Now a large lake they would keep balance in the ecosystem but in a smaller farm pond/small lake they really unbalance it. Their will be mounds of just otter scat on the lakes. Its crazy
even those you would be surprised, its extremely hard to unbalance an ecosystem with external predators like that as they really don't really focus on bait fish, one of the best fishing ponds I knew had more turtles then you've ever seen and plenty of otter too. I'd say almost any lake with fishing issues has either A. too little bait fish population/wrong type or B. and most common too many types or too many predator fish. Best thing you can ever do for a lake is actually pull out every small bass (i'd say under 10inch)