Fish Kill in Biscayne Bay: A Report and a Plan

Executive Summary

Biscayne Bay is an ecological and aesthetic jewel, with rolling seagrass meadows, tangled mangrove forests, and diverse marine life. However, between August 10-15, 2020, community members reported thousands of dead and dying marine life in Biscayne Bay from North Miami to Virginia Key. Affecting over 27 thousand marine creatures from 56 species, this was the largest recorded fish kill in Biscayne Bay. Following the mass mortality of marine life, severe algae blooms covered the Bay for several days.

Response efforts among research groups and agencies led to a  consensus statement  with 43 signatories that the likely cause of this crisis was low dissolved oxygen conditions. These conditions are often associated with nutrient pollution, such as nitrogen and phosphorus, from  septic tanks ,  sewage leaks , stormwater runoff, and  fertilizer  overuse. 

Miami Waterkeeper, with input from the  Biscayne Bay Science Coordination group , compiled a report summarizing the events of the fish kill and scientific data collected. Miami Waterkeeper also assembled lessons learned from the community's response into a response plan for future crises that may arise. 

Submitted photo from the public that aided in identifying the species impacted by the fish kill.

Total fish reported to Miami Waterkeeper each day of the fish kill between Aug. 10-26.

How many fish were affected?

Miami Waterkeeper staff analyzed 497 photographs submitted by the public and found that at least 27,640 fish are estimated to have died as a result of the low oxygen event. These fish only represent the fish that were photographed and reported to Miami Waterkeeper. Of these fish, 8,835 could not be identified. Public reports were crucial to understanding the scope of the fish kill.

Which species were affected?

Affected species ranged from bottom dwellers to more mobile, water column species. Bottom dwellers consisted of 69.5 percent of the identified fish, while water column species consisted of 30.5 percent of identified fish.

Where was the fish kill reported?

The fish kill was most prevalent around northern Biscayne Bay. Reports appear to be centered around Morningside Park and the mouth of the Little River. Miami Waterkeeper maintained a database of all public reports for further analysis.

Species most affected by the fish kill in August 2020.

A total of 56 species of fish were identified in the reported images. Toadfish comprised 12,373, or 44.8 percent of the total counted fish. This species naturally exhibits a “burrowing” behavior when they encounter stressful conditions, making them less likely to flee.

Reports of Foam and Algae

As the fish kill began to dissipate and dead fish began to decompose, water quality conditions worsened. Swathes of foam and algae blooms were reported between Aug. 20-25. These reports began in the morning as a white or brown foam, which is likely a build-up of protein in the water, and turned into algae by the afternoon. The blooms disappeared overnight, likely sinking, and returned the next morning. Since these blooms occurred after the fish kill and not prior, they are considered to be a result of the factors that led to the fish kill, and not a cause. 

Transformation of foam (pictured left) to algae (pictured right) once the fish kill dissipated.

Transformation of foam to algae once the fish kill dissipated.

Efforts to aerate by the City of Miami with stormwater pumps at Morningside Park.

Interventions to Address Acute Hypoxia

In an effort to aid marine life struggling to breathe, experimental emergency aeration efforts were undertaken by the Phillip and Patricia Frost Museum of Science using oxygen rings, Miami-Dade County using fireboats, and the City of Miami using stormwater pumps. Bacteria levels declined, oxygen levels increased, and mortality reports and reports of wildlife struggling for oxygen slowed or stopped in these localized areas after interventions.

Efforts to aerate Biscayne Bay by the City of Miami and PortMiami.

While there were no controlled experiments around these interventions, taken together, data suggest that these interventions were beneficial, increasing dissolved oxygen levels and water quality in the immediate areas where they were deployed. While we recommend that aeration resources are identified for use in possible future crises as they may provide small refugia to allow wildlife to survive until conditions change, it is not a solution for large-scale water quality issues, including fish kill events. Instead, resources should be put towards addressing the root causes of the fish kill to prevent future low dissolved oxygen conditions.

