There are a few species of organisms in nature that produce light. Well known are fire flies and the blue light emitting from the sea in summer, due to light emitting algae. Another group is the light emitting bacteria of the genus Photobacterium. Species of Photobacterium are very common in the sea and thus can also be found on fish. The bacteria are harmless.

When a fish is fresh, Photobacterium is still present in large numbers. The older the fish, the less Photobacteria will be present. The fish in the refrigerator probably was still very fresh.