All fish are much more sensitive along the full length of their body than we humans are.
Fish possess a myriad of nerves and nerve endings just under the skin, which transmit electrical impulses to the brain. The lateral line, running the full length of the body, around the centre of the body is the focal point for all of the fish's touch sensory organs. This simply means that the fish feels much more through the water, than we do.
So what does this have to do with safe gaffing techniques?
In fact, it has everything to do with gaffing techniques. Any object floating on the surface of the water or positioned within the water column creates a disturbance in the natural flow. While this water disturbance may be invisible to humans, it is a high stimulation field to every fish.
Any movement of the boat through the water increases the turbulence and disrupts the water even further. As the water impacts on the hull, it creates a shock wave, which bounces back throughout the water column. As the motor pulses out power to move the boat forwards or backwards through the water, the propeller creates more disruption and pressure waves across the water surface.
The fish, with their incredible sense of touch along the lateral line, are able to pick up the numerous impulses and disruptions to the natural flow and respond accordingly.
Although it seems a long way around, it does have a considerable deal to do with safe gaffing techniques.