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Low Dissolved Oxygen and Fish Kill Alert for Sanibel Lakes

June 23, 2023
dead fish in Sanibel Lake

The storm surge from Hurricane Ian drastically changed the characteristics of most freshwater lakes and ponds on Sanibel.

“As a result, current conditions add up to an increased potential for fish kills in Sanibel lakes this summer, especially following cloudy days,” said SCCF Marine Lab Research Associate Mark Thompson.

Here’s why:

  • Fresh water was immediately displaced by the storm surge and saltwater from the Gulf of Mexico took its place.
  • Freshwater fish that were resident in those lakes and ponds died immediately or over the next few months.
  • The storm surge also introduced estuarine and marine fishes within some lakes and ponds. The marine fish are accustomed to tidal exchange and water with high levels of dissolved oxygen.
  • Deeper, larger lakes on Sanibel, such as those in The Dunes, East and West Rocks, Lake Murex, The Sanctuary, and Beachview Estates, will be very slow to change back to freshwater – possibly taking years.
  • Most of these salty lakes are now experiencing blooms of marine and estuarine microorganisms, known as phytoplankton or algae.
  • The phytoplankton produce oxygen during sunny days due to photosynthesis and add oxygen into the water.
  • During the nighttime, phytoplankton use oxygen out of the water, and oxygen levels drop.
  • Cloudy days, like the weekend of June 16-18, disrupt the ability of phytoplankton to photosynthesize, and they are not able to produce oxygen and add it to water. However, they continue to use oxygen from the water to respire along with fish and other lake inhabitants.
  • Low levels of dissolved oxygen in Sanibel lakes result from the combination of the above events.
  • Marine fish are not adapted to low dissolved oxygen levels and die during the low oxygen events brought on by cloudy days.
Fish kill at Lake Murex on June 20, 2023.

In summary, there is an increased potential for fish kills during cloudy days in our lakes post-Ian.

“Over the course of years, these lakes which are now marine waterbodies will become more fresh and marine fish like mullet, snook, tarpon will either adapt while others such as shad, herring, redfish, porgies will die off,” said Thompson. “Meanwhile, freshwater fish such as non-native cichlids, bass, and sunfish will begin to reappear.”


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