Systems and methods for object enumeration in a culture tank. The methods comprise: obtaining an array of two dimensional images of an illuminated scene in a culture tank; correcting non-uniform background illumination in the two dimensional images; improving a color contrast between a background and object edges shown in the two dimensional images; performing a defocus analysis using the two dimensional images to generate a focus map image; generating a modified focus map image by eliminating isolated spots shown in the focus map image and merging relatively small spots with adjacent relatively large objects in the focus map image; classifying each object shown in the modified focus map image as an object of interest or an object of no interest; and counting the number of objects classified as objects of interest.