Complex sentence: Whirling disease afflicts juvenile fish (fingerlings and fry) and it causes skeletal deformation and neurological damage that causes the fish to "whirl" rather than swim forward, making feeding difficult and the fish more vulnerable to predators.
Split into simpler sentences: Whirling disease afflicts juvenile fish (fingerlings and fry) and causes skeletal deformation and neurological damage. Fish "whirl" rather than swim forward, find feeding difficult, and are more vulnerable to predators.