If the fishing in a lake goes deteriorates sharply with no obvious environmental reason, what usually causes this? I suspect that the cause is a series of poor spawning years, but are there other causes? Examples of this phenominom include Swartzwood Lake in northern New Jersey and the New Croton Reservoir in Westchester in New York. If a lake goes downhill, how long does it take for it to come back? Thank you for your responses.