Greta Jimenez
Leopard sharks are one of the most common shark species in San Francisco Bay, and they live there all year round. However, in recent years, there have been several instances of mass die-offs of leopard sharks in the Bay. The first of these occurred in 2011, and there have been further die-offs every year from 2016 to 2019, with the largest number of deaths occurring in 2017. The cause of these deaths is believed to be a combination of factors, including an infection caused by the parasite Miamiensis avidus, which enters the sharks' bodies via the nose and ear canals, as well as pollution from street runoff, heavy rains, and lower salinity levels in the Bay's waters.
| Characteristics | Values |
|---|---|
| Year of Pollution | 2017 |
| Location | San Francisco Bay |
| Species Impacted | Leopard Sharks, Bat Rays, Halibut |
| Cause of Death | Amoeba Miamiensis avidus, fungal meningitis |
| Contributing Factors | Low salinity, stagnant water, human pollution, heavy rains |
| Impact | Hundreds of leopard shark deaths, potential ecological disruption |
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What You'll Learn
Parasite Miamiensis avidus
Miamiensis avidus is a species of parasite that is known to infect marine fish, causing mass mortalities in certain cultured fish species. It is a scuticociliate parasite, which means it belongs to the Ciliate Order Hymenostomatida. This parasite has been observed to cause fatal infections in farmed large yellow croaker (Larimichthys crocea) in Fujian province, China.
Miamiensis avidus has a life cycle with three forms: macrostomes, microstomes, and tomites. It infects its host by entering through the nose and ear canals, eventually infecting the brain. In 2019, it was discovered that this parasite was responsible for the deaths of large numbers of Bay leopard sharks in San Francisco Bay. Mark Okihiro, with the California Department of Fish and Wildlife, examined the brains of the sharks and found evidence of Miamiensis avidus infection. The parasite was found to be the likely cause of the die-offs, which occurred after rainier-than-usual winters when leopard sharks migrated to the Bay's shallow waters.
The heavy rains may have made the sharks more susceptible to infection due to lower salt levels and higher levels of pollutants in the Bay's waters. Miamiensis avidus has also been implicated in the deaths of older white seabass in the same region, where it was found to cause lesions in the noses and brains of the affected fish.
Miamiensis avidus is a well-known pathogenic scuticociliate, with its external morphology resembling the original descriptions provided by Thompson and Moewus in 1964. Phylogenetic analyses based on SSU rDNA sequences have suggested the presence of cryptic species within Miamiensis avidus, indicating the need for further molecular data to fully understand its genetic diversity.
The parasite's ability to infect hosts appears to be influenced by environmental conditions. For example, in cultured fish, the rapid multiplication and spread of Miamiensis avidus have been attributed to high stocking densities and obstructed water exchange. The parasite causes serious infections in aquatic animals, resulting in significant economic losses for the fish farming industry.
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Low salinity levels
Leopard sharks are the most abundant shark species in San Francisco Bay, and they are found from Oregon to Mexico. They are not endangered, but they are a key part of the bay's food chain, eating clams, worms, crabs, and small fish.
In recent years, there have been several instances of leopard shark deaths in the San Francisco Bay. The first major incident occurred in 2011, with other die-offs reported every year between 2002 and 2006, and again in 2016 and 2017. The largest known shark death toll was in 2011, and the events in 2016 and 2017 were the second and third major die-offs, respectively. In 2017, at least 80 leopard sharks washed up dead, with experts estimating that the total number of deaths could be as high as 1,000.
One of the suspected causes of these die-offs is low salinity levels in the bay, which may be related to heavy rainfall. Scientists have found that leopard sharks prefer salinity levels of about 20 to 25 parts per thousand and can tolerate levels down to 50% of seawater. However, when exposed to lower salinity levels for prolonged periods, the sharks can experience a variety of impairments. For example, their kidneys may stop functioning, their immune systems may fail, and their breathing may become limited, making them more susceptible to infections and diseases.
In the case of the San Francisco Bay, heavy rains may have contributed to lower salinity levels, particularly in the slow-circulating South Bay. The bay water near San Francisco International Airport was measured at 9 parts per thousand in March 2017, the lowest in 31 years, and far below the historic average. This reduction in salinity could have weakened the sharks, making them more vulnerable to infections and other health issues.
While the exact cause of the die-offs is still under investigation, low salinity levels are considered a significant factor, especially when combined with other stressors such as pollution and the presence of parasites or fungi.
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Human pollutants
San Francisco Bay has faced a number of environmental challenges over the years, with human pollutants being a significant concern. In 2017, hundreds of leopard sharks washed up dead or dying on the shoreline, the second year in a row of mass shark deaths in the Bay. This event highlighted the issue of human-caused pollution and its impact on the delicate aquatic ecosystem.
One of the main sources of human pollutants in the Bay is street runoff, which includes polluted water from streets that flows directly into the Bay. These tidal lagoons and waterways receive a significant amount of pollution and excess nutrients from street runoff, as well as human and animal waste. The high levels of pollutants in the water can have devastating effects on the wildlife that calls the Bay home, as evidenced by the leopard shark die-offs.
