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Individual fish may look very different. Consider the flat, pan-shaped Bluegill, the slender Eel and the huge Sturgeon. Nevertheless, all fish share some body forms, functions and habits.

Most fish are ectothermic (cold-blooded) animals that live underwater. “Ectothermic” means they tend to assume the temperature of their surroundings, instead of generating their own body heat. Most fishes have a bony backbone and skeleton. They are grouped together under the scientific class Osteichthyes. A few fish, like the lamprey, skates, sharks and rays, have a skeleton of rubbery, gristly tissue called cartilage. Instead of legs, fish have fins to help them get around in their liquid habitat. They breathe by taking up oxygen that is dissolved in the water through their gills. The gills also allow the release of waste gases, like carbon dioxide and ammonia.

What else do most fish have in common? Their skin is protected and lubricated by mucous glands. The skin is also generally covered by scales, or in a few fish, bony plates. Catfish and Bullheads also have only skin and no plates or scales. Fish have a two-chambered heart and a blood circulatory system. Fish don’t have eyelids, so they can’t close their eyes. Their ears are all internal, with nothing showing on the outside. Most fish reproduce by laying small, round, jellylike eggs in the water. They are all adapted to living in water, a medium that is very different from the air in which we move about. Water is denser and generally colder than air, and fish must also withstand the forces of water pressure.

With water covering so much of the Earth, it’s no surprise that fishes are found worldwide, both in the salt water of oceans and the fresh water of lakes and rivers on the continents. With so many different places in which to live, it’s no wonder that so many different types of fish developed. Worldwide there are about 29,000 species of fish, grouped in 445 families. Here, in North America, there are over 800 species, which biologists have assigned, because of their similarities and differences, to nearly 40 freshwater families and about 10 marine families.

In Pennsylvania, with some 83,000 miles of streams and rivers and hundreds of lakes, there are currently over 160 species of fish in about 24 different families. The numbers are approximate because non-native fish species may be stocked or accidentally released. In addition, fish that were once present may have disappeared from the state (extirpated), or they may have become extinct. Fish species that were thought gone are also occasionally rediscovered in the state.

Fish are very old forms of life, with some species seeming to be unchanged in body form from their fossilized ancestors found in rocks hundreds of millions of years old. Other fish have changed dramatically over time, in body shape, function and probably behavior, to survive or prosper in habitats that have been altered or to take advantage of new habitat opportunities. The science of the origin of fishes, paleoichthyology, investigates the fossilized remnants of these ancient fishes to learn how this evolution happened..

Dunkleosteus, 400-million-year-old prehistoric fish

The oldest fish fossils that scientists have identified date from 425 million years ago. These fish were jawless underwater creatures, with armorlike plates covering the body. Sharks, like the prehistoric Dunkleosteus, appeared about 400 million years ago. Fossils show that this prehistoric fish was as big as a school bus. Bony fish are more recent in the time scale than cartilage-skeleton fish such as sharks. About 200 million years ago, the Ichthyosaur, a fishlike reptile, swam in ancient seas. By 10 million years ago, fish had developed into such forms as the sabertooth salmon, which grew to eight feet long. Fish like bass and walleyes are more modern than trout or bowfins, which retain some body characteristics that were typical of “primitive” fishes.


Smallmouth Bass
Smallmouth Bass


Pennsylvania has many different types of fish because it offers many types of aquatic conditions. Its water habitats range from deep, cold lakes to shallow, swampy lakes, to chilly, clear mountain streams, to slow-moving, warm, silty rivers. There are lots of places that fall between these habitat descriptions, plus some tidally changing waters. Some of the state’s fish are similar to fish, or are also found, in Eurasia, like the Northern Pike and Brown Trout. Others are native only to North America, like the Sunfishes and the Catfishes.

Even if appropriate habitat exists, a fish species may not be in a lake or stream simply because it doesn’t live in that watershed. When rain falls to the ground in Pennsylvania, it drains into one of six major watershed systems. In the northwest, it might flow toward Lake Erie. In most of the rest of western Pennsylvania, water flows into a tributary of the Ohio River, which feeds the Mississippi River. Most of central Pennsylvania’s water empties into the Susquehanna River and its branches, which flow to the Chesapeake Bay. The smallest Pennsylvania watershed is the Genesee River, which collects water along the central part of Pennsylvania’s boundary with New York state and sends it north to Lake Ontario. Along the central section of Pennsylvania’s southern boundary, streams flow to the Potomac River, which eventually meets the Chesapeake Bay. Eastern Pennsylvania is in the Delaware River watershed, which opens into the Atlantic Ocean.

