Why Fish Look Bigger Underwater

One of the attributes of water is that when viewed through a prism of air, objects appear to be larger than they really are. So when I’m underwater looking through my diver’s mask, large fish, like a 45-inch musky for example, appears to me to be a 60-inch fish! Fish and anything else viewed underwater are only ¾ of their apparent size. Fisherman often ask me how big the fish are that I see. I do my best to adjust for the optics of underwater viewing, but the fact is, I just don’t know. Fish look really big underwater, and big fish look positively huge when viewed underwater. For the first few years, I got really excited whenever I saw muskies. “Wow! That’s gotta be a world record!” I would say to myself. But over the years, I’ve come to better understand this illusion and now I don’t pee my wetsuit quite as often when I see what looks like Moby Dick.

What this means sadly, is that all those reports you hear of 6 foot long monsters swimming next to the boat, or huge fish that got off before they could be netted are really just ordinary sized muskies. Some may argue that the fish lined up exactly with something on the boat that’s of known size and therefore, that’s evidence that the fish was really a whopper. This of course is nonsense. Since you would still be viewing the fish in water through a space of air, the magnification illusion is still in play. Your boat is a poor yardstick since it’s in air, and the fish is underwater. (If you don’t quite grasp this idea, lean over the side of the boat sometime and stick your hand underwater about a foot. Then take a ruler and hold it above the water’s surface and take a good measurement of what you see. Next, take your hand out of the water and measure it. If you did this right, your actual hand will measure only ¾ of the size of your estimated measurement while it was underwater.)