Though the University isnt sure how they got there, USF Physical Plant employees pulled at least 12 dead fish out of Castor Pond on Monday.

Karla Willman, assistant director of communications for administrative services, said the man-made pond lost the unnatural inhabitants to natural causes.

Physical Plant does not stock ponds, whether theyre natural lake ponds or retention ponds, Karla Willman, assistant director of communications for Administrative Services, said. We have no idea how or when the fish were introduced into Castor Pond.

Willman said the sighting was reported to the USF Division of Environmental Health and Safety, who consulted with the U.S. Fish and Wildlife Service.

Based on our observations, they determined that the fish kill occurred due to low dissolved oxygen within the pond, which was a natural phenomenon, she said. However, if there is another occurrence within the next few days, we will need to contact them again.

Willman said no chemicals were added to the pond, which is lined and was built by the University.

It serves as a retention pond and adds aesthetic value, she said. Besides the rainfall, the pond receives runoffs from the parking lots and roof drains. The design allows water to percolate into the ground when the water level increases above the lining height.

David Lewis, an integrative biology professor, said though fish can sometimes be introduced into an environment without trouble, it often leads to problems for the species.

Its not uncommon for these introductions to happen through people trying to develop a population that they can later go fishing for or people just getting tired of their aquarium pets and dumping them in the first lake they find because they feel guilty about flushing them down the toilet, he said. (With) the consequences of introducing a species to an ecosystem, almost anything can happen.

Though a fountain in the pond oxygenates the water, Lewis said oxygen scarcity can be caused by a multitude of factors.

One of the primary ways oxygen is depleted through lake water is through something called biological oxygen demand, he said. What that is, is that if theres a lot of organic matter on the bottom of the lake or suspended in the water, microorganisms will basically feed on that stuff as an energy source, the same way (humans) would get energy from sandwiches. That is decomposition, the same way a gardeners compost pile shrinks over time. The process of decomposition burns up oxygen dissolved in the water.

Lewis said the organic matter is often a result of nutrient runoff from fertilizer used in nearby vicinities.

A lot of it gets washed away into runoffs during rainstorms, he said. The organic matter (that decomposes) comes from lots and lots of algae dying. If fertilizer makes plants grow on land, it makes it grow in water. It makes algae grow. Eventually, that algae dies. Without those nutrients thered be less algae, so thered be less dead algae, so thered be less organic matter, so thered be less decomposition, so thered be more oxygen. The conditions in the lake or pond are very much linked to the activities on the landscape around the pond.