PA watersheds
PA watersheds

These watersheds flow to different destinations. Even though fish can’t cross mountain tops, they can spread upstream and downstream in a watershed simply by swimming. Although Pennsylvania’s watersheds share many fish, like smallmouth bass and white suckers, other fish are found only in one or two watersheds, like the johnny darter and flathead catfish. Some fish species entered new watersheds when glaciers advanced and retreated, over about a half-million years, ending about 15,000 years ago. Across northern Pennsylvania, the huge ice masses blocked and rerouted north-flowing streams and sometimes added their flow to already southgoing rivers, mixing the fish life. The Allegheny River and its tributaries in northwestern Pennsylvania have a diverse aquatic community because the glaciers changed the state’s landscape and redirected those streams and the species of fish that lived there.



A fish’s outside appearance can reveal a lot about where and how it lives. The shape, color and form of parts of a fish’s body, its external anatomy, suit it to its habitat and its feeding methods. Some fish bodies are laterally compressed. That is, when the fish is viewed head on, it’s narrow side to side. Others are dorsally compressed, which means they’re more flat top to bottom. Others may be roundish in shape, viewed from the front, or somewhere between these extremes. When viewed from the side, some fish are long and slender. Others appear blocky or circularshaped in the body, like the pan-shaped Sunfish.

Most fish have the same basic parts. Except in the Eels and Lampreys, which are slender their whole length, fish have a definite head portion and a longer, wider body section, and they narrow to a tail. They have a mouth, which may range in size and shape from large-jawed with strong teeth, as in the walleye; toothless and capable of being extended for “vacuuming” small food items from the bottom, like Suckers; or even a round, jawless disk for attaching to a host fish, like some Lampreys. The snout has nasal openings, or nares, to let them sample the chemical “odors” in the water. The eye is usually on the upper portion of the side of the head.

Most fish have a flat cheek area behind the eye and jaw, and a flap behind that, which covers the gills, called the operculum. The presence or absence of scales on the cheek and/or the gill cover is also often used in determining the difference among Esocid (pike family) species.

Along the back, at the end of the tail, on the undersides and just behind the lower portion of the gill cover, most fish have fins. How many there are, their length, location and shape, vary greatly among fish families and species. Some fish, like the Perch, have distinct, well-developed, separated fins. Other fish, like the Eel, have a long, continuous fin curving around their length from top to bottom.

Soft-Rayed Trout
Soft-Rayed Fish (Trout)

The fin or fins along a fish’s back are called the dorsal fin. This may start just behind the head, as in the darters, or it might be positioned far back on the body, as in the pikes. There may be one or two dorsal fins, the second either widely separately or close against the first, along the back toward the tail. Some fish, like Trout, have a small, fleshy or fatty lobe that grows from the back behind the dorsal fin, which is called the adipose fin. The fish’s tail is finished off by the tail fin, or caudal fin. Fish show many different forms of caudal fins, from small and roundish to large with long, pointy lobes, and many shapes in between.

Spiny-Rayed Yellow Perch
Spiny-Rayed Fish (Yellow Perch)

On the underside of the fish’s body, in front of the tail, is a rudderlike fin called the anal fin. Farther forward along the belly is generally a pair of small fins called the pelvic fins. And on each side of the fish, usually just behind the lower part of the gill cover, is a fin called the pectoral fin.

Fins are supported by filaments called rays. Rays can also spread out, saillike, or draw in a fish’s fins. In some fish, like trout and pickerel, all of the rays on all of the fins are soft, so these are called soft-rayed fish. Rays can also be hard, spiny and sharp, as in the Bullheads and Sunfish, which are called spiny-rayed fish. Counting the number of spiny rays and noting their position can be helpful in identifying which fish is which. When present, spiny rays are usually in the first dorsal and/or the front edge of the pectoral, pelvic and anal fins, and can inflict pain if the fish is handled carelessly.

A fish’s skin may be naked, but most often it is covered with thin, fingernaillike, roundish, overlapping scales. Scales may be large and coinlike, as in the Carp, or embedded in the skin and easy to overlook, as in the Brook Trout. The scales of soft-rayed fish are roundish, cycloid and smooth, which give the fish a slick feeling. Spiny-rayed fish have ctenoid scales, which have tiny “teeth,” and give these fish a rough feel. Scales can also be diamond-shaped, called “rhombic,” as in the Gars. Instead of scales, primitive fish like Sharks and Sturgeons have bony plates, especially on the head.

