My bottom teeth have cavities. Are these fixable? (Photo)

Thank you for asking a great question. Based on the picture you provided it looks more like a tartar then decay. In either case, most likely it is fixable. I would definitely advise you to see a dentist asap.
Good luck!

A Thorough evaluation  is needed to determine your exact needs.  With that said, it would appear your gums are inflamed and you have a massive amount of tartare built-up on your teeth at the gumline.   removal of this tartar should allow your gums to become healthy. At that point, the dentist may find cavities  at the gumline. Our experience is that  once the tartar is removed,   There usually are not any cavities at the gumline. But, with that amount of tartar and that amount of gum inflammation, the bone may be starting to melt away. The bone is what holds the teeth in, so you don't want to let the bone keep melting. Get into see your dentist and have your teeth cleaned. 

Good Question--The real question from your photos is that if the dark areas down by the gumline is decay--in the photo it looks more like stained tartar build up more than decay--this would also correlate with the smoking history. Without an examination and x-rays it is impossible to determine the long term prognosis. However I would expect that you will be able to treat and save these teeth. See a reputable dentist soon. Good luck