16-20cm and 8-14cm in total length. These schools likely emigrate from the inside of the bay towards the entrance of the same in season of November-December. The larger fishes are considered to be mature there and become the spawners.
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