Pigment cells give the fish’s skin its color and pattern. These can be intensified or reduced by the nervous system in response to the fish’s environment or hormonal changes. Salmon, for instance, “color up” in spawning season. Their colors fade and their scales just reflect and refract the light in silver and rainbow tints when they are in deep, open water. Some fish are colored in patterns that hide or camouflage them in their surroundings, like the Pickerel in its weed beds. Others have markings that advertise which sex they are, like the male Bowfin’s dark spot at the base of the tail.

Many fish have an interrupted line marking, often visible on the skin, that runs along the middle of the side, from head to tail, called the lateral line. This is actually a sense organ that detects vibrations and changes in water pressure. A slimy covering, produced by glands in the skin and called mucous, helps to protect the fish against disease and scrapes. A fish’s skin may also have poison glands, as in the madtoms.


Internally, bony fish have some organs that are like our own, and some that are not. Yet, we share most of the basic animal body functions. From head to tail, bony fish have a spinal cord encased in a backbone, and a skeleton that consists of a row of thin, flattish bones above the backbone, supporting the muscles and giving the fish’s body shape. Below the backbone, protecting and making room for the fish’s internal organs, the row of bones is paired and curved, forming a rib cage. A row of single bones completes the skeleton behind the ribs and below the backbone in the tail. Fish bones are shaped like a curve. In others, single bones branch into “Ys,” which branch into even smaller “Ys,” as in Suckers and Shad.

Fish have a brain, a two-chambered heart that is small for their size, a closed circulatory system, digestive and elimination system, liver, kidneys, pancreas and reproductive organs such as ovaries and testes, much like other more advanced animals. Most also have an internal air-filled sac, called an air bladder, which helps the fish maintain and adjust its position in the water, providing it with neutral buoyancy, so that it can suspend itself and not sink. The air bladder also aids in hearing by intensifying sound, and in some fishes it is used as a sort of “lung,” for gulping air above the water’s surface.

Anatomy - Internal
Illustration of internal fish anatomy

Multi-page PDF of internal fish anatomy illustration

Gills are the organ fish use to “breathe.” This process is different from that of land animals, although the result is the same. The gills take in life-giving oxygen and remove waste gases, like carbon dioxide. The gills consist of blood-filled filaments with lamellae, which look like teeth on a comb, supported by bony arches. The lamellae are covered with a single layer of cells and have a large network of tiny blood vessels. This is where the gas exchange takes place. Fish take in water by opening the mouth, allowing water to pass over the gill filaments, and out through the opening of the gill flap. It’s a one-way trip, so the fish doesn’t “breathe” in and out, as people do. Some fish, like Trout, need lots of oxygen. Because more oxygen can be dissolved in cold water, that’s where Trout live. Other fish, like Carp, can live in warm, silty water, because they don’t require as much oxygen. In the breathing function, a fish’s mouth works as our noses work. A fish can open its mouth to take in oxygenated water to flow across its gills, without opening its throat to swallow.


A Bluegill waves its fins as it moves slightly up and down, back and forth, deciding whether to take a bait or lure. This is proof any angler can see that fins are made for fine adjustments. But when the fish decides to swim away, it’s the muscles of the fish’s body that produce that strong, swift motion. If the skin is stripped away, the fish’s body muscles can be seen following one another as thin, W-shaped segments from head to tail. The muscle segments contract in succession, first along one side and then along the other, so the fish swims by pushing against the water side to side. The tail fin adds an extra kick. The other fins aid in balance. The dorsal, anal and caudal fins act as rudders. The pelvic and pectoral fins help in turning, as a banking airplane. The pelvic and pectoral fins can also be spread against the forward motion of the fish, to “put on the brakes,” or they can be folded against the body, to streamline the fish when it wants to swim fast. Fish with air bladders can move easiest up and down in the water, or keep their position in the water column. Fish without air bladders have to dart from place to place along the bottom, like the darters, because they are heavier than water and sink.


Habitat influences the species and numbers of fish found in a waterway. If the habitat meets a fish’s needs, it can survive there. If the habitat doesn’t meet the fish’s needs, it won’t be found there. One very important habitat factor is water temperature. The temperature of a waterway is determined by many variables. Water temperature is influenced by the time of year, the amount of sunlight reaching the water, the amount and speed of the water (flowing water and currents), the source of the water (springs or runoff), and the amount of material suspended in the water.

