A friend and I were scuba diving near San Diego. He was spearfishing and I was just sightseeing. He shot a halibut that looked pretty good-sized to me. When we swam back to our boat, he measured the fish and found that it was a half-inch short. He threw the dead fish overboard, and we returned to shore empty-handed. It seemed like such a waste to me. The only winners that day were the scavengers on the bottom. I don’t know what could be done differently. Unlike a lobster, you can’t catch a halibut with your hand to measure it alive.
Here is what can be done differently: Have your spearfishing friend cut out the shape of a halibut in a hard plastic an inch or two over legal size. Weigh it so it lays on the bottom and practice shooting it from various angles and distances. Not only will your friend become a better shot, they will get a “feel” for what is legal sized. The same can be done for fish with size restrictions that reside off the bottom only you make them float with a line and weight to the bottom. And when actually spearfishing, and in doubt of the size, do NOT shoot!