Manatees In Danger: Pollution's Impact On Marine Life

Manatees are large aquatic mammals, also known as sea cows, that are native to Florida and are currently listed as vulnerable to extinction. Pollution is a significant threat to manatees, as it leads to habitat loss, algal blooms, and debris in the water. These issues cause health problems for manatees and can even result in death. For example, in 2013, a record number of manatee deaths in Florida were linked to algae outbreaks caused by sewage, manure, and fertilizer runoff. Additionally, manatees often ingest plastic debris, such as discarded fishing lines and plastic bags, which can cause fatal infections and injuries.

Man-made debris in waterways

Manatees are facing a growing threat from man-made debris in waterways. Human-generated trash is a significant problem for aquatic wildlife and habitats, and manatees are no exception. Manatees are susceptible to ingesting or becoming entangled in marine debris, which can be fatal. For example, discarded plastic packing straps or bicycle tires can wrap around a manatee's body and, if not removed, will embed into its flesh as it grows. Monofilament fishing line is the most common foreign object found in the digestive systems of manatees that have died from ingesting marine debris.

Manatees are naturally curious and often explore items in their environment with their flippers or mouths. This innate curiosity often leads them into dangerous situations, such as swallowing objects or becoming entangled and unable to remove the item. The loss of a flipper is possible if the entangled animal is not discovered in time. Experienced rescuers are often able to remove entanglements and either release the animal or send it to a rehabilitation facility to recover.

The presence of man-made debris in waterways is a direct result of human activities, such as accidentally dropping or intentionally discarding items into rivers, bays, or oceans. This debris can have fatal consequences for manatees and other aquatic wildlife, and it is crucial that people take steps to prevent it from ending up in the water. Properly disposing of trash, securing items on vessels and in recreation areas, and collecting any plastic items or fishing line seen in the water are essential to reducing the impact on manatee populations.

Algal blooms

Red tides occur when there is a higher-than-normal concentration of microscopic algae, specifically a type of microalgae called Karenia brevis (K. brevis), in the water. This algae produce a toxin known as brevetoxin, which is harmful to manatees and other aquatic life. The brevetoxin can be ingested by manatees when they eat affected seagrass, or inhaled when the churning of the sea and waves create an aerosol containing the toxins.

In Florida, the Indian River Lagoon (IRL) has been severely impacted by algal blooms. Excess nitrogen and phosphorus in the waterways fuel the growth of algae, and a combination of improperly treated sewage, leaking septic systems, fertilizers, and stormwater runoff has led to eutrophication. This has resulted in frequent harmful algal blooms that block the light necessary for seagrass photosynthesis, leading to a loss of seagrass biomass within the IRL. As manatees rely on seagrass as their primary food source, the loss of their food source due to algal blooms has resulted in starvation and death for many manatees.

The issue is further exacerbated by climate change, as higher temperatures, increased sunlight, and rising carbon dioxide levels contribute to the growth and spread of toxic blue-green algae. Drought conditions also make freshwater saltier, causing marine algae to invade freshwater environments. Additionally, rising sea levels create more shallow and slow-moving coastal waters that are more susceptible to algal blooms.

Efforts are being made to address the problem of algal blooms and protect manatee populations. The U.S. Fish and Wildlife Service (FWS) declared an Unusual Mortality Event (UME) for manatees in 2021, triggering an immediate response to the significant die-off of manatees. Conservation organizations like the Save the Manatee Club are advocating for stronger protections, improved water quality, and the restoration of seagrass beds to support manatee populations. Local governments have also implemented slow-speed zones in known manatee habitats to reduce the risk of boat collisions.

Sewage, manure and fertiliser run-off

Sewage, manure, and fertiliser run-off is a major contributor to the pollution that is affecting manatees. These pollutants enter the water when new developments are built along waterways. This has resulted in the destruction of manatees' natural nesting areas and has led to algal blooms, some of which are toxic and can kill manatees if ingested.