Fish can’t maintain their body temperature at a constant level as humans and other warm-blooded animals can. They are what biologists call “ectotherms.” The temperature of their surroundings influences their body temperature and bodily functions. This is why water temperature is such an important habitat factor for fish.

Each fish species has a specific range of water temperatures in which it can live. If the water is outside that range, it can’t survive. Within that range is a narrower range of temperatures. This is called a fish’s “preferred temperature.” Fish can live, grow and reproduce when they are within their preferred temperature range. The preferred temperatures are ideal for the fish’s survival.

Biologists group fish with similar temperature preferences into three groups: Cold, cool and warm. Fish in a group often have other similar habitat needs. The groups do overlap, however. They overlap because temperature preferences among groups overlap. The habitats where they overlap are called “transition waters.” Transition waters may be ideal for one group, but not for both. Brown Trout, for example, are considered coldwater fish. However, they can tolerate warmer water than Brook Trout. Because they can tolerate higher water temperatures, they may be found with coolwater fish and in transition waters.

Temperature and Fish Communities

Coldwater Fishes

Group of fishes that thrives and reproduces in water temperatures less than 70 degrees. The preferred temperature range for these fishes is between 50 and 65 degrees.

Coldwater Fishes—Brook Trout, Slimy Sculpin, Rainbow Trout

Coldwater Transition Fishes—Brown Trout, Blacknose Dace, Longnose Dace

Coolwater Fishes

Group of fishes that thrives and reproduces in water temperatures less than 80 degrees but warmer than 60 degrees. The preferred temperature range for these fishes is between 65 and 70 degrees.

Coolwater Fishes—Northern Pike, Muskellunge, Fallfish, Creek Chub, Northern Hogsucker, Tessellated Darter, Yellow Perch

Coolwater Transition Fishes—Walleye, Smallmouth Bass, Chain Pickerel, Grass Pickerel, Rock Bass, Redbreast Sunfish, Margined Madtom

Warmwater Fishes

Group of fishes that thrives and reproduces in water temperatures warmer than 80 degrees. The preferred temperature range for these fishes is between 70 and 85 degrees.

Warmwater Fishes—Largemouth Bass, Bluegill, Common Shiner, Channel Catfish, Yellow Bullhead, Carp



Although made to work underwater, fish eyes have a lot of similarities to ours. Fish eyes and human eyes work like a camera and lens. They also have differences, such as no eyelid (fish do sleep, but they don’t have to close their eyes) or tear ducts (a fish’s eyes are always moisturized). Fish don’t have an iris like ours that contracts and expands to regulate light. To focus, the fish has a lens in its eye that moves back and forth.

In addition, fish that have to look for food or danger at or above the surface have to deal with a property of light called refraction. Light bends when it goes from air to denser water, or vice versa, so a mayfly at the surface or a fisherman on a bank appears higher than it really is. This idea is like seeing around a corner, which is why an angler should stay low when approaching fish in clear, shallow water. Generally, if you can see the fish, the fish can see you.

Fish have both binocular and monocular vision. With eyes on the sides of the head, they can see in nearly all directions, except for small blind spots directly in front and in back of them. Because most fish in flowing water face upstream, watching for incoming food, an angler approaching from downstream of the fish has less chance of being seen. Where the area of the fish eyes’ vision overlaps, a narrow cone of about 30 degrees in front and above its snout, the fish can see in three dimensions. This binocular vision helps it judge the distance to something like drifting food or a baited hook. But the fish’s monocular vision, when just one eye is looking more directly at an object, is best. Fish are near-sighted and see best close-up, which is why they may move up to an angler’s fly and look it over with just one eye.

Biologists believe freshwater fish see in color and can tell the difference between bright and dark. Fish are thought to have three sets of lightsensitive cells, called cones, in each eye, which can detect colors and even ultraviolet light (which we can’t). Fish eyes also have rods, cells that are especially good at detecting movement and seeing contrast. Some fish, like Walleyes, have a reflective layer in their eyes that helps them see better in the dark or in cloudy water. When fish take a black lure at night, they are seeing the solid lure’s outline against the lighter sky above.

As light penetrates ever-deeper water, it loses its color, red first. Yellow, white and bright lime-green stay visible in deep water and are popular lure colors for anglers after deep or bottom-dwelling fish.


