From the looks of your picture you seem to have an infestation of a fruitworm. I'm assuming that no other part of the trees are not exhibiting signs of damage.  You need to identify the pest. Cut into a plum and see if you can find an insect larva inside. if you find one, take it to your local extension and have them identify it. You didn't describe the extent of the problem.  If you have just a few plums infected, just pick them off. Being that it's rather late in the growing season for plums other controls may not be effective. For larvae of butterflies and moths you could spray trees with Bt (Bacillus thurengensis), but that won't kill the caterpillars that are already in your plums. To control this problem for next year spray tree in the dormant season with a dormant oil, available at your garden supply store.
   If you find no insects inside your fruit, look for signs of Pseudomonas seringii like yellowing and dying of leaves and branches,leisions on the bark with or without oozing gum. Control this with proper pruning and sanitation along with a dormant spray schedule of lime/sulfur.

I have 6 plums trees and one crabapple tree.  All of them are infested and it appears that all the fruit will 

Another thing is I have zero squirrels on my yard this year.  I always have dozens.  The only thing I can think I did differently was use fish fertilizer but that was months ago.  Any ideas.  It worries me because my dog got sick suddenly and died.   I don't know if something bad is going on and we are getting a new puppy soon. 

Ask your neighbors if they have seen any squirrels around this year. This problem is unrelated to your plum problem. The possible causes for this are; someone is spraying trees which the squirrels ate, hunting or trapping or the squirrels, lack of food for the squirrels to eat. If there are no squirrels in your yard it is likely that there aren't any in your neighborhood. Call the extension office or the city parks dept. and see if they have noticed anything. At any rate I don't see how your yard conditions could be the culprit. Sorry about your dog! It could be that someone was baiting squirrels and you dog got into it.