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Think back to your elementary school science classes. While explaining the properties of light, your teacher probably brought out a lens, a prism, or even a glass of water, and showed how these clear objects bend rays of light. While the concept of refraction is interesting to observe, you may have discounted it as something that wouldn't affect your life outside the classroom. However, if you ever try spearfishing or bowfishing to put food on the table in a survival situation, you'll learn immediately that it's an important variable to consider.
When you're viewing an underwater object from above the surface, refraction causes that object to appear in a different position than where it actually is. Just as a drinking straw seems to bend when you place it into a glass of water, a fish below the water's surface will appear to be somewhere it isn't. If you're trying to spear a fish for survival, this small difference in visible and actual position can spell the difference between an empty stomach and a delicious dinner.
Fortunately, there's a simple rule of thumb for spearfishing and bowfishing: aim low. Well… that's a good starting point, at least. How far below the visible fish you'll need to aim depends on your angle of approach, the water depth, and even the size of your target.
The diagram below from The Fisheries Blog shows how it works:
If the fish is nearer to the surface, or you're aiming almost straight down by your feet, you won't need to aim much lower. If it's swimming deeper below the surface and you're at a shallower angle, you'll need to compensate more for refraction.
The following 1-minute video from Diamond Archery explains the basics of this concept:
So, for every foot of depth between the fish and the water's surface, you can aim 6 inches below your target, assuming you're shooting or spearing from approximately a 45-degree angle.
This will take practice, but it's important to keep in mind in case you ever need to spearfish or bowfish in a survival situation. If you aim directly at the fish you see, you'll almost certainly miss, and may end up frustrated and hungry after many failed attempts. So remember: when you're trying to hit an underwater target, aim low.