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Manatee Migration Update: April 12, 2000

Today's Report Includes:

Field Notes from Cathy Beck

"Dear Students,

"I can't believe it is already April and the Journey North manatee tracking season is almost ending. It's probably for the best, because we really are having a hard time keeping tags on our manatees!! There will be one more report from me with my final comments for the season. But, in the meantime, here's the latest news from the field.

More Alligator Hors D'oeuvres?
"Of the four manatees tagged with PTTs and released from Blue Spring State Park this winter, Xoshi is the only manatee still carrying a tag. (Remember, Xoshi's tag had to be replaced once already-๑her first one became an alligator Hors d'oeuvre).

"The other manatees--Brian, Calista and Comet--all have either lost or suffered damage to their tags since the last report. Jim, Susan, Dean, Bob and I have all been out in the field searching for tags and manatees. Here are all the details from the field.

Brian's Near Miss!
"In my last report on 3/29, Brian's PTT had been off the air (stopped transmitting) since the prior weekend. Jim traveled a good distance searching, and listening, for Brian. Unfortunately, he was not able to locate Brian in that first week after the transmitter stopped. He attempted VHF tracking in the areas where Brian was last located, but received no signal.

"On 4/5, Dean and Susan did find Brian! He's fine, but his transmitter had been hit by a boat; the top was split by a prop and it filled with water and sunk. The tag was destroyed and they cut it off Brian, but they were unable to retag him. (He is still wearing the belt and we will continue to try to retag him).

No Tag, But No Trouble for Calista
"In my last report, I also said that Calista's tag hadn't moved--it was sitting in the same location, with no "tips".

"Jim Reid went into the field, and he recovered her tag on 3/28 at the Sanford Municipal Marina. It was broken at the weak link--likely due to entanglement. Jim then tracked her by the belt-mounted VHF, and found her with Comet!! They were both feeding on bulrush (Scirpus sp.) at the east end of Lake Monroe.

The Quest for Comet
"During the following week, Comet's tag also stopped transmitting. We suspected that Comet's tag might have been damaged by either an alligator or a boat strike. So on Friday morning 4/7, Susan and I headed to Lake Monroe to try to find him!

"It was an absolutely beautiful day and we saw lots of alligators, osprey, terns, gulls, ducks, egrets, herons, pelicans, etc., but... no Comet. Actually, we didn't see any manatees!

"But, we did retrieve Comet's tag. Surprisingly, it was just a few meters from where Calista's tag was recovered! It had broken free at the weak spot in the tether--just what it should do if it becomes entangled. It had become wedged between the boards of a hyacinth barrier fence near the Sanford Marina on the south side of Lake Monroe. There was about a 5 ft. clearance between the bottom edge of the fence, which was under water, and the bottom of the lake. Comet apparently swam under this fence and his tag became wedged between two boards.

"Then, for the next 4 hours, we tried to find Comet! We were getting only an intermittent signal on his belt-mounted VHF transmitter, not constant enough to home in on. But, since we had heard the VHF signal since we began our search, we thought he was still nearby. We searched along much of the south side of the lake and in the marinas. Before giving up (at dusk) we circled the entire lake, listening in on the frequencies for his and Calista's signal, but did not pick up any signal beyond the southern portion around the town of Sanford, where we initially "heard" Comet.

"Dean will go back to Sanford/Lake Monroe on 4/11 to try again to find Comet. But he's not likely to have time to track for Calista and Brian too. Depending on his results, Susan may go again on Thursday. Toward the end of May we had planned a capture, for medical reassessment, of Comet, Xoshi, Brian, and Calista. However, since we are losing tags so quickly over there, we may wait a few weeks before intensely trying to locate and retag these manatees again. The logic being if we retag now, we may have to do it yet again before a re-capture!

Mark Your Calendar for April 2001!
"Hopefully we can "expect" good news from Xoshi next year. Jim tracked her, and she is back in Lake Woodruff--and she's the focal manatee in a mating herd, with 4 other adults! Jim says 'mark your calendar for April 2001!'

