What If All The Fish Die?

The Trout-in-the-Classroom program is a learning experience about watersheds and raising awareness of California fisheries. It is important to understand that sometimes, no matter how much care is taken, none of the fish survive. The program is not intended to be a stocking program. Also, know that Salmonids in the wild face a difficult lifecycle and they naturally have low survival rates. For every 2,500 eggs laid, only two will survive to become spawning adults. In the classroom/aquarium environment, we usually expect survival rates to be higher than they would in the wild, but it is common to lose a few fish at each lifecycle stage. Eggs, alevin or fry may not survive for a variety of reasons, including genetics, environmental factors and general defectives.

What we do know is that Salmonids must have cold, clean, highly oxygenated water to survive. Common reasons for sudden fish loss include:

  • Power outages (depleting oxygen levels and increasing water temperature)
  • Unplugged or broken chiller (increasing water temperature)
  • Unplugged or broken pump/power-head (depleting oxygen levels)
  • Introduction of outside chemicals (contaminating the water)
  • Changes in water quality (possibly decaying fish that have not been removed or overfeeding)
  • Improperly assembled equipment (fish get drawn into pump or burrow under the under-gravel filter)

The exact reason for fish die-off cannot always be pinpointed. Use the experience as a teaching tool, a mystery the kids can learn valuable lessons from trying to solve.


Could it have been temperature?

Questions for Students: What could have caused the temperature in the tank to rise or fall out of the salmonids preferred range?

Salmonids requirement: Salmonids prefer water temperatures between 45 – 55 degrees F. While they are tolerant of slightly warmer or colder temperatures, a sudden change in water temperature can shock or kill the fish.

In the Classroom: A power outage or unplugged unit may have caused the chiller or incubation unit to turn off for an unknown period of time, thus causing the water temperature to rise.

In Nature: Talk about what things in nature could cause river or lake water to fall outside of the desired temperature range. Common factors are loss of shade trees, low water level during drought, lack of snowmelt or thermal pollution from nearby industry.


Could it have been low oxygen?

Questions for Students: How do low dissolved oxygen levels affect fish? What might have caused a change in oxygen levels in the tank?

Salmonids requirement: Like all living things, salmonids require oxygen to survive. Dissolved oxygen (DO) in the water allows fish to breathe, and adequate oxygen flow is necessary for the development of fish, eggs and alevins require an even higher level of DO than fry.

In the Classroom: A faulty powerhead or interrupted electrical power can cause the dissolved oxygen levels in the water to become very low. An insufficient flow of oxygenated water to eggs and alevin during early life cycle stages can cause delayed or stalled development. Also, some powerheads may be too small and unable to meet the needs of the tank size. Furthermore, warmer water holds less DO, so tank temperature can affect DO levels.

In Nature: Silt and sedimentation of water through erosion causes higher turbidity (cloudiness), which warms water by increasing the absorption of sunlight and lowers DO levels. Loss of shade and factors listed above that would increase water temperature would decrease DO. Increased levels of phosphorus or nitrogen cause algae and plankton growth; decomposition of dead algae and plankton utilize oxygen and decrease DO levels. Forest fires or use of fertilizers in a watershed can also cause changes in DO.


Could it have been a pollutant or poor water quality?

Questions for Students: Was the die-off sudden, or did it happen gradually over time? What could cause sudden changes in water quality? Could the water have been contaminated by a chemical? How is the closed system of a classroom aquarium different than the river system?

Salmonids requirement: Salmonids require clean water with a close to neutral pH and are sensitive to changes water chemistry.

In the Classroom: Removing dead eggs, alevin and fry, regularly cleaning pea-gravel and changing water prevent the buildup of wastes and toxins in the tank. The presence of dead or waste materials may lead to increased nitrogen levels or decreased pH, causing gradual die off. Overfeeding fish leads to excess food that is not consumed, causing changes in water quality. The introduction of outside chemicals (intentionally or by accident) causes rapid change in the water quality, which can shock or kill the fish.

In Nature: Pollutants from roads, developments and landscapes are transported to waterways by runoff. Pavement and other impervious surfaces collect oils, metal compounds, and chemicals that run into streams during rainfall. Agriculture and landscape maintenance practices use pesticides and herbicides that contaminate runoff and soak into groundwater. Pollutants and chemicals can affect salmonids directly, causing health problems and developmental impairment, or indirectly, by depleting native insect populations and altering the ecosystem.


Using Games as a Teaching Tool

Using natural survival games and activities to help illustrate the difficulties of the Salmonid life cycle is a good way to address high mortality rates. The Project WILD Aquatic activity ‘Hooks & Ladders’ is a good activity that illustrates natural survival in a fun and engaging manner.


In Conclusion

Losing all your fish is an unfortunate occurrence, but as shown above it doesn’t have to be the end of your teaching experience. You can still take your field trip and engage in other lessons. If near one, you could visit a hatchery. Just encourage the students to apply the knowledge they learn to their everyday life and the world at large. Things like conserving water, cleaning up litter, and keeping pollution out of our storm water systems (gutters, roads and drains) help all the living organisms in our watershed.


This article was written by Molly Schnur, CDFW

About Ethan Rotman

Ethan Rotman (Bayareatic) manages and coordinates programs that hatch fish in classrooms for the San Francisco Bay Area. He also chairs the committee that manages these programs throughout the state. Ethan has worked with the California Department of Fish and Wildlife for over 20 years.
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