In one sentence, describe what the following article is about:

Salmon, unlike most other kinds of fish, is well known for the thin white lines which separate the layers or flakes of its meat. These lines indicate that the fish is still fresh and edible. If you notice that these white lines have disappeared—or if they’ve turned to a more gray color—the salmon has likely gone bad. Edible, fresh salmon should be firm to the touch. If salmon in your refrigerator has become squishy, mushy, or unexpectedly soft in texture, it’s likely expired. The white lines between flakes of salmon can indicate its firmness in addition to its freshness. Once the lines have faded, the meat is all but guaranteed to be mushy. Unlike other types of fish, as salmon ages and begins to go bad, it will develop discolorations. Look over the surface of the meat. If you see any spots that are not the healthy pink color of most salmon, your fish has likely expired. Most of the discolorations you’ll find on salmon will be dark. However, spoiled salmon can also have small white-ish patches.

Summary:
Note if the white lines in the meat disappear. Press the salmon to see if it’s still firm. Inspect the salmon for discolored spots on the meat.