Ivan Unplugged
"Ivan has continued to spend most of his time near the mouth of the Chassahowitzka River. On 4/6 Jim Reid went out to get one last, good look at him, and cut off his tag and belt if all's well. Jim located Ivan in the mouth of the Chassahowitzka River (no surprise). He was fine and feeding with five other manatees. Jim cut off his entire tag assembly (belt with tether and transmitter attached). We'll just watch for him to return to the Homosassa River next winter!

"I've sent the latest data for all the manatees, which you can look at below. Be sure to plot these locations on your map to track their latest movements."

Today's Satellite Migration Data
(Courtesy of U.S. Geological Survey's Sirenia Project)


Cathy Beck
Sirenia Project
Gainesville, FL

News Flash: Manatee Mortality at Record Pace in 2000
The Florida Fish and Wildlife Conservation Commission (FWC) just reported that manatee deaths in the first three months of 2000 have already exceeded the number of deaths for the same time in 1999.

According to the latest press release, 100 manatees died in Florida during the first quarter of 2000, compared with 80 during the same time period in 1999. The leading known cause of death was collisions with watercraft, which accounted for 32 of the deaths (a 41 percent increase over the same time period in 1999.) Cause of death could not be determined in 31 of the cases. Also, red tide returned to southwest Florida this year, and scientists believe some manatees have died as a result of exposure to the tide's toxins.

Officials are particularly disturbed with increasing numbers of manatees killed by boats. "There is no question that the manatee population can not sustain this continued increase in the number of human-caused deaths," stated Kipp Frohlich, biological administrator for the FWC's Bureau of Protected Species Management.

As a result of the continued high number of manatee deaths, the FWC will step up law enforcement in key manatee areas. Reduction of watercraft-caused deaths is a critical component to saving the this endangered species.

Challenge Question #16
"How do you think the increased manatee deaths so far in 2000 will affect the manatee population? Why? Are there any other population facts you will need to know to answer this question?" (For guidance, be sure to read Dr. Ackerman's answer to CQ #15 below)

Challenge Question #17
"If the 32 watercraft deaths so far in 2000 are a 41 percent increase over the same time last year, how many watercraft deaths were there by this time last year?"

(To respond to these questions, please follow the instructions below.)

A Meal or a Snack? Discussion of Challenge Question #13
Photo: U.S. Geological Survey, BRD, Sirenia Project
Two classes really "sunk their teeth" into the correct answer to Challenge Question #13, which asked what an alligator would be most interested in-๑"a very large manatee meal or the transmitter tidbit?"

"If I were an alligator, I would be thinking, 'What is that thing floating along?' I would be interested in this tidbit. I would go and try to bite it because I never saw anything like it before. The alligator probably wouldn't go after the manatee, who is swimming below it, but the transmitter floating above water is intriguing." Aziza, Andrew, Brooke, and Sean, Iselin Middle School 7A, (iselin5@injersey.infi.net)

"I think that the alligator would probably go after the little thing floating on top of the water. He would probably try to bite it, and wouldn't be much interested in the manatee floating below it. The little thing is more intriguing and seems like and easy catch." Katie Vernali, (KitKat2V@aol.com)

Photo courtesy of the Birmingham Zoo
Cathy Beck explained just how right these answers were. "We think the alligators mistake the transmitters for prey. Alligators and manatees share much of the same habitat and we have never observed a gator behaving aggressively toward a manatee. Manatees pretty much ignore alligators. We have seen a mother manatee position herself between a large gator and her small calf; but generally the only defensive move we have observed a manatee take is to simply move away. In contrast, the transmitters, bobbing along on the tether about a meter away from the manatee's body, must look like an easy meal. They are grabbed by gators, but not ingested. Once they realize it's not food they let go, but the teeth have by then punctured the housing."

