Dr. Tanguay suspects something in the stream water is harming the fish. She collects water from the stream and takes it back to her lab. She hypothesizes that one of two things may be happening:

  1. Something in the water is directly harming the adult fish, causing their numbers to decrease.
  2. Something in the water is harming the baby fish as they develop into adult fish. This would also reduce the number of adult fish.

Dr. Tanguay starts by doing a simple experiment to guide further investigation.

To set up the experiment, Dr. Tanguay sets up four fish tanks:

  1. Stream water + adult fish
  2. Clean water + adult fish
  3. Stream water + newly fertilized fish eggs
  4. Clean water + newly fertilized fish eggs

Zebrafish eggs,
two hours after fertilization

Adult zebrafish

Tanks 1 and 3 are the experimental treatments. Tanks 2 and 4 are the control treatments. Dr. Tanguay knows the water in Tanks 2 and 4 is free of any contaminant that could harm fish. She will compare the fish or eggs in these control treatments to the fish and eggs in the experimental treatments. If they turn out different, She will know the problem is something in the water. It is necessary to use clean water that is known to be free of contaminants to compare the effects of the stream water.

For the experiment, Dr. Tanguay uses zebrafish, a small black and white striped fish native to India. Zebrafish live in warm freshwater streams. They are often found in pet stores in the United States.

Adult zebrafish

Why Use Zebrafish?

Zebrafish are often used in lab experiments for several reasons:

Transparent developing
zebrafish embryo
  1. Zebrafish reproduce very quickly and their embryos develop into young fish within days. Scientists can see results of their experiments fast.
  2. Like most fish, zebrafish eggs develop outside the mother’s body. Scientists can watch the embryos develop without disturbing them.
  3. Zebrafish embryos have clear bodies. Scientists can watch the fish as they develop and see the organs inside their bodies.
  4. Zebrafish biology is very well understood. Scientists have studied zebrafish for a very long time. (photo: transparent zebrafish embryo)

What Happened to the Fish?