Complex sentence: While drifting, the fish weakly beat its pectoral fins in a behavior known as sculling, which is suggested as necessary to keep the fish in a neutral position in the water and to counteract any displacing currents.
Split into simpler sentences: While drifting, the fish weakly beat its pectoral fins in a behavior known as sculling. The sculling behavior observed is suggested as necessary to keep the fish in a neutral position in the water and to counteract any displacing currents.