PRELIMINARY STUDY ON THE VERTICAL DISTRIBUTIONOF THE COMMERCIAL FISH FRY IN LOWER YANGTZE

The fish fry referred to are those of Mylopharyngodon piceus, Cteno-pharyngodon idella, Hypophthalmichthys molitrix and Aristichthys nobilis, all of which fail to spawn in ponds or lakes, and their fry, being vital to pisciculture, have to be secured from the rivers year after year. The gears employed in fry-collecting are diversified, but all of them agree in work- ing only in upper layer of water, reaching hardly more than 1 meter below the surface. It has been the aim of the writers to find out whether these fry are restricted to the upper layer, and, if not, whether they are always more abundant in the upper than in other layers. By installing fry-collecting nets at different depths but at the same vertical column of water it has been possible not only to ascertain the occurrence of these fry in different layers but also to compare their re- lative abundance. Out of 11 cases of collections, 5 cases pointed to the upper layer rank- ing above the other layers in catch, 4 cases the middle layer and 2 cases the lower layer. The yield of the upper layer, however, was the highest, contributing 43.5% of the total catch, while the middle and the lower layer gave 27.6% and 28.9% respectively. A kind of pisciculturally obnoxious fry under the vernacular name of "Moon-Tse", the definitive characteristics and systematic identification of which are yet to be followed up, enters the collecting nets along with the commercial fry as an adulteration. Observation reveals that Moon-Tse is highly concentrated in the upper layer, as 93.2% of the total catch of this fry came from this layer. Evidence for diurnal change in distribution is presented for this fry.