The present paper is a study on the basis of the investigation ofhydrographic data of the middle fishing ground off Zhejiang in 1977--1982. The occurrence of hairtails in the fishing ground belongs to seasonal migration,and the shoal is more sensitive to the change of environment conditions. Thereforethe season alternation of hydrography in fishing ground, the annual distribution ofwater temperature and the latitudal fluetuation of mixed water system ect, should beused as indices in determining the advance of fishing season and the variance, orshifting of central fishing ground in practice. If the alternation between summer and winter of the fishing ground hydro-graphy appears in adance, the fish occurrence will be early in time, otherwise it willdelay. If the water temperature distribution presents‘cold year pattern', it would delay. The fishing season should be in advance, otherwise. There is an evident rulativity between the latitudal fluctuation of 34‰ isosalinecure (Namely, high isosaline curve) and the central fishing ground, If the highisosaline curve inelines to the west, the fish occurrence tends to inshore, otherwise ittends to offshore.