In order to study the assumed importance of the passes in generating Red Tide outbreaks, the hydrographical tidal conditions were studied in the Boca Grande Pass. The results confirmed previous ones and indicated the two-layer character of this estuary. In the deeper water no Gymnodinium
brevis was found. The heavier concentration was observed during all tidal phases on the side of Gasparilla Island, suggesting that they must have originated somewhere “behind Gasparilla Island” and not in the area of the highest total phosphorus. As a second, simultaneous part
of the study, an effort was made to find the areas from which the G. brevis in this case originated. In two separate spots (Stations 12 and 20) actual, locally limited Red Tide kills were observed. An optimum salinity for the G. brevis appeared to exist between 32 and 33 parts
per thousand. A diurnal vertical migration of G. brevis was observed.
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