Clean-Up Efforts

The abundance of dead marine life gathered on the shorelines and began decomposing, further worsening the water quality. With a call to action, dozens of community members volunteered to clean up fish at accessible areas by kayak and boat. The City of Miami's scavenger vessel also assisted in clean-up by removing floating fish carcasses. Collaboration among Miami Waterkeeper, Fertile Earth Worm Farm, and the City, made it possible to designate compost bins throughout various hotspots such as Morningside Park, Albert Pallot Park, and Margaret Pace Park.

Efforts to clean up the dead fish throughout North Biscayne Bay were made by Miami Waterkeeper, community members, the City of Miami, and Fertile Earth Worm Farm.

What caused the fish kill?

Scientists and experts rapidly coordinated efforts to gather data, which determined that low dissolved oxygen, called hypoxia, was the proximate cause of the large quantities of fish mortality. During the fish kill, abnormal gatherings of fish and marine life were observed near the surface, gasping for air, further indicating that the fish were struggling to breathe due to low dissolved oxygen levels. Low dissolved oxygen is a common and well-documented side effect of too much pollution in the water, specifically nutrient pollution.

See Biscayne Bay's conditions in real-time by checking out FIU's CREST  research buoys .

Contributing Factors

Low dissolved oxygen conditions are more common in the summer months since water holds less oxygen at higher temperatures. During the summer, high amounts of runoff and flow from canals from heavy rainfall can also leave a layer of fresh water at the surface of the water column, leading to less mixing at the bottom. This can also lead to low dissolved oxygen. This fish kill was preceded by high temperatures and very high flows from the Little River canal. These high flow rates are still unexplained. An underlying build-up of nutrient pollution, as well as the decline of oxygen-producing seagrass meadows, leaves Biscayne Bay vulnerable to hypoxia. 

Nutrient Pollution

Scientists have been warning for years that Biscayne Bay is reaching a tipping point for nutrient pollution build-up. Nutrients, like nitrogen and phosphorus, are carried along with runoff from land and pollute the water. In severe cases and under the right conditions, this pollution can lead to fish kills like this one. Nutrient pollution comes from  septic tanks ,  sewage leaks , stormwater runoff, and  fertilizer  overuse. A build-up of leaf litter or sargassum can also contribute. These land-based sources release an excess of nutrients that flow into coastal ecosystems like canals and Biscayne Bay. 

Seagrass Die-Off

Excess nutrients, or eutrophication, can fertilize algae growth and turn water brown or green and can lead to algae blooms. These blooms block the sunlight that seagrass needs to grow and to produce oxygen during photosynthesis. Therefore, nutrient pollution can lead to seagrass die-offs. The increase of nutrients in Miami’s waterways has diminished the coverage of seagrass beds in Biscayne Bay over the last 10-20 years. Coverage has been observed to decrease by up to 90 percent in some areas, according to a 2019 study performed by  Miami-Dade County . These die-offs present a situation where hot, shallow waters become vulnerable to low oxygen conditions.

Google Satellite images of seagrass coverage, represented in darker shading, near Julia Tuttle Causeway in 2011 (pictured left), contrasted with coverage in 2016 (pictured right).

What can you do?

Sustain our work by donating to Miami Waterkeeper and help fund future research and response!

Support from the Frohring Foundation was critical to further our understanding of this unprecedented fish kill.

Collaborator

Samantha Morejon

Collaborator

Rachel Silverstein, Ph.D

Collaborator

Elizabeth Kelly, Ph.D

Total fish reported to Miami Waterkeeper each day of the fish kill between Aug. 10-26.

Species most affected by the fish kill in August 2020.

Efforts to aerate by the City of Miami with stormwater pumps at Morningside Park.

Efforts to aerate Biscayne Bay by the City of Miami and PortMiami.

Google Satellite images of seagrass coverage, represented in darker shading, near Julia Tuttle Causeway in 2011 (pictured left), contrasted with coverage in 2016 (pictured right).

Support from the Frohring Foundation was critical to further our understanding of this unprecedented fish kill.