Inexperienced boat anglers are often told to sit still and be quiet, because they’ll scare the fish. There is some truth to this, not just because the fidgeting gets on the other fisherman’s nerves. Sound waves travel in water, especially low-frequency waves, and fish do have ears. The ears are internal, one inner ear on each side of the fish’s brain. The inner ear receives and interprets sounds and helps the fish maintain its balance. Solid ear bones, called otoliths, are surrounded by a fluid-filled sac that is lined with tiny hairs. The sound waves penetrate the fish’s body and vibrate the otoliths, which stimulate the hairs and attached nerves and send the signal to the brain. The air bladder can also work as a resonating chamber in intensifying sounds.

Fish also “hear” with the lateral line. Most fish have a line of tiny pores, which are openings to tubes, running from the head to the tail along their sides. This is the lateral line. The tubes go through the scales to a large nerve. The fish receives sound waves or low-frequency vibrations, like an oar bumped against the side of a boat, along the length of its lateral line. Fish can tell with great accuracy where the “noise” or disturbance is coming from. The sense in the lateral line has also been called “faraway touch,” and helps schooling fish move together. The lateral line sense works best for the fish when the vibrations are coming to it from far away. Actual hearing with the ears takes over when the object sending the waves is close by and the sounds are of a higher frequency.


Fish live in a world of dissolved chemicals, scents and tastes from the rocks and aquatic vegetation, the stonefly nymphs and minnows, even a fisherman’s hip boots. The senses of smell and taste detect and interpret these chemicals. Smell is something like tasting at a distance. Fish have nostrils, called nares, on either side of the snout. Water flowing through the nares circulates to the fish’s olfactory, or smelling, organ. Nerves in that organ transmit signals that the brain reads. Some fish have extremely sensitive scenting abilities, detecting concentrations as low as one part in one trillion. Salmon are famous for the ability to scent out the stream in which they were born, from all the streams along a coastline, returning to just that one tributary to spawn.

Fish have taste buds, more than 10,000, not only in the mouth and on the tongue, but on the lips and deep inside the mouth. Catfish can even taste through the skin and barbels, because taste buds are also located there. Some fish feed mostly by sight, like Northern Pike. Others, like Carp and Catfish, rely heavily on scent and taste to find food. This is why strong-smelling baits work for these fish. Not coincidentally, Carp and Catfish find food in silty water without much visibility. Sight-feeding fish tend to live only in clear water. Fish have welldeveloped tastes for sourness, bitterness and salt, but many are insensitive to sweetness.


How adult fish produce a new generation is basically the same as in the other higher animals: The female produces eggs in her ovaries and the male produces sperm in the testes. The two combine to produce a new individual, containing genetic material from both parents. Many fish species are live-bearers. However, no live-bearing fish live in Pennsylvania waters.

Most fish use external fertilization, in which the eggs or sperm are released from the fish’s vent, near its anal fin, into the water. Eggs and sperm mix, and new fish life begins. The embryo grows larger as cells in the fertilized egg divide, and a tiny fish develops inside the small sphere. After a time–a shorter period in warm water, a longer time in cold water– the rubbery, thin egg “shell” splits and the baby fish emerges. Now called a “sac fry,” because its underbelly is swollen by a nourishing yolk sac, the young fish gradually uses up its food supply and then strikes out on its own. Eventually, if it is lucky, and good at getting food and keeping from being eaten itself, it will spawn another generation of its kind.

Life cycle of walleye

Before the adult male and female fish spawn, there is often preparation. In some species there is a journey, a spawning migration, to reach the proper water flow, depth, temperature and/or bottom structure, like gravel or weeds. Some migrations are short, the fish traveling just to the shallow riffle above the creek hole in which it spends its whole life. Other fish, like American Shad and Striped Bass, swim many miles through the ocean and upriver to reach their spawning grounds. Fish often engage in a courtship between potential spawning partners, swimming against each other, nipping each other, or engaging in other activity. In some fish, such as American Shad, one of the sexes reaches the spawning grounds before the other, and anglers might notice schools that are all males or all females. Many female fish release eggs during the spawning season with several males, which diversifies the gene combinations.

Exactly where and how fertilized eggs are distributed depends on the fish species. Some take no care at all of their eggs, except perhaps to release them over a suitable bottom for that type of fish, like rock rubble or sunken woody debris. Most fish eggs are sticky, so they adhere where they settle, giving them protection from washing away. Some fish simply release eggs and milt into the water column, allowing the fertilized eggs to drift downstream. These are called random spawners, and they usually produce great numbers of eggs, but suffer a high mortality among their young. Other fish expend much energy in constructing nests for their eggs. The nests can range from dishlike depressions that are cleared in the bottom by the male’s fins, to piles of gravel that the fish carries to the nest site, piece by piece, with its mouth. Eggs may also be placed on the underside of rocks or in holes in stream banks.

