Teaching Tips

Teaching Tips:The Fish Experiment

Synopsis:
In this module students experience a simulation consisting of a goldfish in an aquarium to study the respiration rate of the fish. Students adjust the temperature and presence of algae, while measuring the dissolved oxygen concentration and respiration rate of a goldfish.

Big Ideas:
a. Fish use the process of respiration to uptake oxygen.
b. Gill movement is a measure of respiration rate.
c. The solubility of a gas in a liquid is affected by temperature.
d. As water temperature increases, the DO concentration decreases.
e. As the amount of algae in water increases, the DO concentration decreases.
f. As the DO concentration increases, respiration rate decreases.
g. As the DO concentration decreases, respiration rate increases.

Misconceptions Addressed:
a. Fish breathe oxygen by gulping air.
b. All water contains the same amount of DO.
c. Fish expend the same amount of energy extracting oxygen from water as animals do from air.

NSES Grades 5 – 8 Content Standards Addressed:
a. Life Sciences
i. Structure and function in living systems
ii. Regulation and behavior
b. Science in Personal and Social Perspectives
i. Natural hazards
ii. Risks and benefits

Thinking Like Scientists

When students use PSI Sims™ to investigate how changing certain variables affect an environmental scenario, they are immersed in the scenario. Often, their curiosity is stimulated and they become interested in other questions. In the Fish Experiment, students are cued to notice gill movement, and they may wonder how the gills work, or how fish get oxygen. These questions launch the students on a parallel inquiry, and they begin thinking like scientists. The answers to their questions may be in their textbooks, they may be able to research it on their own, or you may wish to provide outside material for them. An interesting article found on the internet called "Fish Respiration: build a counter current exchange model" explains that:

Fishes use two different methods for keeping a continuous supply of new water available, one is very simple and the other complex.

--Ram Ventilation: Swim through the water and open your mouth (such as a shark would). Very simple, but the fish must swim continuously in order to breathe, not so simple.

--Normal Ventilation: Occurs by the fish taking in water through the mouth. The mouth closes, forcing the water back over the gill filaments and out through the gill slits.

The same web page also shows detailed drawings of how the gills work.

A Teachable Moment

Teachers can often motivate students by using counter-intuitive discussion concepts. In the Fish Experiment, students learn that lowering the temperature increases the amount of Dissolved Oxygen (DO) in the water. They also notice that adding algae to the water lowers the DO in the water. They have learned that manipulating variables changes outcomes in the system. Great. However, their new found knowledge allows them to be easily tricked, and sets up a teachable moment. After the students have gone through the Fish Experiment, ask them this question:

In a 24 hour period, when would you predict that there would be the most Dissolved Oxygen present in an outdoor pond in the middle of summer? Since they will probably deduce that there is more algae generated in the sunlight, and the temperature is higher in the day, both of which have a lowering effect on DO, they are likely to answer "in the middle of the night."

However, they would be forgetting that the sunlight also triggers photosynthesis and affects the "diurnal oxygen cycle" as shown in this graph, found on the University of Florida IFAS extension web page entitled "Dissolved Oxygen for Fish Production":

Oxygen, derived from photosynthesis, is produced during the day when sunlight shines on the plants in the water. Oxygen levels drop at night because of respiration by plants and animals, including fish. These predictable changes in DO that occur every 24 hours are called the diurnal oxygen cycle ( Figure 1 ).

Figure 1. Dissolved oxygen concentration in ponds fluctuates on a 24-hour basis. This fluctuation is called a diurnal oxygen cycle. Dissolved oxygen increases during daylight hours when photosynthesis is occurring and decreases at night when respiration continues but photosynthesis does not.

It is important that students remember that they are engaging in "controlled experiments" that don't account for all variables. In the Fish Experiment PSI Sim™, for example, they are not concerned with light. They mustn't generalize their results to situations that have additional variables.

Topic of Conversation

Students have probably heard of Stephen King's Dead Zone, but may not have heard of real Dead Zones. Real Dead Zones are bodies of water that are so oxygen deprived that very little life can survive. In the Gulf of Mexico a huge Dead Zone is increasing in size. Every summer, a huge area (over 21,000 square miles) hugs the coast of Louisiana and Texas.

Area of the Gulf of Mexico
Source: NASA

The Chesapeake Bay, and nearly 150 other areas of water worldwide become too low in dissolved oxygen to sustain life. This puzzling phenomenon presents a good research topic for students. You can find more information on the Grinning Planet site.

Fun Facts

The biggest fish in the world are: the whale shark at 50,000 pounds, the basking shark at 32,000 pounds, the great white shark at 7,000 pounds, the Greenland shark at 2,250 pounds, and the tiger shark at 2,070 pounds.

The fastest of all fish in the sea is the swordfish, streaming forward at speeds near 68 miles per hour.

The flying fish builds up speed in the water then leaps into the air to escape predators. Once in the air, it can stay airborne for up to 325 feet (100 meters).

The blue fin tuna swims with its mouth partly open, relying on ramjet ventilation, unlike slower fish, which force water through their gills to remove oxygen and release carbon dioxide. Since seawater contains only about 2.5 percent as much oxygen it needs from the volume of water flowing through its mouth, the blue fin has proportionately one of the largest gill areas of any fish.

There are more species of fish than mammals, reptiles and birds combined.