If the gators do not bother manatees, should we humans, measuring about the same length as a manatee, be afraid? According to Cathy Beck, "there really are not a lot of cases of alligators attacking humans - especially when you consider the large number of alligators and people using the same waters in Florida. The cases I am familiar with involve children playing or flailing in shallow water. This likely attracts an alligator's attention and perhaps is interpreted by a gator as a struggling animal (easy prey!). A human appendage (like one of our radio transmitters) splashing in the water could look as attractive as a fish to an alligator, and be easily mistaken for one. Probably the most important thing to remember is that alligators are particularly dangerous during breeding and nesting season (spring and summer) when males are defending territories and females are protecting nests. We humans do occasionally, and unknowingly, blunder into their territory."

Troubled Transmitters: Discussion of Challenge Question #14
Photo: U.S. Geological Survey, BRD, Sirenia Project

In the last report we also asked you to tell us "What reasons can you think of for Brian's tag being "off" (no signals), and Calista's tag being in the same location, no "tips"? By the time of this report, Comet's tag also had stopped transmitting.

Cathy's report today explained what happened to the transmitters, but long before today many of you already had figured out the possible reasons for the transmitter problems:

"I think that maybe Brian's tag could be "off" (not giving signals) because it may have been struck against a hard object, such as a rock or boat or another manatee. Calista's satellite transmitter may have been pulled off and caught on something." Kristen Basiaga, Griswold Middle School, Rocky Hill, CT (Seishigirl13@cs.com)

"The reasons that Calista and Brian have disappeared are: An alligator might have bitten Brian's tag off, like Xoshi's was. He might also have swam through a cluttered area and got it stuck or broken. Calista's tag might have fallen off, or there might be a lot of food, so she is staying in the same area." Ryanne, Allison, Sumara, Iselin Middle School, 7th Grade, (iselin5@injersey.infi.net)

Millennium Manatee Count: Discussion of Challenge Question #15
Dr. Bruce Ackerman
In Challenge Question # 15, we asked: "How do this year's manatee counts differ from previous years? What reasons can you can think of for these different counts?"

As many of you saw, the number of manatees counted this year were less than some of the earlier counts, but more than other counts. Just what does a lower manatee count mean? Are there actually fewer manatees than in 1996 and 1997 and 1999, or do other factors affect the counts?

According to Dr. Ackerman, "the lower count may not necessarily signal a sudden decrease in the size of the population, which we still estimate to be at least 2400." In fact, the number of manatees counted is often affected by the weather conditions at the time of the count. "We expect the counts to vary depending on the weather conditions and the manatee response to cold weather."

Researchers normally want a prolonged cold front followed by a clear, sunny, windless day to conduct the best count. The colder water temperatures cause the manatees to seek out warm water sites, and the warmer air encourages them to float. "The best conditions encourage the manatees to float at the surface, which makes it easier to see and count them", notes Dr. Ackerman.

This year's first count was hampered by the warm weather and only 1,629 manatees were counted. In contrast, the second count recorded 2,222 manatees. Dr. Ackerman explained that "we achieved a higher manatee count than the previous survey probably due to the very cold weather and better survey conditions throughout the state."

Dr Ackerman reports that Manatee scientists are starting to use computer population models to estimate the changes in the size of the population. These models should be better at estimating the actual small changes in the size of the population from year to year, and less subject to differences in weather conditions from day to day. The actual size of the population does not change very fast, depending on how many manatees are born and how many die in each year, but the counts can vary quite a bit depending on the weather.

As you can see, the number of manatees from one count to the next can be the result of many things, and a count can vary by several hundred manatees simply due to weather conditions. Be careful with numbers. They are very exact--but sometimes their meaning may not be so clear.

How to Respond to Today's Challenge Questions

Please answer ONLY ONE question in each e-mail message!:

1. Address an E-mail message to: jn-challenge-manatee@learner.org
2. IMPORTANT: In the Subject Line of your message write: Challenge Question #16 (OR #17)
3. In the body of the EACH message, give your answer to ONE of the questions above.

Coming Next Time!

In our FINAL Manatee Update on April 26, watch for:

  • Final Comments From Cathy Beck & Bruce Ackerman
  • Answers to the Challenge Questions
  • Your Chance to Write to the Scientists
  • Teachers, Share Your Thoughts


The FINAL Manatee Migration Update will Be Posted on April 26, 2000

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