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Saltiness of Manatee Habitats Changes with the Seasons
Can That Affect Manatee Migration?

Contributed By Susan Butler and James Reid, USGS-Sirenia Project

During the dry season, the saltiness or "salinity" of coastal and inshore habitat waters rises because there is less rainfall and thus less fresh water flowing from the canal and riverine systems. Because manatees are mammals and need to seek out fresh water to drink, the high level of salinity in the habitat waters can affect manatee movement patterns.

The Faka Union Canal is part of an extensive canal system in southwest Florida designed to prevent flooding by allowing the water to drain south to the Ten Thousand Islands. At Port of the Islands (POI), which are residential canals located 5 miles up the Faka Union Canal, there is a spillway, or low dam. During the dry season the flow over the spillway is minimal so the manatees need to swim all the way up the Faka Union Canal to POI to get their "drinks" of freshwater.

Watch a Manatee Calf Drinking Fresh Water

Wondering how this calf can find freshwater in the ocean?

Try This!
With this simple experiment, you can explore how and why fresh water floats on saltwater--it's all about bouyancy:

In a similar situation, the inland rivers and creeks discharge fresh water into the coastal bays. During the dry season the flow is minimal so the salinity of the coastal bays is quite high. Therefore, manatees have to travel further up the rivers and creeks to get sufficient fresh water.

As the rainy or wet season approaches, the salinities will decrease because more fresh water enters these systems and dilutes the salinity with fresh water. When we are well into the wet season we rarely see the manatees go all the way into POI to drink because the water is often fresh enough at the southern end of Faka Union Canal. Likewise, we see fewer manatees traveling far up into the rivers and creeks because the water becomes fresh enough for them to drink near the mouths of the rivers.

How Do You Define and Measure Salinity?
At POI, it is typical during the dry season to have salinities of 32 parts per thousand or more (32/1000), while in the wet season it will often become 0 parts per thousand in POI and all the way out the Faka Union Canal! This same idea holds true for the rivers, creeks and bays.

During the dry season we will typically see the manatees make a journey toward fresh water about once a week. During the wet season we see more extended periods of time using the outer islands areas with short trips to river mouths, mouth of Faka Union canal or the canals of Marco Island about once every 1-2 weeks.

Another interesting note is that manatees can actually sip the fresh water off the surface after a good rain that may let them stay in the salt water for a longer period of time.

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