Manatees primarily feed on seagrass, which is a vital part of the health of marine ecosystems. Seagrass acts as a habitat for juvenile fish and other marine life, improves water quality, and protects coastal habitats by stabilising sediments and reducing wave energy. However, sewage and fertiliser run-off into these ecosystems adds nitrogen, phosphorus, and other nutrients that drive excessive algal growth. This, in turn, blocks sunlight and depletes oxygen levels, both of which are essential for seagrass to photosynthesise.

Between 2011 and 2019, over 50% of all seagrass in the Indian River Lagoon was lost due to sewage and fertiliser run-off. This has led to an increase in macroalgae and a change in the animal communities that live in the lagoon. Manatees have had to shift their diet to include more algae, which has negatively impacted their health. Researchers found that manatees consumed 45% less seagrass and 74% more algae after the seagrass decline.

The decline in seagrass is leaving manatees malnourished and is believed to be a contributing factor to the increase in manatee deaths. The rehabilitation of these ecosystems must focus on improving and maintaining water quality to prevent further harm to manatee populations.

Loss of warm water habitat

Manatees are tropical marine mammals that migrate to warmer waters during the colder months of the year. They cannot tolerate prolonged exposure to very cold water (below 68º F), and in the winter will migrate to areas of warm water to survive. This is because prolonged exposure to lower water temperatures causes manatees to lose body heat and inadequately digest their food, which can lead to a condition called "cold stress" and can eventually be fatal.

Historically, manatees relied solely on warm water springs and other natural areas for refuge in winter. However, with the advent of power plants and other industrial sources of warm water effluent, many manatees began using these discharge basins as winter refuges. It is estimated that currently about 60% of the manatee population is dependent upon industrial sources of warm water, with many hundreds found together at some sites during the cold season.

The loss of warm water habitat is a serious long-term threat to manatees. The projected operational lives of power plants currently used by manatees do not extend beyond 35 to 40 years. As power plants are retired, it is unclear whether natural warm-water refuges will be sufficient to support the current manatee population. Warm-water springs are believed to be the best natural winter habitats for manatees, but most have been blocked by dams or other structures, altered by construction, degraded by overuse, used intensively for recreation, or are subject to declining spring flow due to groundwater pumping for human consumption or agricultural use.

To address this issue, the Florida Fish and Wildlife Commission (FWC) and the U.S. Fish and Wildlife Service (FWS) have begun efforts to improve access to and protection of natural springs, many of which are currently underutilized by manatees. This includes dredging spring runs to improve manatee access, replanting seagrass to provide food sources, and purchasing lands around springs to prevent development and manage them as part of wildlife refuges.

In addition, the FWC and FWS are working on a warm-water refuge management plan to ensure that manatees have adequate warm-water refuges when power plants creating outfalls now used by manatees are eventually retired. It is recognized that manatees have become dependent on power plants, so it is imperative that these plants continue to provide safe and consistent refuge until alternative sources of warm water are available.

Crab traps and fishing gear

Fishing gear, such as nets, traps, and lines, can also become lost or abandoned in the sea, becoming what is known as "ghost gear". This ghost gear continues to entangle and harm manatees and other marine life long after it has been discarded. It is considered the most deadly form of marine plastic debris, responsible for harming marine mammals, seabirds, and sea turtles.

Manatees are naturally curious and often explore items in their environment with their flippers and mouths. This innate curiosity makes them more susceptible to entanglement in fishing lines, crab trap ropes, and other plastic debris. The entanglement can cause severe injuries, and in some cases, lead to the loss of flippers or even death.

Fishing lines, in particular, pose a significant threat to manatees. Monofilament fishing lines can get tangled around their flippers or be accidentally swallowed, leading to fatal infections and digestive issues. A 2017 study found that over an 11% period, 20-year of manatees that died had ingested trash or shown evidence of entanglement in marine debris.

To protect manatees from the harmful effects of crab traps and fishing gear pollution, it is essential to practice responsible boating and fishing practices. This includes properly disposing of trash, recycling used fishing lines, and reporting any sightings of entangled manatees to the appropriate authorities.

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