Trout Dissection

designed for the NYC and NYC Watersheds Trout in the Classroom program

Here is a thorough instructional video on Trout Dissection.

Objective:
To give the students a chance to get a first hand look at the inside of a trout and how their bodies work.

Background: 
    Trout exist in abundance in the streams throughout New York City's three watersheds.  The Catskill, Delaware, and Croton, which supply New York City with it's drinking water, are healthy and clean sources of water.  Because trout are indicator species, scientists measure their populations and health to determine the health of a stream.

Materials:
A whole fresh or frozen (thawed) trout, Dissecting tools (scissors, tweezers or forceps, probe, magnifying glass), Newspapers and paper towels, Trout Anatomy diagram/ reference sheet to explain the function of each organ or characteristic.
 
Procedure:

  1. Choose a location where students can view the dissection. Cover the surface with newspaper.
  2. Study the external anatomy.  Have students feel the fish’s skin and discuss what purpose the slime serves. (It protects against growth of fungus and it allows the fish to glide more easily through the water.) Have students use the magnifying glass to examine the scales and note how they are arranged. Have students describe the color patterns and discuss function of coloration (camouflage).  Observed the lateral line. Discuss what it is used for and the way it works. Have students describe the fish’s overall shape and how this adaptation is beneficial to the fish. Look at the placement of the fins and ask students to imagine the fish swimming in the water. How does it move? How are the fins used. Note the range of movement of each fin. 
  3. Allow students to feel the bony rays that support the fins. Have a volunteer count the number of rays on the anal fin. This is one of the most distinguishing characteristics of salmonids. 
  4. Notice the relatively large size of the eyes along with the large pupil tells us how important vision is for this animal. Have students note that there is no eyelid. Have them observe the tough, clear membrane that covers the eye. Rotate the eye in the socket with a finger. 
  5. Locate the nostrils. Describe the large olfactory lobes that are located in the brain. Ask students to speculate why a salmonoids smell receptors are so highly developed. 
  6. Open the mouth, note the color of the gums, feel the teeth along the gum margins and on the roof of the mouth. What function these teeth perform? (The teeth are used for grasping and holding onto prey.) Note how wide the mouth opens and comment on why this is.  Point out that the mouth is also used for breathing. In low oxygen conditions, fish actively pump water over gills by opening and closing the mouth. Demonstrate this action with the fish’s mouth. 
  7. Point out the gills arches by having students look down the fish’s mouth. Use a probe to separate the arches and explore how they are arranged. 
  8. Place the fish on its side and look at the operculum—the bony plates which protect the gills. Lift the operculum to see the gills. Cut the operculum away from its base, exposing the gills. 
  9. Remove the gills by cutting the upper and lower attachments of the arch. Look at the gill rakers, the bony projections along the inside curve of the arches. Observe the large surface area provided by the gill filaments, and the thin tissue which allows blood vessels to come into contact with the oxygen in the water. Compare and contrast gills and lungs. 
  10. Carefully cut the fish open using the scissors or scalpel. Let students observe how all the internal organs fit together. Look for the thin transparent membrane that encloses the organs. Cut away the flap of skin and look for fat deposits, which are found around the stomach.
  11. Look for the swim bladder. It is made of very thin tissue and is located in the upper body cavity, below the kidneys. It will be less developed in small fish and will not be inflated.   It may be hard to find. If you can’t find it, point to its location and discuss its function.
  12. The male reproductive organs will be a flaccid white or orange tissue near the intestines. Eggs may or may not be noticeable in females and vary in size depending the fishes maturity. 
  13. Put the fish on its back to find the kidney, located under the backbone. They are thin, dark in color and run the length of the body cavity. Ask a volunteer to discuss kidney functions.
  14. Investigate the digestive track by starting in the mouth and following the route that food would take. Put the probe through the mouth and esophagus to show the route. Follow the course of the stomach using a finger or probe. The first area of the stomach is called the cardiac stomach (where digestion begins.) Notice the different kinds of tissue that make up the stomach.
  15. The pyloric stomach (portion from which the pyloric ceca project) is made of different tissue and begins at bend below the cardiac stomach.  Stomach area is increased by the pyloric ceca, how does this improve the function of the stomach.
  16. The intestines extract nutrients from food. A network of blood vessels are used for nutrient exchange. Follow the intestines to the anal opening, where waste products are eliminated. 
  17. Lift the stomach to show/discuss the spleen (reddish organ near end of the cardiac stomach.)
  18. The liver is in front of the stomach. Discuss it's role in the digestion of fats. Point out the gall bladder, a mass of darker tissue on the liver.
  19. Locate the heart, found near the fish’s mouth. Make out the different chambers. The gills, heart, and liver close together for a reason. Blood pressure is best near the heart (pump). Blood is filtered by the liver, and absorbs oxygen from the gills; both are vital functions. 
  20. Cut through the fish to expose the backbone, muscles and muscle mass (this is the part we eat.) 
  21. Carefully cut away the skin by lifting it while running the scalpel along the skin-muscle interface. The skin may be thin enough to place under a microscope or magnifying glass to observe the pattern of the scales and the growth rings. Remove scales to look at the rings. 
  22. Compare trout anatomy to a mammal's.

Wrap Up:
    Distribute the Trout Anatomy diagram to students. Have them label the organs and include the diagram in their Trout Journals. See if they can recognize any of the organs as ones that we also have in our bodies.

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