FISH ANATOMY
Fish are less complex animals than many others, at least in my opinion. Think about it, most of the fish is muscle, which is perfect for us who want to have a bite. However, there is more to fish that can be eaten or used in cooking than just its flesh. First of all, the skeleton bones that help create the shape of the fish and are involved in movement and swimming. They are not really edible, although they can be used with good effect in stocks. Even if it's bone, it is not as hard and mineralized as our bones. Actually, they can dissolve when we cook them for some time. In result, we get out some nice gelatin that will give some body to the stock. And in some countries, fish bones are actually fried and eaten - they get crunchy and fairly soft! Another thing to use in stocks, if you need some gelatin, is the fish skin. Of course, skin can be eaten, especially if fried or grilled - it becomes crispy. Skin can also be cooked, and then the gelatin inside it will melt and give a moist texture. On the surface of the skin we have scales that are not used in cooking, they are scraped off with a knife, moving the knife against their grain. Some more unusual parts of fish that you may find on restaurant menus include liver, tongues, or swim bladders. These are more commonly prepared in the exotic places.
Setting aside the less famous parts of fish, let's focus on the main course itself - the fish meat. Contrary to other meat, fish flesh consists of small muscle units, arranged in flakes, separated by thin layers of connective tissue. This connective tissue is not very preponderant, and it doesn't become tough with age. It dissolves pretty fast in moderate heat. When it does dissolve, together with the fat that also lies in-between the muscle flakes and under the skin, the overall cooked meat texture becomes moist (unless you overcook!). Most fat, thus succulunce, can be found in the widest region of the fish, around its belly. Most connective tissue can be found in the most exercised areas, usually near the tail.
In some cases, if you happen to buy fish that has entered its migratory phase of life, its flesh becomes very soft and mushy. This is because it uses up its flesh proteins as energy, so the meat is becoming more degraded and thinner. So, watch out for these unpleasantly soft fish meats! The same thing may happen if you freeze and thaw the fish. This is a perfect recipe for destruction - once the fish is thawed, the cells become disrupted and the degrading enzymes flow out from the cells and into the muscle and break it down!
The taste of fish depends on many factors, including the way you prepare it. However, its taste is largely affected by what kind of fish it is, with all that comes into play during its life. Normally, seafood is more tasty than fish caught in freshwater. The reason is the ability of sea fish to counterbalance the high salt content of sea water by accumulating a vast amount of amino acids (that stimulate our taste buds) in its flesh. Now, sea water is not the same everywhere. It's salt content is usually higher in the open seas, and lower near the coastline, especially where the rivers join the sea. Therefore, fish caught in the open sea and ocean tastes more (presuming that it has lived there for enough time to create its tasty meat). Fish with high amounts of these nice amino acids include mackerel and herring, but other species are also worth trying.
Interesting to notice is that when a fish is caught and killed, its meat will start to accumulate another tasty molecule called IMP. Therefore, sometimes it may be worth letting a newly caught fish sit on ice before you cook it. However, don't wait for too long, as IMP will soon start to decrease! Of course, fish will start to go bad after some period of time as well. The strong smell that we all associate with bad fish will start to develop as bacteria (they are everywhere, wherever there's food!) break down some non-smelly substances of fish into this disgusting molecule that we want to stay away from. Ocean fish has much more tendency to develop such smell, as it contains more of this molecule. When you freeze a fish, additional smell of ammonia will develop, now created by the inherent enzymes of the fish. In other words, consume your fish fresh! (I guess this rule goes for almost all food.)
If the fish is in a bad fishy smelly shape, there is still someting you can do. First, just rinse the fish in cold tap water. If it doesn't help, use lemon juice or vinegar (or some other acidic ingredients) and either wash the fish again and then rinse with tap water, or cook it with the acid. The trick with acid is that it provides some protons for these smelly molecules, so that they become more sticky to other molecules in the fish (thereby becoming trapped and cannot reach our nose smell receptors).
Seawater fish also have the smell that reminds us of the sea. Freshwater fish on the other hand can taste like mud, especially the ones that live and predate on the bottom of lakes and rivers. All fish also produce substances that smell like a new-cut grass or plants, but this smell is disappearing fast and must be enjoyed with a freshly caught fish.
One thing has also to be said about the color of the fish. It's important to know because it will help you in your cooking. Most fish meats are slighly translucently white, right? What happens when you cook such fish meat? It becomes more opaque and solid (due to denaturation of the proteins in the heat). So, as you watch your fish fry in a pan, look how the opacity of the meat increases from bottom to top, as time passes by. Then turn the fish over when the opacity has come almost halfway through the fish. If you have a red-meat tuna instead, watch for the browning of the meat (pretty much as beef meat!). This browning, in other turn, is caused by oxidation of myoglobin - an oxygen-storage molecule found in tuna meat and beef.