Last updated: February 19, 2021
Article
How improved freshwater flow will help American crocodiles in Everglades National Park
NPS photo
Below is a summary of a research paper that looked into the potential effects of Everglades restoration on American crocodile populations in Everglades National Park. It is intended to a provide a more accessible, plain language description of the paper. To read the full research paper, use this reference:
Mazzotti, Frank J., et al. "Influence of salinity on relative density of American crocodiles (Crocodylus acutus) in Everglades National Park: Implications for restoration of Everglades ecosystems." Ecological Indicators 102 (2019): 608-616.
Brief Summary: As historic water flow to the park is restored by improving water delivery into the estuaries, we expect to see many changes. These changes include an increase in water flow, more attenuated seasonal changes, and an overall decrease in salinity throughout areas of Florida Bay. The researchers wanted to study how this might affect the population density and distribution of American crocodiles found in the park. In order to answer these questions, twelve years of spotlight survey data were used to determine American crocodile population trends within the park from 2004 to 2015. The results are consistent with the hypothesis that restored water flow and lower salinities in Florida Bay will result in an increase in American crocodile population size.
Background Information:
The American crocodile (Crocodylus acutus – Figure 1) is a protected species, at both the state and federal level, that lives in Everglades National Park. American crocodiles are responsive to measurable hydrologic changes within the greater Everglades ecosystem, and so the management and restoration of the park is critical for their survival and success. Scientists monitor ecological trends, such as American crocodile population size, because they provide insight into how the Everglades ecosystem is doing.
This is important because the Everglades has been profoundly impacted by water management practices. In the past, the northern Everglades were drained and compartmentalized for development and flood control, redirecting water to the coasts. Canals and ditches were built to divert water flow from the southern marshes and mangrove transition zones. The more northern canals and ditches extended into the freshwater interior, cutting off the natural sheet flow that once flowed into the area now designated as Everglades National Park. The more southern canals allowed access for navigation and other endeavors, but also further drained the freshwater marshes and allowed unnatural saltwater intrusion. Substantial changes to the environment resulted, which negatively impacted ecosystem function and the wildlife that depends on it.
People started realizing the negative effect of these water management practices on the Everglades ecosystem and began to think about restoring freshwater flow. In the 1980s, scientists tried to make water flow to the park more closely replicate former hydrologic conditions. A large-scale effort to comprehensively improve water flow became known as Everglades restoration. Florida Bay and the western, or Gulf Coast, estuaries, where American crocodiles are found, are the final receivers of that additional and restored fresh water. Scientists hypothesize that Everglades restoration will improve habitat quality and further increase the American crocodile’s overall growth and survival in the park, resulting in a higher population density.
The researchers in this paper were especially interested in how restoration efforts aimed at increasing freshwater flow to Taylor Slough, such as the C-111 Spreader Canal Western Project, will affect American crocodile populations. That’s because Taylor Slough is the main source of freshwater for northeastern Florida Bay, where they tend to live.
The American crocodile (Crocodylus acutus – Figure 1) is a protected species, at both the state and federal level, that lives in Everglades National Park. American crocodiles are responsive to measurable hydrologic changes within the greater Everglades ecosystem, and so the management and restoration of the park is critical for their survival and success. Scientists monitor ecological trends, such as American crocodile population size, because they provide insight into how the Everglades ecosystem is doing.
This is important because the Everglades has been profoundly impacted by water management practices. In the past, the northern Everglades were drained and compartmentalized for development and flood control, redirecting water to the coasts. Canals and ditches were built to divert water flow from the southern marshes and mangrove transition zones. The more northern canals and ditches extended into the freshwater interior, cutting off the natural sheet flow that once flowed into the area now designated as Everglades National Park. The more southern canals allowed access for navigation and other endeavors, but also further drained the freshwater marshes and allowed unnatural saltwater intrusion. Substantial changes to the environment resulted, which negatively impacted ecosystem function and the wildlife that depends on it.
People started realizing the negative effect of these water management practices on the Everglades ecosystem and began to think about restoring freshwater flow. In the 1980s, scientists tried to make water flow to the park more closely replicate former hydrologic conditions. A large-scale effort to comprehensively improve water flow became known as Everglades restoration. Florida Bay and the western, or Gulf Coast, estuaries, where American crocodiles are found, are the final receivers of that additional and restored fresh water. Scientists hypothesize that Everglades restoration will improve habitat quality and further increase the American crocodile’s overall growth and survival in the park, resulting in a higher population density.
