A computer image processing system for quantification of zebrafish behavior.

The tropical zebrafish (Danio rerio) has frequently been used for investigating developmental biology. Here, we developed a computer image processing system for quantifying zebrafish behavior. We could acquire an image of zebrafish freely moving in an aquarium using a CCD camera through a graphic I/O board. To acquire the image of moving zebrafish in real time, we required high spatial (256 x 256 pixels) and temporal (10 frames/s) resolution. Such a high speed of data analysis was accomplished using a skipping search method. By using a small aquarium, trackings of newborn zebrafish could be traced. The velocity of adult zebrafish (7.2 cm/s) was far faster than that of newborn zebrafish (1.8 cm/s). Furthermore, by separation of occluded images of two fish, we could acquire images of the two zebrafish. They behaved as in a school in which one fish chased the other. The chasing was defined by the distance, angle and approach of the two fishes. The chasing ratio of pairs of zebrafish was 37%, whereas those of pairs of different fish were significantly reduced to less than 20%. The present image processing system is a very useful tool for quantitatively scoring the schooling behavior of multiple fish.