While fish share many of the same basic organs as you and I, they do have some unique attributes.

Cold-Blooded—To begin, fish don’t maintain a higher body temperature than their surroundings, as do mammals. Instead, their bodies are the same temperature as the surrounding water—warm in summer, cold in winter. This often leads to increased activity and heavy feeding in summer, and lethargic behavior in winter, but once again, it depends on the individual species. Some species remain quite active in winter, providing good ice-fishing opportunities.

Temperature Preference—This can be a somewhat misleading topic, because most fish can tolerate a wide range of water temperatures. Still, each species tends to function best within a certain broad band of temperature. Certain species must avoid excessively warm or cold temperatures in order to survive. You don’t, for example, find bass in the icy waters of northern Canada, nor lake trout in the warm waters of the southern U. S.

Gills—Passing water over the gills reoxygenates blood flowing through the gill filaments. Fish expel water through the backs of their gill openings even at rest, so they don’t have to be swimming in order to breathe. Never touch a fish’s gills; they’re prone to injury.

Swim Bladder—This interesting organ functions much like a balloon, allowing fish to either retain or dissipate sufficient air to maintain buoyancy when they change depth levels and experience different water pressure against their bodies. Most freshwater fish are limited to doing this rather slowly over an extended period, although some can expel air rather quickly when they move rapidly to the surface, as the air within their swim bladder expands due to reduced pressure. While most species can make significant but brief depth changes to feed on an easy meal, they tend to return rather quickly to their original depth to equalize pressure. This may explain why fish tend to remain at or near specific depth levels with comfortable surroundings.

Fins Galore—Look at any fish, or accurate artwork depicting a fish, and you’ll probably be surprised at how many different fins they have. All serve a function, chiefly for steering and maneuvering, like a combination of adjustable flaps on an airplane and thruster rockets on a spaceship. Their main propulsion, meanwhile, comes from sequentially coiling and uncoiling their bodies and caudal fin (commonly known as a tail fin) to each side.

Body Shape—Long, slender fish like pike and muskies are typically known for bursts of speed, but tend to be less maneuverable in and around cover. Shorter, flatter fish like panfish (bluegills, crappies) are better able to weave through cover and obstructions, but lack the long body to propel themselves at high speeds. Intermediate body shapes, like those of bass and walleyes, tend to fall in between. Trout, salmon and stripers tend to be the fastest freshwater species, largely due to their body shape and muscular nature.

Physical Size—With freshwater species ranging from half-pound sunfish to 500-pound sturgeon, there’s plenty of variation. While each species grows to a typical size range, most freshwater fish are less than ten pounds, with the vast majority under thirty. Two prominent exceptions are stripers and salmon, which have their roots in saltwater.  Big catfish, alligator gar and the aforementioned sturgeon can reach and surpass 100 pounds.

Mouth Size And Shape—You might not think this is a big deal, but it is. After all, fish can only eat what will pass through their mouths. Fish with large mouths tend to prefer large meals (usually comprised of smaller fish), while smaller fish like panfish are limited to eating insects or small minnows, chiefly because that’s all that fits between their gums. Most fish, however, tend to feed by inhaling a vortex of water along with their prey, expelling the water through the gills, and trapping the prey in their mouths. They don’t simply swim up to and bite something with their jaws.

Teeth—Toothy critters like pike and musky tend to be ferocious predators, and can easily cut your line if you don’t use a wire leader to deflect their rows of sharp teeth. Yet some fish, like walleyes and sauger, have rounded teeth that won’tyou’re your fishing line; the line rests between their teeth, rather than being sawed and severed. Other fish, like panfish, bass and catfish, have roughened surfaces along their gums, rather than teeth, yet still function as efficient predators. Big cats have powerful crushing jaws, however, so be careful where you put your hand. In all cases, keep your fingers out of mouths with teeth.

