How Do Hurricanes Kill Fish?

Pictures in the various news media make it clear that these immense storms can devastate the landmasses in their path, but it may be difficult to see how hurricanes and tropical storms could affect fish. Here are some reasons you may see fish kills following a major storm event:

Changes in salinities
Freshwater flooding from rain or saline storm surge may trap fish in an inappropriate salinity. If this happens rapidly and the fish have no escape, species that are intolerant to changes in salinity may die.

Low dissolved oxygen
Low dissolved oxygen is by far the most common cause of post-storm fish kills. When oxygen levels get too low, fish are unable to obtain the required amount of oxygen necessary for metabolism. Several factors may occur in concert to cause this condition:

• Wind-In small lakes or ponds, wind action may push surface waters to one side of the lake. Water from the bottom comes to the surface to fill the space the surface water used to occupy, bringing with it sediments and organic materials from the bottom. This water from the bottom is naturally low in oxygen. The bottom materials may include hydrogen sulfide; in high enough concentrations, hydrogen sulfide can be lethal to fish and is responsible for any "rotten egg" or "sewage" odors. Bacteria in the sediments are also brought to the surface; these bacteria decompose the organic material from the bottom, using up oxygen in the process. This whole event is termed a "turnover," since literally, the bottom comes to the top.
  
  
• Long periods of cloudy days-In aquatic ecosystems, the oxygen manufacturing system consists of microscopic organisms and aquatic plants that carry out photosynthesis: using energy from sunlight to create carbon-based nutrition for themselves with oxygen as a by-product. When there are long periods of cloudy days, these organisms produce less oxygen. At night, photosynthesis doesn't occur at all, and these same oxygen-producers are actually using up oxygen during respiration, just like fish and other animals. Under these conditions, it does not take long before there is little oxygen left for fish. Low-dissolved-oxygen fish kills often occur early in the morning, when oxygen levels are at their lowest.

Strandings due to flooding
Rising water may flood areas that normally do not contain water. After water levels recede, fish can become trapped if they are cut off from the connection to the main body of water. When the small ponds the fish have been trapped in dry up, the fish die.