II. Freshwater Fish General Characteristics

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II. Freshwater Fish

  1. General Characteristics

Fish are cold-blooded animals, meaning their body temperature changes with the temperature of their surroundings. They have backbones and live their entire lives in water, breathing with gills and moving with fins. There are 247 species of freshwater fish in Texas alone. Some of these species are able to live in brackish (partly salty) water, such as in estuaries. All fish play an important role in the aquatic ecosystem.
The three main groups of fish are

• jawless fish (lampreys),

• cartilaginous fishes (sharks and rays),

• bony fishes (most fish species).

The first two groups are primitive, ancient fishes, whereas the bony fishes are more recent, advanced fishes.
Fish extract dissolved oxygen from the water using their gills. In breathing, fish first gulp a mouthful of water, then close their mouths, and pressurize the water, forcing it over the rich red blood supply of gills and out the opercula (gill-flaps). Oxygen is absorbed directly into the fish’s blood supply and distributed throughout the body via the circulatory system.
Some fishes have an internal, inflatable air (swim) bladder that evolved as an outgrowth of the intestine. The air bladder can be inflated or deflated to regulate buoyancy and depth. Some fish use their air bladders to amplify underwater sound, and thereby increase their ability to hear. Gar and lungfishes use their air bladders to gulp and breathe atmospheric oxygen. They can survive in low-oxygen water for long periods.
Nearly all fish have protective slimy mucus covering their skin. This outer coating reduces friction and increases swimming speed. It also protects fish from parasites and diseases, and permits salt balance (osmoregulation). Removing the mucus layer by netting and handling fish can increase their susceptibility to disease and disrupt their salt balance.

  1. Fish Habitat

Each species of fish has its own distribution and range of tolerance in which it can survive or thrive. The basic requirements for a fish’s survival include prey, cover, suitable water temperature, and dissolved oxygen levels.
Species such as common carp and channel catfish can survive in a variety of habitats and are found throughout the state. Others, like sunfish, prefer calm, quieter waters, while darters prefer very swift flowing streams with riffles. Some species have a very limited geographical distribution in the state. For example, the Devils River minnow is found in only a few streams in southwest Texas. Other species have specific water quality criteria for survival, such as the rainbow trout (nonnative species), which is restricted to the cold water of the Guadalupe River below Canyon Lake dam.

How they Move:

The source of propulsion for virtually all fish comes from:

1. Undulation of the body

2. Paired Fins: (2 of them)



3. Unpaired Fins: (only 1)




4. A combination of the above
Fish use their fins to stop, start, steer, turn, swim backward and forward, chase and catch food, and migrate. The dorsal, caudal, and anal fins are median, unpaired fins. The pelvic and pectoral fins are paired.


Each fin on a fish is designed to perform a specific function.

    • Dorsal fin. Lends stability in swimming.

    • Ventral fin. Serves to provide stability in swimming.

    • Caudal fin. In most fish, the Caudal or tail fin is the main propelling fin.

    • Anal fin. Also lends stability in swimming.

    • Pectoral fins. Locomotion and side to side movement.

    • Adipose fin. Stability.

Fin shape and location are important for swimming and maneuvering. Fast swimming fish (tuna and swordfish) have long pointed and crescent-shaped fins that fold into body slots to reduce drag. Flying fish have long, broad wing-shaped fins that allow them to jump and glide long distances above the surface. Puffer and box fish have small, rapidly-beating fins for fine maneuvering

Crescent-shaped: Fish with crescent-shaped tails are fast swimmers and constantly on the move.

Forked: Fish with forked tails are also fast swimmers, though they may not swim fast all of the time.  The deeper the fork, the faster the fish can swim.

Rounded: Fish with a rounded or flattened tail are generally slow moving, but are capable of short, accurate bursts of speed.caudal

C. Feeding Groups

Fish feed at all levels of the food chain, from bottom scavengers to top-level predators. Fish can be categorized into groups based on food preferences: piscivores, invertivores, omnivores, and herbivores. The groups are referred to as trophic guilds

Fish in each of the trophic guilds have morphological (form and structure of an organism) adaptations to allow more efficient feeding. For example, piscivores generally have large eyes for better sight and a well-developed lateral line to detect the vibrations of prey in the water. Omnivores and invertivores feeding on the bottom usually have fleshy lips (suckers) or taste buds on their barbels (catfish) to aid in detecting prey. Herbivores use pharyngeal teeth (a modified gill arch in the throat) for crushing plant material (minnows).

flattened bottom surface

sitting on the bottom

bullet or torpedo shaped

for fast swimming

thin, flat, vertical disc shape

feeds above or below

thin, flat, horizontal disc shape

bottom feeding


stable in moving waters

Body Shape


large mouth

surrounds prey

little mouth

nibbling on small plants/animals

mouth near top

eating near surface or on prey above

mouth in middle

eating directly ahead

mouth on bottom

eating off the bottom

Pollution Tolerance

Fish can be used to evaluate the water quality and health of an aquatic ecosystem. As part of this evaluation fish are placed into categories based on their tolerance to pollution.

The three general categories are intolerant, tolerant, and intermediate. Freshwater fish in Texas have been classified into pollution tolerance groups
Many factors contribute to the loss of fish species and the degradation of their habitat. These include:

• dams and impoundments;

water pollution, especially spills of toxic wastes (i.e., oil and petroleum products, industrial acids, pesticides, and fertilizers);

sedimentation from agriculture, construction, and logging and mining;

• introduction of exotic species; and

• overfishing.

Dams block fish spawning migrations and isolate fish from upstream spawning and nursery areas, causing populations of anadromous and catadromous fish to decline. As streams and rivers are transformed into lakes and reservoirs, alterations in downstream water flows and water temperatures, negatively impact fish communities. River fish that have evolved and adapted to inhabit free-flowing rivers may not survive in lakes and reservoirs. Water pollution threatens fish. Heated water (thermal discharge), low dissolved-oxygen levels, toxic chemicals (gasoline and oil), and coal-mine acids impact water quality and fish. Fish may temporarily avoid water pollution by swimming into small, clean tributary streams. However, they cannot live continuously in a polluted stream.

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