Native Freshwater Fish

 

Print this worksheet then choose a fish to study in an aquarium.
Photo courtesy G. Schmida

Common Name:

Scientific Name:

Freshwater Environment
The freshwater environment of fish has the following characteristics to which they are adapted:

Viscosity
Water is between 50 and 100 times more viscous than air. This affects animal movement, and shape. Fish are generally streamlined to assist movement. They have a pointed front, bulky middle and tapered tail.

Why are some fish more streamlined than others?

Draw a general side view of a fishes body.

Fish have fins for movement. The largest fin is the tail or caudal fin for rapid forward movement. Dorsal fins on the top and anal fins underneath assist with lateral stability. Pectoral fins behind the gill covers (operculum) assist with hovering and slow turning. Pelvic fins are often small for open water swimmers but larger on bottom dwellers which use them for resting on.

Add fins to your sketch of the body.

Vertebrae are hinged to allow movement of the backbone, mainly in the horizontal plane. Fish also have a coating of mucous which has less friction with the water and their protective armour, scales allow flexibility of the body so they can swim.

Density
Water is about 770 - 1 000 times denser than air so the body of a fish is partly supported externally by the up thrust of water. The shape and structure of a fish is supported internally by its skeleton. - backbone, ribs, head and fins.  The gills rely on support from water for efficient functioning. If a fish is taken out of water it will die from lack of oxygen. The gills are made of many fine parts which collapse together if not supported by water.

externally:

Gases.
Water contains dissolved gases, mainly oxygen and carbon dioxide. Fish require oxygen for respiration and produce carbon dioxide as a waste gas. Gills are structured very similarly to lungs, however, they are much more efficient. A typical human lung may extract up to 25% of the oxygen from the air however a fish will extract 80% of the air from water. It is also a lot more difficult to obtain oxygen from water. It has only one thirtieth the amount of oxygen as air and oxygen diffuses 300 000 more slowly through water than in air.

* The surface area of gills is much increased by many filaments which are lined with blood vessels close to the surface.

* Water only goes in one direction and its movement is continuous. It enters through the mouth, goes over the gill filaments and out the gill covers.

* Gills operate on the principle of a "counter current".

What would happen to its breathing if the fish were very active or stationary?

Osmosis
Water molecules will diffuse through a semi permeable membrane such as the wall of a fish cell from the dilute to the more highly concentrated side of the membrane (osmosis). Water tends to move by osmosis into the cells of freshwater fish. This makes it in danger of taking in too much water and swelling. It has no need to drink and what water does enter the body through the skin and gills goes to the kidneys and is used to carry away waste products in large amounts of dilute urine. In saltwater the water diffuses from the cells into the sea so fish do need to drink water.

How do freshwater fish overcome the problem of too much water entering their body?

Temperature.
Fish are cold blooded, they are ectothermic meaning their body temperature depends on water temperature as well as the level of their metabolic activity.

What will happen to the activity level and food intake of fish during winter.

Sense Organs
Fish have a number of sense organs including:

• lateral line running the length of the body for detecting vibrations
• touch senses beneath the scales for detecting vibrations
• nostrils for chemosensing
• taste organs in the mouth and body cover for sensing odours
• eyes for visual sensing
  Add the sense organs to your basic fish diagram.
  

Feeding.
Most fish are carnivorous. The size and position of the mouth indicates food size and mode of feeding. Bottom feeders usually have down positioned mouths, surface feeders have upward positioned and open water feeders have symmetrical mouths.

Look at a fish in a tank and determine where it normally feeds. Draw its mouth on the diagram.