Hurricane Risks to Manatees

Author: Emma Verville, Edited By: Emilyann Ashford

Figure 1. Graph by NOAA showing the number of storms per 100 years and when they peak in early fall.

Human-induced climate change has multiplied the impact of hurricanes. Rising ocean temperatures are fueling hurricanes' need for heat, increasing their strength and capability to intensify (4). As the air temperature warms, it is able to hold more moisture causing for there to be heavier rainfall during hurricanes (4). Lastly, rising sea levels are also occurring because of climate change which leads to higher storm surges (4). Storm surges are produced when strong wind forces push water towards the shore causing walls of water, which are larger than predicted astronomical tides (4). Florida experiences a six month long Atlantic hurricane season, occurring from June to Late November, with peak months between August and October due to warmer waters at this time. One of the many endangered species in Florida, is The Florida Manatee, pictured in Figure 2, which can be greatly affected by these storms and their gradual intensification due to climate change. 

Figure 2: Image of the endangered Florida Manatee in the wild by Keith Ramos. 

Manatees, also known as sea cows, are marine mammals that have large bodies with a snout, flippers, and a flat tail. They are herbivores feeding primarily on sea grasses which is how they were nicknamed. Some manatee species can grow to be over 14 feet long and weigh over 3,000 pounds. There are three different species of manatees, each related to the region of the world in which they originate: the Amazonian, the West Indian, with two subspecies of Antillean and Florida, and the African manatee. Manatee habitats range from riverine to coastal areas in both freshwater and saltwater environments. The Florida manatee, or Trichechus manatus, has a low tolerance for temperatures below 68 degrees Fahrenheit, and thus their habitat range extends further north in the summer and further south during winter to accommodate their need for warmer waters, which is seen in Figure 3.

Florida Manatees are considered an endangered species, and thus are protected under both the Endangered Species Act and the Marine Mammal Protection Act to help conserve the wild population. Currently, the biggest impact to manatees causing the most deaths and injuries is collisions with boats but as climate change affects the oceans it will also likely disrupt manatees lives.

Figure 3: Map of the Florida manatee’s range spanning from the east coast all the way down to Brazil depending on the temperature. Taken from Jane Cooke, USFWS.

As manatees live primarily on the coastlines, they will receive the brute force of storms moving inwards to land. Typically, they will hide out in sheltered waters as storms brew, but as hurricanes become stronger, they can push the manatees further out to sea, or leave them stranded on land after the recession of water post-storm. Also, the manatees primary food source can be highly affected or destroyed from hurricanes passing through, adding yet another stressor to their daily life (4). In one long term study, a subpopulation of Florida manatees were monitored for 19 years using photo identification data and annual variation in weather to determine if there were lower adult survival rates for years that had large storms (2). As shown in figure 4, they found that lower estimates of manatee survival were seen in 1985, 1993, and 1995 which were all correlated with major storms of category 3 or higher (2). 

Figure 4: Table showing the estimated survival rates of manatees during years with major storms of category 3 or 4 from the Langtimm and Beck Study.

In years without major storms the estimate of survival was above 0.97 but when there was a storm that estimate dropped to ranging from 0.817 to 0.936 (2). This study notes that effects of major storms could have been preventing the return of a marked individual back to the winter site due to death or emigration to a different site. In other words, manatees recorded as dead may not actually be dead, but reside in a new area that is not being included in the study. Four variables in storms could be responsible for manatees not returning; the storm surges pushing sea water and debris into the estuaries and rivers, strong winds causing high energy waves, parallel long-shore currents, and cooler water at the surface that can persist for days (2).

Figure 5. Photograph of red tide after a storm taken from a helicopter by P. Alejandro Diaz.

As storms continue to intensify with global change, other issues can arise in the oceans. For example, upwelling from the storms can lead to red tide which is caused by toxic algal blooms that deplete the oxygen levels in the water. In 1996, there were 149 Florida manatees that died from red tide resulting from Hurricane Opal which had the lowest estimate of survival rate of 0.817 (1,2). Pathology results showed that constant inhalation of brevetoxin, a neurotoxin responsible for shellfish poisoning, was the cause of their death (1). Theories of the brevetoxin being stored in the manatees lungs and significant sensitivity to the toxin were tested with two captive-born manatees used for research (1). This was done by reinforcing manatees to breathe into a mask, in order to measure the maximal effort and true vital capacity of their lungs, to define the normal breathing pattern of the mammal (1). In turn, this information allowed for scientists to determine volumes and flow rates per breath to estimate toxin deposition in the lungs (1). There was another large case of mortality in 2002-2004 where 34 manatees and 107 bottlenose dolphins died in Florida’s coastal waters. Water research showed there were low concentrations of brevetoxins within the water but analysis of the organisms stomachs showed high levels of the neurotoxin. The animals died from bioaccumulation of the toxin from eating fish and seagrass which was verified with laboratory experimentation (3).
Increasing hurricane impacts due to climate change will further impact lifestyles of the already endangered species, Trichechus manatus or Florida Manatee. Manatee females are not sexually mature until they are about five years old and for males it is not until they are nine. Due to long gestational periods, females only mate once every two years which could impact their population size if the storms push manatees out to sea or inland from where they usually reside. More intense hurricanes will also increase the upwelling of the ocean causing red tide which causes large mortality events as the manatees breathe in the brevetoxins. It is important to be aware of how our actions affect human-induced climate change to limit further impact on the world to help protect this endangered species from reaching extinction.

Citations:

1. Kirkpatrick, Barbara, et al. “Florida Red Tides, Manatee Brevetoxicosis, and Lung Models.” Harmful Algae 2002 : Proceedings of the Xth International Conference on Harmful Algae, St. Pete Beach, Florida, USA, October 21-25, 2002. International Conference on Harmful Algae (10th : 2002 : St. Pete Beach, Florida), U.S. National Library of Medicine, 2004, www.ncbi.nlm.nih.gov/pmc/articles/PMC4593611/.

2. Langtimm, Catherine A. & Cathy A. Beck, Lower survival probabilities for adult Florida manatees in years with intense coastal storms, 13 Ecological Applications 257 (2003).   https://www.jstor.org/stable/3099964?seq=1#metadata_info_tab_contents.

3. Rabinowitz, Peter, and Lora Fleming. “Toxic Exposures.” Human-Animal Medicine, 2010, pp. 50-104., doi:10.1016/b978-1-4160-6837-2.00008-7.

4. Wolf, Shaye. “Blown Away.” Biological Diversity.org, Center for Biological Diversity, Oct. 2019, www.biologicaldiversity.org/programs/climate_law_institute/climate_change_is_here_now/pdfs/Blown-Away.pdf.