The researchers in this paper were especially interested in how restoration efforts aimed at increasing freshwater flow to Taylor Slough, such as the C-111 Spreader Canal Western Project, will affect American crocodile populations. That’s because Taylor Slough is the main source of freshwater for northeastern Florida Bay, where they tend to live.
Study Methods
Crocodile relative density counts were conducted through nighttime spotlight surveys. Researchers looked for American crocodiles by boat along canals, shorelines, coves, ponds, and creeks between February 2004 and December 2015. While surveying, researchers would drive slowly and shine a high-powered flashlight to try and see a reflection from the crocodile’s eye that would shine back as red, orange, or yellow.
When an eye shine was observed, the researchers approached as closely as they could to try and identify if it was a crocodile or an alligator since both produce the same color eye shine. All crocodiles are recorded, but only juvenile and adult crocodiles were counted for analysis in this study, since hatchlings have low mobility and high mortality. When a crocodile was spotted, the water salinity was measured along with the air and water temperature at that location.
The study area was divided into three main regions ranging from high to low salinity. Sampling areas were along the southern coastal mangrove region and in Florida Bay, a shallow estuarine bay found off the southern tip of mainland Florida with an average depth of less than one meter. Each of the three focus areas had different salinities. The study areas were:
Crocodile relative density counts were conducted through nighttime spotlight surveys. Researchers looked for American crocodiles by boat along canals, shorelines, coves, ponds, and creeks between February 2004 and December 2015. While surveying, researchers would drive slowly and shine a high-powered flashlight to try and see a reflection from the crocodile’s eye that would shine back as red, orange, or yellow.
When an eye shine was observed, the researchers approached as closely as they could to try and identify if it was a crocodile or an alligator since both produce the same color eye shine. All crocodiles are recorded, but only juvenile and adult crocodiles were counted for analysis in this study, since hatchlings have low mobility and high mortality. When a crocodile was spotted, the water salinity was measured along with the air and water temperature at that location.
The study area was divided into three main regions ranging from high to low salinity. Sampling areas were along the southern coastal mangrove region and in Florida Bay, a shallow estuarine bay found off the southern tip of mainland Florida with an average depth of less than one meter. Each of the three focus areas had different salinities. The study areas were:
- Northeast Florida Bay - a northeastern survey section of mainland coastline, shallow bays and creeks,
- a central section of creeks, bays, and inland brackish lakes, and
- a western section of brackish lakes, canals, creeks and mainland coast.
Figure provided by Frank Mazzotti
Figure provided by Frank Mazzotti
Crocodile surveys were repeated for 12 years along 22 distinct routes (each approximately 10 linear km) and each route was repeated two to four times per year. This resulted in a total effective study area of 1,800 km2 and 1,449 confirmed crocodile sightings. Statistical analysis showed that detectability increased with air temperature, decreased with water temperature, and was highest during the moon’s first quarter. Overall crocodile relative density increased as salinity decreased within this study.
Survey route was found to be a major factor in relative density (see Figure 3). Buttonwood Canal (labelled BWC on Fig. 3) had the highest crocodile density, which may be due to increased detectability because of the manmade linear habitat and crocodile habituation to boat traffic in the heavily used corridor. Crocodiles have become accustomed to boat traffic and do not attempt to hide when boats approach. There was also a higher number of crocodiles in the Flamingo route (green on Figs. 2 and 3) compared to the Northeast Florida Bay route (purplish-blue in Figs. 2 and 3).
The lowest relative density of crocodiles occurred along routes in Northeast Florida Bay (NEFB) and the central lakes between West Lake and Seven Palm Lake (WEST) shown in Fig. 3. These central and eastern estuarine areas are most impacted by upstream freshwater diversion. The completion of Everglades Restoration projects that will most directly benefit northeast Florida Bay should result in lower salinities in these areas, which may increase the relative density of American crocodiles found there.
As hydrologic restoration efforts continue, scientists hypothesize that American crocodile densities will continue to increase because there will be more favorable salinity regimes and increased prey for them to eat. Both of these factors result from the improvements in overall ecosystem function.