vertical lines in the blacks of my photos

I took photos of a doggy swim in late afternoon light.  The dogs' colors are from white to black and every color in between.  I am shooting fast and the pool is half in bright light and half in shadows.  When I bring up the exposure and the shadows of some black dogs so you can see their facial features, vertical black lines (noise) appears.  Anyone have any good ways to lessen this? See dog on right.