Today's News
A/CPB
Manatee Migration Update: April 16, 1997
James Reid
My, What A Lovely "Transmitter" You Have!In previous updates, you have read how important it is to learn as much as possible about manatees if we hope to help them survive. Scientists try to do this by using many different techniques to study the manatees. Along with Photo-identification techniques, scientists also utilize radio transmitters for tracking manatees. But, have you ever wondered HOW these transmitters are used--what do the transmitters look like, how do the scientists attach them to the manatees, what kind of radio signals are sent out, how long do the transmitters last, and what happens if the transmitter gets caught in something
Floating Radios
The radio transmitters are contained inside of floating plastic cylinders
about 39 cm long and 9 cm in diameter. The cylinders have a 20 cm wire
antenna on the top of them, and every cylinder is color-coded with large
identifying letters and engravings on them. Our thanks to James Reid and
Robert Bonde of the Sirenia Project for the photo of the manatee tracking
devices. Each cylinder actually has two different transmitters inside it,
one called a VHF transmitter which sends specific radio frequency signals
to scientists in the field using a portable receiver, and another called a
Platform Transmitter Terminal or "PTT" that sends out location signals to
an orbiting satellite. Typically, the PTT will send from two to six
different location points for each tagged manatee every day.
Backpacks, Collars, And Belts!
Journey North students tracking the migration of the Bald Eagle know that
it wears a "backpack" transmitter, and students studying Caribou know that
they wear a "collar" transmitter. Manatee transmitters are attached with a
"peduncle belt" and a "nylon tether." The belt goes around the manatee just
in front of its tail, in an area called its peduncle, and the tether is a
stiff nylon rod (about 10 mm in diameter and 130 to 200 cm long) that
connects the belt to the transmitter. Each tether has a "weak link" built
into it, which will break and allow the manatee to swim free if the tether
or transmitter ever become snagged on something such as a dock, a boat,
thick vegetation, or even a hungry alligator!! Yes, that's right,
scientists have reported seeing an alligator pull a transmitter off an
manatee, and also have found alligator teeth marks on other transmitters.
An entire manatee radio transmitter assembly is shown in this diagram Courtesy of the Sirenia
Project, Biological Resources Division, USGS.
"Tag" A Manatee By The Tail
There are several methods the Scientists use to capture and "tag" manatees
with radio transmitters. Many are captured with nets after the scientists
lure them with fresh water running from a garden hose; some are actually
cooperative and can being tagged by scientists with snorkles who gently
swim near the manatees and put a belt around their tails; lastly, some were
tagged after they had been captured for rehabilitation (like Dakota).
Speaking of Tracking.....
Thanks to Cathy Beck and Jim Reid of the Sirenia Project, here is the
latest data on the manatees we are tracking.
- Link to Today's Data
Science Rules!
Were you able to rule out the answer to last week's Challenge Question #8?
"Why do you think that mainly adult manatees died from red tide and not
young manatees?" (This was widely reported in press coverage about last spring's die-off.)
As you will read in the answers below, our young student scientists were hard at work trying to figure out a theory about manatee deaths and red tide. It's important to keep in mind, that just like you, scientists can come up with many different theories and opinions about an area of science. Our Manatee Scientist Cathy Beck also wrote to us and gave us what you might think is a surprising answer to the Challenge Question #8: "Why do you think that mainly adult manatees died from red tide and not young manatees?" Here's what Cathy said:
"Unfortunately, there was quite a bit of misleading information disseminated about the red tide event in 1996, including the idea that mostly adults were affected. A total of 158 deaths were documented during the event and 152 animals were recovered for necropsy. Of these, 55% were adults, and 45% were immature (subadults and calves). Yes, this is more adults, but not mainly (or mostly) adults. Greg Bossart, a vet at Miami Seaquarium, hypothesizes that the red tide brevetoxin accumulates in the manatees due to chronic exposure, and perhaps that would make older animals more susceptible to re-exposure. However, due to lack of evidence, other experts do not support this. (Humans, however, can be more sensitive to subsequent exposures.)"The bottom line is, not that many more adults were affected! Investigations into the effects of red tide on many species is a developing science. There are many questions and much work to be done on the effects of this toxin.
"What is interesting is that 90 were males and 62 were females, not a normal distribution. But when you think about it, females with calves would more likely be in quiet, more sheltered waters, and perhaps less apt to be exposed to the red tide."
No Large Families Here!
In Challenge Question # 9 we asked, "If a female manatee is born in 1997, what is the earliest
year that she could bear her third calf, assuming that no twins are born?"
In the e-mail version of the question, we also asked you to assume that the female would pair up with a male born the same year, but in real life manatees do not actually form pair bonds. If they did, then the year they would have been able to have their third calf would have been 2010. This is because the first calf could not have been born until 2006 when the male became sexually mature at 9 years old, and then the second and third calves could have come as early as 2 years apart, in 2008 and 2010.
However, as we said, manatees don't pair off for life, and a female manatee will always have mature adult males to mate with so the female's ability to produce her third calf would be based on when she matures and how soon after her first calf she might give birth to another calf. In that circumstance, the manatee could have her third calf as early as 2006, because the female could give birth to the first calf in her fifth year--2002-- and then one more calf every two years.
What you can see from this example is that the manatee birth rate is very slow, so injuries or deaths to even a few manatees can have a serious potential impact to the population. Compare this to other animals' reproduction rates such as cats or dogs, which have multiple births.
The Next Manatee Migration Update Will Be Posted on April 30, 1997