Rotenone is the most commonly used pesticide to eradicate fish populations.  For many years, introduced fish species have caused desirable native species to suffer because of the unwanted species’ competitiveness.  When a pond or lake has an unbalanced fish population and is therefore undesirable to recreational fishermen, a common solution is to eradicate the fish population completely and introduce new fish in a more desirable combination.

However, as with any pesticides, rotenone must be used carefully.  In California in 1997, a lake that was a primary water source for 2,300 people was treated with rotenone in order to kill Northern Pike.  The pike had been illegally introduced into the lake and were threatening to destroy prized salmon and trout fisheries.  However the pesticide killed all the fish in the lake.  The residents were forced to find an alternative water source, as the seven-mile lake was still contaminated six months later.

Some fish are more resistant to rotenone than others and require a higher concentration to have the same effect.  Resistant fish include bullheads, goldfish and bowfin.  Rotenone treatments will usually kill fish within 24-36 hours.  When poisoned with rotenone, fish swim erratically and move to shallower water or come to the surface gasping for air.  After that, their ventilation rate slows and they sink to the bottom where they remain until death.  This happens not because rotenone removes oxygen from the water, but because it inhibits a process that occurs during cellular respiration, called oxidative phosphorylation.  The specific site of action of rotenone is in the electron transport system where it blocks a mitochondrial enzyme called NADH ubiquinone reductase (or complex 1).  This means that the blood oxygen content of the fish will increase because oxygen is now unavailable for respiration.

Rotenone is able to inhibit cellular respiration in almost every living organism, including mammals, fish, amphibians, insects and even plants.  So why is rotenone only toxic in this way to fish and insects?  Fish are highly susceptible because rotenone can efficiently and quickly enter the blood stream through the gills.  This is compounded by the fact that rotenone is very insoluble in water and the gills have a relatively high lipid content that the rotenone will favour.  The trachea of insects acts in a similar way to the gills of a fish meaning rotenone can also act quickly on insects.  Mammals are not usually affected by rotenone as it is inefficiently absorbed from the gastrointestinal tract.

Rotenone poisoned fish can be revived by simply putting them into untreated fresh water.  Rotenone can also be detoxified by solutions of potassium permanganate, chlorine or methylene blue.  The use of methylene blue is not recommended because it increases bacterial infection on fish.  Potassium permanganate is the most effective at detoxification and is used to reverse the toxicity in the affected fish or to accelerate the natural breakdown in water.  It is often used on water in outlet streams flowing out of treated lakes so that rotenone will not kill desirable fish species that live in the stream.

Rotenone treatments in municipal water supplies have often been criticised because they can cause water to have a bad odour and taste.  Adding activated carbon to the treated water can control these effects.  Also, if the rotenone is applied during the summer it will degrade faster due to the increased temperature and light levels.