Like any food, the more oysters you eat, the better you'll understand the flavor. "Well, that was salty!" is not an uncommon reaction after first eating a raw oyster. Yes, oysters are salty, and they taste of the sea, which makes sense. But they also taste like unexpected things, like mushrooms, melon, carrot, seaweed and butter. Sometimes, when approaching something so foreign, it helps to have a framework. Here's a method (borrowed very heavily from Rowan Jacobsen) that we use frequently in our pairing classes and tastings. Like any framework, it's a great starting place: Where you go with it is up to you.
Oyster Flavor Primer
1. Salinity is often what you notice first about an oyster, and it will vary with growing location, with season, and from oyster to oyster (skinnier oysters tend to be saltier).
2. Body can refer to the mouthfeel of the oyster... was it crunchy, soft, or skinny? But it can also be more abstract, similar to wine: did the oyster linger on your palate, or come and go? Body is determined by shell shape, season, age, and species. In Puget Sound and Hood Canal, oysters tend to be skinny in the late summer to fall after they've spawned, nicely balanced in the winter, plump in the spring, and soft in the summer (during the spawning season).
3. Finish is the most elusive component of an oyster's flavor. It's determined by growing location, growing method, species, age, and season. Sometimes, you don't even really "taste" the oyster until after you've swallowed it. This is one of the reasons why oysters pair so interestingly with wines... you often have time to take a sip of wine before you even taste the oyster, so you can really influence and expand an oyster's flavor with the right wine pairing.
Growing location is perhaps the most famous of these factors. The biophysical properties of an estuary ... the nutrients, temperature, salinity, chemistry... will influence what kinds of algae grow there, thereby determining the oyster's diet. But the upstream environment also plays a role. Yes, oysters eat algae and plankton, but they can also consume tiny bits of organic material, of bark, grass, or leaf floating down a river. Read more about growing location here.
Growing method influences an oyster's shape and flavor. When we tumble an oyster, for instance, we grow it up off the bottom where it has access to an entirely different community of algae. Tumbled oysters also spend more of their time feeding, so they have access to different kinds and quantities of food, and will fatten and spawn at different times of the year.
Different oyster species exhibit different ranges of flavors. Pacific oysters tend to be on the sweet and fruity side of the oyster spectrum, although some growing locations will produce savory, full-flavored Pacific oysters. Virginicas (the East Coast species) tend to be saltier and less fruity than a Pacific. Olympias (native to the West Coast) are renowned for their coppery flavor, although sometimes we're struck by their strong citrus notes. Eat a robust, chewy European Flat (native to Europe) and you'll understand why the French people invented mignonette. The dainty Kumamoto (native to Japan) are famous for a sweet, buttery, mild flavor.
An oyster's flavor develops with age, gaining complexity and depth. We try not to sell any oyster that's under 18 months old.... we like them to go through at least one reproductive cycle before hitting the marketplace. Some of our slower-growing varieties, like the Hama Hama, might be 2.5 to 3 years old before they reach extra small size. Age also plays a role in determining an oyster's size and shape. The sweetest part of an oyster, in our opinion, is the adductor muscle, which tends to be larger on older animals.
Finally, seasonality is very important. Each of the oysters profiled above will taste differently throughout the year... saltier in the skinny seasons, sometimes bitter in the spawning season, sweet and flavorful in the plump seasons. You can get a good idea how an oyster will taste by looking at it (gray = skinny, white = sweet) but the only real way to learn is to keep tasting.