Oxygen—All fish need to extract oxygen from the water, but some species require higher oxygen levels to survive. Most unpolluted waters provide adequate oxygen content above five parts per million throughout the year, although in some cases, the deepest portion of a lake may suffer oxygen depletion during the extreme heat of summer, or beneath the ice during a severe winter. If so, some species are unable to survive until the water is reoxygenated by natural causes. Carp, catfish and some warmwater species appear best at withstanding poor oxygen levels, while coldwater fish are usually the first to perish when stressed.

Scales—Most fish have distinctive scales, although some, like catfish, have more of a skin. Fish typically have a slime coat of some sort to help stave off infections. While most coatings are inoffensive, some are slimy and rather strongly scented!

Coloration—Fish display an amazing range of colors and distinctive patterns, although there are some universal principles. Species that suspend in open water are usually silvery, white or light in color, with sunlight reflecting off their bodies helping them to blend in to the coverless environment. Those that chiefly live in and around cover are often mottled with dark colors, functioning as a camouflage to protect them from larger predators and to disguise them from smaller prey. When they switch from one area to another, they tend to adjust the intensity of their coloration, turning lighter or darker. They’re not chameleons, but they do adapt.

Fish also tend to be dark in color along their backs, and light or white along their bellies, with a transition between. This is the principle of countershading: Sunlight shining down on their backs brightens the upper portion of their bodies, while their shaded underbelly blends in with the colors above, rendering them less visible to predators.

Males versus Females—Ah, the battle of the sexes, even in fish! Not really, but you will find that the females of most freshwater species grow to a larger average size than males. Differences between the sexes are most apparent near spawning time, although during the rest of the year, larger fish (mostly females) may tend to school together and feed on a certain forage type or size, while smaller fish (mostly males) may school in other locales and feed on smaller food items.

Spawning/reproduction—Female freshwater fish lay eggs, with the eggs being fertilized by males in close attendance. Eggs tend to develop more rapidly (perhaps a few weeks) in warmer temperatures, and more slowly in colder water (possibly months). Most freshwater gamefish spawn in the spring, although salmon, char and some trout spawn in fall. Water temperature is a key trigger to the timing, although other factors like sunlight penetration into the water are likely involved.

Where and fish lay eggs, however, is highly dependent on the species. Some deposit eggs on rocks, other atop weeds or flooded brush. Nesting fish like those of the sunfish family (bass, panfish) sweep out pit-type nests to spawn in, with males remaining on the nest for several weeks to protect the eggs and newly-hatched fry from predators. Broadcast spawners like walleyes simply dump their eggs into rocks swept by current, and then leave the area unguarded. Trout and salmon, meanwhile, create crude nests in rocky current areas, but don’t guard them after spawning. To each its own.

Schooling behavior—Most fish are not loners; instead, they often school together in groups, large or small, to help defend them from predators (safety in numbers), or to increase their feeding effectiveness. They display a social lifestyle not because they’re friendly, but to enhance efficiency and survival. The result is, you’re often trying to locate schools of fish rather than individuals, and when you locate and convince one into biting, chances are there are more nearby. Fish that suspend and roam in open water tend to form the largest schools, while those that function in shallow cover tend to function in smaller groups, all else being equal.

Pea brain—They may say that you need to think like a fish in order to catch a fish, but you can put that saying out of your mind. Fish have small brains and aren’t about to outthink your best efforts. Instead, they tend to react by instinct, which is what you must appeal to most. You’d never get an entire roomful of smart people to agree on anything, but when a weather change suddenly affects the entire fish population and they go off-bite, well, that’s an instinctual reaction to an outside stimulus.

That’s not to say that fish can’t learn, however. Studies in the lab and fishing on the water show that fish can and do learn to avoid certain negative stimuli, and to experience positive reinforcement when feeding successfully. Fish can eventually become conditioned to certain lures or presentations, reducing their effectivness. This partially explains why a hot new lure can be so hot at first; either the fish haven’t yet learned to avoid it, or the ones most likely to respond to it haven’t yet been caught and released, recaught, or kept. Thus the constant urge for fishermen to experiment and discover a new wrinkle to provide an advantage over their fellow anglers—and the fish.