Anal gland problems affect millions of pets and are a very common and frustrating problem. Anal gland issues arise when the anal glands of dogs and cats becoming over-filled, blocked, or irritated. All dogs and cats have these two small glands sometimes referred to as anal sacs near the anal opening. These glands which are typically the size of a small grape normally release a few drops of scent marking fluid whenever your pet defecates observed near the end of defecation.
Why Your Dog Smells Like Fish
The anal glands or anal sacs are small glands near the anus in many mammals , including dogs  and cats. They are paired sacs on either side of the anus between the external and internal sphincter muscles. Sebaceous glands within the lining secrete a liquid that is used for identification of members within a species. These sacs are found in many species of carnivorans ,  including wolves ,  bears ,   sea otters  and kinkajous. The anal glands are in the wall of the anal canal. They secrete into the anal canal via anal ducts which open into the anal crypts along the level of the dentate line.
The Real Reason Your Dog Smells Like Fish
Dogs can smell like many things, depending on what they have most recently rolled in, but one of the more unpleasant aromas dogs commonly develop is one of fish. If your dog smells like fish, it is probably not because he figured out how to spray himself with Eau de Sardine. A fishy odor is usually the result of secretions from the anal glands.
Putnam North Animal Hospital. Anal Gland Disease. The Basics.