What Happened to That Fish?


Via Mr.TinDC on Flickr http://bit.ly/2lZoFGc

Lake and pond habitats are very complex habitats. They are influenced by soil conditions in the watershed, water chemistry and temperature changes from rainfall and resulting runoff, heating from the sun and cooling from groundwater inputs. No two bodies of water are the same, but a common question we get from visitors in the summer is, “What happened to that/those fish?”

Here are the top five things that contribute to summertime stress to fish populations; any one or a combination of several can lead to die-offs.

  1. Not Enough Oxygen: Low oxygen is the number one killer of fish in the summer. One of the physical properties of water is that warmer water doesn’t hold as much dissolved oxygen as cooler water.
  2. Daily Cycle of Photosynthesis: Oxygen comes from photosynthesis of algae and rooted plants below the surface. Plants produce much more oxygen than is consumed during the day, but respiration and decay of plants and animals consumes oxygen when it’s not being produced at night. Photosynthesis can’t occur when the sun isn’t shining, so an oxygen crisis is most likely to occur at night.
  3. Catching and Releasing: Just like in any animal population, there are always a certain number of fish that battle pathogens. These normally don’t cause large die-offs, but occasionally it happens. Sometimes stress from handling can be a root cause. Over-handling of fish, which rubs off their protective slime coat, can cause infections that don’t cause mortality until days later…they may swim away after being caught, but days later turn belly-up. Handle gently and release quickly.
  4. Yard Clippings: Any decay of organic matter consumes oxygen. Common problems are runoff that is carrying manure, mowing grass with the discharge of grass clippings going into a pond or even runoff from a mulch pile.
  5. Cold and Warm Water Don’t Mix: Nearly every pond or lake in our region stratifies in the summer, which means it has layers of water of varying temperature that don’t mix easily. Cooler water is more dense, so it stays at the bottom, while warmer water is less dense and is heated by the sun to make the temperature and density difference even more significant. A cold rain on the warm water at the surface can sometimes cause a summer lake turnover which can bring unoxygenated water from the bottom into the top layers. Bottom waters sometimes have an oxygen “debt,” in the form of decaying plant matter. It’s like having below-zero levels of oxygen, because organic matter consumes available oxygen during mixing which was previously limited to the upper layer of water. This normally happens over the course of days or weeks as temperatures cool in the fall, but a fast summer turnover can cause suffocation of fish.

Sometimes several of these items occur simultaneously. Imagine how many times we experience this in the summer: hot weather, warm water temperatures at the surface, combined with a heavy night-time thunderstorm which brings could possibly wash organic matter into a lake.

Each circumstance is different, but you can see how so many cultural, physical, chemical and biological processes influence lake and pond habitat.

Bret Henninger
Chief of Conservation and Parks