Fisheries Management

Have you ever walked along your pond or shoreline on one of spring’s first sunny days or the morning after a particularly strong storm, only to notice scores of dead fish in your pond floating or washed up? Not a pretty sight – or smell. Furthermore, it can be alarming to residents or patrons, leading to frantic phone calls, social media comments, and other headaches that distract from your other important duties. 

This instance of many fish dying in a lake or pond is known as a “fish kill.” While it is unfortunately often too late to stop the progression of a fish kill once you start observing dead fish, there is plenty that you can do to prevent them from occurring in the future.

Fish kills often occur in the summer due to low levels of DO, and it should be noted that, in general, warm waters hold less DO than cold. Therefore, fish kills in the summer usually occur alongside the following variables:

• Weeds and algae utilize too much DO overnight when they can’t photosynthesize
• Aquatic weeds or a large algae bloom dying and decaying after being treated
• Oxygen-deficient water from the bottom of a waterbody being pulled upwards due to strong winds or thunderstorms
• Species of fish that spawn in the late spring or early summer, crowding out habitat and causing competition for oxygen
• Run-off from livestock, fertilizers, and other organics after heavy rains

Another common cause of fish kills is toxic algae, which most commonly develops in mid to late summer. Toxins produced by cyanobacteria (also known as blue-green algae) are not only lethal to fish, they can kill animals and dogs, and harm humans.

Fish kills that occur in the spring can be deceiving – often, these fish actually die off over the colder winter months. When snow and ice cover the surface of your pond or a portion of your lake, sunlight is blocked out. Without adequate sunlight, aquatic plants die and no longer produce oxygen. Dead plants then feed undesirable bacteria within the pond, which go on to further use up the oxygen in the waterbody. In these situations, DO falls to lows that cannot sustain fish populations throughout the colder months, even though fish use less oxygen in the winter.

To prevent fish kills all year round, we recommend the following strategies and solutions:

• Install floating fountains, surface aerators, or submersed aerations to maintain healthy oxygen levels and circulation, prevent sudden DO fluctuations during algae and plant treatment, and limit ice formation on the water’s surface over the winter. This equipment is also integral to prevent the development of a thermocline – which refers to naturally occurring layers in the water based on temperature and DO. If sudden mixing of these layers occurs, DO levels can fluctuate significantly, triggering a fish kill. 
• Partner with a company that strives to use herbicides in moderation by treating only those areas that need it, when they need it. 
• Be cautious when distributing lawn or garden fertilizers that are susceptible to running off into a pond, as well as keeping livestock downstream to prevent waste run-off.
• Look for a fisheries management partner that practices responsible fish stocking, taking care to avoid pushing populations beyond what can be healthily maintained by the waterbody.
• Repair damaged shorelines to prevent erosion and muck accumulation, which reduces depth and habitat for fish.