In addition to street runoff, wastewater treatment plants and stormwater are also major contributors to the problem. The wastewater treatment plants are not equipped to handle and remove all the chemicals that enter through stormwater, leading to a buildup of contaminants in the Bay. High concentrations of contaminants such as flame retardants and pharmaceutical products have been found in the South Bay, posing risks to both wildlife and human health.
Furthermore, the historical use of elemental mercury in gold mining during the Gold Rush era has left a lasting impact on the Bay. Mercury is considered one of the most significant legacy pollutants, found in high concentrations in Bay sediments. This has led to fish consumption advisories throughout the region due to the potential health risks.
The issue of human pollutants in San Francisco Bay is not just limited to the water itself. More than 5,200 toxic sites have been identified along the lip of the Bay, and rising groundwater levels pose a severe risk of moving contaminants towards the Bay and nearby communities. Many of these vulnerable sites are located in lower-income communities of color, exacerbating existing social vulnerabilities.
While efforts have been made to reduce toxic pollutants, such as the implementation of the Clean Water Act, the approval of new chemicals and the potential risk of contaminants in pharmaceuticals, food, and consumer goods remain a concern. As the Bay Area continues to face the challenges of population growth and increasing water demand, addressing human pollutants and their impact on the ecosystem will be crucial for the long-term health and sustainability of the region.
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Stagnant water
The presence of stagnant water can have detrimental effects on the environment. The build-up of pollutants, toxins, sewage, and chemical wastes deteriorates soil and groundwater quality, making the soil unsuitable for agriculture and threatening local ecosystems. Stagnation can also cause erosion, sedimentation, and unwanted accumulation of organic matter, leading to the destruction of flora and fauna habitats.
To mitigate the risks associated with stagnant water, various measures can be implemented. Regular cleaning and desilting of drains, efficient solid waste management, and proper drainage systems are essential. Additionally, participating in local clean-up drives, planting vegetation to absorb excess rainwater, and checking for potential water traps around homes can help reduce stagnant water formation.
In the context of San Francisco Bay, leopard sharks have experienced mass die-offs in recent years, including in 2011, 2016, 2017, and 2019. The die-offs have been attributed to various factors, including parasite infections, polluted street runoff, and lower levels of salt in shoreline waters after heavy rains. While the specific mention of stagnant water in relation to these events is not evident, it is reasonable to assume that stagnant water, if present, could contribute to the pollution and environmental conditions impacting the leopard shark population in the Bay Area.
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Die-offs in 2011, 2016, 2017, 2019
Leopard sharks in San Francisco Bay have been affected by pollution and die-offs in recent years. While the exact causes are still under investigation, several factors, including pollution, fungal infections, and low salinity, have been implicated in these events.
Die-offs in 2011
The first recorded die-off of leopard sharks in San Francisco Bay occurred in 2011. Over 1,000 dead sharks were found inside and outside the Redwood Shores Lagoon and along Richardson Bay in Marin County. This event prompted state regulators to warn people against consuming leopard shark meat due to accumulated toxins in their blood and flesh.
Die-offs in 2016
In 2016, there were reports of another leopard shark die-off in San Francisco Bay. While details are scarce, this event marked a continuation of the challenges faced by the leopard shark population in the region.
Die-offs in 2017
In the spring of 2017, hundreds of leopard sharks and bat rays washed up dead or dying on the San Francisco Bay shoreline. This was the second consecutive year of mass shark and ray deaths in the Bay and the third major die-off since 2011. Biologists were alarmed by the scale of the die-off, which was attributed to various factors. One theory suggested that the sharks were exposed to toxins in stagnant saltwater marshes and man-made lagoons in Foster City and Redwood City. Additionally, the rainy season was believed to have exacerbated the issue by washing out toxins accumulated during the drought.
The role of fungal infections was also significant. Dr. Mark Okohiro, a pathologist with the California Department of Fish and Wildlife, attributed the cause to an amoeba, *Miamiensis avidus*, which entered the sharks' noses and ear canals, infecting their brains. The tidal lagoons, such as those in Foster City and Redwood City, are particularly susceptible to polluted street runoff, including human and animal waste, providing a breeding ground for harmful organisms.
Die-offs in 2019
I could not find specific information regarding a leopard shark die-off in San Francisco Bay in 2019. However, it is worth noting that leopard shark deaths have been an ongoing concern, with reports of struggling or dead sharks in various locations within the Bay Area in recent years.
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Frequently asked questions
Leopard sharks in San Francisco Bay have been affected by pollution for many years. There were die-offs every year between 2002 and 2006, and again in 2011, 2016, 2017, and 2019.
The exact cause of leopard shark die-offs is not always clear, but a combination of factors, including pollution, parasites, and low salinity, are believed to contribute.
Pollution can make leopard sharks more vulnerable to infections and can also directly poison them. Human pollutants such as fertilizer, pesticides, motor oil, and heavy metals are washed into the Bay during storms.
The parasite Miamiensis avidus has been identified as a cause of leopard shark die-offs in San Francisco Bay. This parasite infects the brains of the sharks, causing disorientation and death.