Depending on the species, the eggs and emerging young fish may or may not be guarded by a parent fish, usually the male. Some fish, like Largemouth Bass, Sunfish and Catfish, are vigorous at tending their nests and eggs, chasing away other fish and picking up and removing items that fall onto their nest areas. Many other fish, such as Trout and American Shad, once they have spawned, abandon the eggs to develop on their own.


It’s true that big fish eat little fish, and that little fish eat smaller fish in turn. But fish species also eat algae and other plants, insects, mollusks and crustaceans. They also consume organic bottom material and even eat birds and small mammals that happen to get into the water. Some fish, like Bullheads, are generalists, consuming a wide variety of foods. The feeding habits of other fish are specific, especially as adults, such as the fish-eating (and frog-eating) Muskellunge. Fish fit in many places on the food web, and as a group may consume just about everything edible that’s available in or on the water. The form of a fish’s body, especially its mouth, is suited for obtaining the foods it prefers. Some fish have toothless mouths that distend to form a sort of sucking tube, by which they can “vacuum” up their food. Some use their heavy snouts for turning over rocks to gobble up what’s underneath. Many have toothy jaws and catch other fish and larger aquatic life. Some strain microscopic animal and plant organisms, called plankton, from the water with hundreds of closely spaced gill rakers. Even fish that don’t seem to have teeth have toothlike structures in the throat, called pharyngeal teeth, which grind food against a horny pad.

Lampreys are classed as jawless fishes. They have an oral opening that is a circular sucking disk, with rasping teeth, for attaching to and feeding on a host fish’s body fluids.

What a fish eats may also change from when the fish is small and immature to when it becomes a full-sized adult. Many fish begin life as plankton-eaters. Then they switch to eating small aquatic insects and other underwater invertebrates and small fish. Then they progress to eating larger insects, crustaceans and fish.

Fish use many methods for getting a full stomach, actively hunting their food, chasing or stalking and attacking it, or lying in wait and striking at passing prey. They may hang about in the current waiting for food to flow to them, root it out of bottom debris, or graze on underwater plants. Some fish feed mostly by day, and are called diurnal feeders, like Bluegills. Others feed mainly at night. They are nocturnal feeders like Walleyes. But most fish actively feed in the lower light hours around dawn and near sunset, and that’s generally when fishing’s best.


What affects a fish’s behavior? What makes it do something other than just go about its daily round of eating and trying to keep from being eaten? The rhythm of the seasons is one of the biggest influences on both the fish and their watery home. Fish may change their location in their home water, whether stream, river, pond or lake, with the season. This movement is a response to the water’s warming and cooling, as the fish tries to stay in the temperature range that it is most suited to, and as it seeks out or follows food sources. So not only “structured” habitat influences where a fish is found. Thermal habitat also influences its location.

There may be winter and summer migrations, either to different places in the home water, such as off the mouth of tributary streams or near springs, or to different depths in the water column. In winter, the warmest waters are generally on the bottom. In summer, the coolest waters are there. In spring and fall, there is a mixing of these water layers as the temperature becomes more uniform, and fish that had been found deep, for instance, may be found shallow as well.

When the water becomes too warm or too cold, and a fish can’t get away, the fish’s activity level decreases. Water that’s too warm is more stressful on a fish’s body than water that’s too cold. In summer, some fish, like Brown Trout, switch to being mainly nocturnal, night-feeders, to avoid the effects of heat. They spend the daytime in deep water or they hide in shade under stream rocks and boulders. In spring, fall and winter, fish that would normally be active in early morning or toward evening when the water is warm, may be seen feeding during the sunlit middle of the day. They may also be in shallower water than usual, where they can take advantage of the sun’s warmth.

Fish change their behavior as the seasons roll around to years, and they become old enough to feel the spawning urge. That, too, makes them leave their usual haunts and travel, or it makes them become aggressive toward, or associate more with, their own kind. Spawning time may even mean a change in the type of water in which a fish lives. Some fish, like Salmon and Shad, begin their life in fresh water and travel to salt water when still quite young and small. They grow up and spend most of their lives in the ocean. These anadromous fish return as adults, on a migratory run, upstream to flowing fresh water to spawn, and become available to inland fishermen. Other fish, like Eels, are catadromous. They live as adults in inland freshwater streams and go to the ocean to reproduce.