The black and white picture separated most fish into 2 or 3 stripes.  As an estimate I took the results and divided by 2.  The result from Image J was 659 individual strips so I estimate about 330 fish.

Slice Count Total Area Average Size Area Fraction Mean
tropical fish.jpg 659 178569 270.97 9.3 254.447-

The image was downloaded from  http://thundafunda.com/33/underwater-animals-fish/School%20of%20Tropical%20Fish,%20Tahiti%20pictures%20underwater%20photos.php