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Red Tide on the Bronx River

For the past three years, we have seen numbers of menhaden fish (also known as bunker) and red tide float upon our estuary water and shorelines. We have seen first signs of these natural occurrences these past weekends during our public estuary paddles from Concrete Plant Park to the mouth of the river. Although these unusual sights may cause concern, there are some natural reasons why they happen in the Bronx River described below.

Menhaden swim in large schooling groups to avoid predators, such as Bluefish and Striped Bass. These giant predators chase the menhaden into shallow bodies of water causing excessive numbers of menhaden ending in the Bronx River. Large amount of menhaden in the river can deplete oxygen present in the already fragile ecosystem leading them to suffocate and die.

In addition, the river has also shown a large bloom of the marine dinoflagellate (dino – two; flagellum – whips) identified as ceratium. These single-celled aquatic organisms have both plant and animal characteristics and have spiked horns which help them stay afloat closer to the river surface. Certain ecosystem conditions may cause the ceratium to reproduce quickly and in vast numbers causing the water color to look reddish brown, sometimes referred as a “red tide”. The ceratium are not considered harmful and do not produce toxic chemicals; however, since they float higher up in the water where oxygen is more abundant, they cause deeper waters to lack oxygen which is needed for species survival, such as the menhaden, adding yet another cause for dead menhaden found in the river.

While there are not many ways to control these natural occurring systems, you can still help the Bronx River in your everyday activities by:

• Conserving Water: Help reduce the amount of pollutants and bacteria that enters the river by reducing the amount of wastewater that flows down our drains

• Reporting What You See: Report any observations of dead fish or a red tide on the river to the Bronx River Alliance and your local Department of Environmental Protection

• Getting Involved: Volnteer as a citizen scientist in one of our many different and exciting Bronx River projects!

(Top) Microscopic sample taken of start of red tide noticed on 6/30/2017 from Starlight Park docks. Millions of dead ceratium floating in river medium.


About the Bronx River Alliance

The Bronx River Alliance is a coordinated voice for the river that works in partnership to protect, improve and restore the Bronx River corridor so that it can be a healthy ecological, recreational, resource for the communities through which it flows.

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