Experimental procedure. Prior to triggering vertical motion, the organisms were attracted towards the bottom of the tank (a). Subsequently, vertical motion was triggered by activating the green laser sheet and translating the blue beam upwards (b). Measurements were taken for different stages in the migration: the stage referred to as the beginning of the migration occurred when the first organisms in the aggregation reached 1/2 of the height of the tank (c), whereas the middle of the migration occurred when the first migrating organisms have reached 3/4 of the height of the tank (d).