Aggression in 7 month old puppy

Question
Hi Jill,
I have an almost 7 month old male Australian Shepherd. He has always been very sweet and I have been taking him to our local dog park almost every day since he had his final shots. Tonight though he got a little aggressive. I brough his own water since they turned the water off at the park and he growled when another dog tried to drink from it. He then growled when another dog tried to take a frisbee from him. Then he actually bit and was very aggressive with the same dog when it tried to take a stick from him. I have never heard him growl with another dog before and normally he plays great and there are never any problems. When he did this I grabbed him and told him "NO" and we left right away. I am just wondering if you know why he might be doing this and how I should handle it? Thank you so much.

Answer
You need to find a "growl class".  Since you live in a place where I think this might be possible, ask your veterinarian (or call around to others) for a credentialed (national) positive reinforcement trainer with a training facility.  A "growl class" is a place where dogs with problems such as your dog has can interact and learn to "meet and greet", with counter conditioning, over several weeks.  By grabbing him and telling him "no" you are not addressing the dog's fear but, in fact, might make it worse.  He is resource guarding.  This breed is known for aberrant aggression, it is not uncommon.  

Your dog is NOT a candidate for the dog park.  DO NOT GO THERE.  At seven months, this behavior is a serious problem in terms of temperament and/or training and socialization.  I am not a great lover of dog parks.  Many dogs get into trouble there and develop behaviors they would not otherwise have had.

If you are unable to find a good, credentialed trainer (and you must ask for credentials and check them, ask for references and check them, and audit a few classes before bringing the dog) then I suggest you find a Certified Applied Animal Behaviorist (CAAB) who can assess the dog and do whatever s/he determines is necessary in order to evaluate the problem.  You can hopefully find one within reasonable distance from the following sites:

http://certifiedanimalbehaviorist.com/page6.html
http://www.animalbehavior.org/ABSAppliedBehavior/caab-directory

Dr. Ian Dunbar, a very well known and highly respected Veterinary Behaviorist, has his headquarters in Berkeley, CA.  His office might be able to help you find someone to help you, since you are on the "west coast" (more or less):
http://www.siriuspup.com/about_founder.html