The manatee of Florida, originally called "the sea cow", is a placid slow moving wild animal, except at mating time. Manatee are not territorial, but are free- roaming, sometimes over great distances. The areas originally occupied by the manatee included arms of the ocean, rivers and springs, covering essentially all of Florida, and at times the waters of other states. They are sensitive to cold, so move to southern waters, to spring runs, and to underwater springs during the winter months.
Humans have invaded essentially all of the original manatee areas by filling waters to get cheap construction sites, by destroying the grass beds which provide feed, directly or by pollution, and by boats moving over the water areas. As a result, there are encounters between the species. Originally, this was hunting for food, to the point that the species was becoming rare. Encounters between the boats of humans and manatee increased as the number of boats multiplied and their average speed increased, and is now the largest form which exists.
To borrow some nomenclature, the present form of encounter can be classified as the first, second and third kinds, all including the term, "close encounter" and all being between a manatee and a human directed boat. For the first kind, there is no physical contact between the two. For the second the encounter involves appendages to the boat, the propeller(s), rudder and other sharp objects if present. The third close encounter involves actual impact between the boat hull and a manatee. Let us look at some factors of these.
Very little seems to be known about the first kind of encounter. Power boater operators often claim that have never seen a manatee. Actually this is difficult from a fast moving boat, partly because the manatee is submerged most of the time, and the short duration swirl at surfacing for breathing is not large. Then too, the helmsman's attention is not on where the boat is, but on where it is going to be, a requirement for safe navigation. Vessels under sail, canoeists and rowers do report sightings, but they also report that there are so many high speed boats that they have given up this mode of boating.
There are two possible effects of close encounters, again with little known. Wild animals are observed to spook and flee in fright. No studies, accounts or even estimates of this in manatee or its effect have been found. There is an appreciable "neo-natal mortality", the death by abortion. Beyond annual number counts, there is no data on causes. It seems reasonable that some fraction could be from fright induced abortion.
Practically every adult manatee shows a pattern of cuts or healed cuts resulting from an encounter of the second kind with a propellor, so encounters must be a common phenomena. Surprisingly, it is not possible to determine the size of boat or the speed with which it was moving from the cut pattern. This is due to the hydrodynamic laws which govern propellor design and use. A propellor rotating too fast causes the pressure on its face to drop to the point the water vaporizes, called "cavitation". Propulsion efficiency is lost and there may be damage to the metal, causing further loss. The larger propellor of a big boat must turn slower than for a small one to keep the tip speed below the cavitation point.. Further, a propellor is a section of a screw, and the pitch or the distance between threads does not change as the rotation speed or "speed of advance" changes. A big propellor could cut deeper, but without surgery it is not possible to determine depth: a shallow cut could come from a very close encounter with a small boat, of a more distant one with a large boat.
There have been many proposal to stop propellor cuts by putting a shroud just larger than the propellor diameter around it. These neglect a design factor: to maintain efficiency, the shroud must be thin with a fairly sharp leading edge. The shroud would cause change from a series of cuts to a gouge, the width of the cuts plus a little, and as long as the pattern of cuts. The trauma to the manatee would be much greater with a shroud.
The only known way to eliminate the cuts is to place the propellor in a tunnel, a technique developed in the 1920-30 period. A jet ski does precisely this, but the other types of encounter remain. The only saving factor is that the manatee seems to have a remarkable healing ability.
For close encounters of the third kind, the effect varies with the weight and speed of the boat. Very slow speed of a small boat may amount to nothing more than a nudge. A heavier impact can produce a bruise, still heavier torn tissue and bruises on internal organs. A real heavy impact can cause almost instantaneous death by damage and shock. For a given size boat, the impact energy varies with the square of the speed. Going from 5 to 35 mph increases the impact by almost 50 times. But slowing from 35 to 25 mph just cuts the energy in half, helpful but not really a correction.
There seems to be no data on the effects on the manatee, but there is considerable research relating to humans, often with the pig as a test subject. The summary statement is, speed kills.
Reasonably good data on deaths by manatee have been available since 1976. The trend has been increasing, as shown by statistical analysis, and is now just under 400 in a year. The rate of increase can only be called alarming. If present trends continue, it will reach 500 per year in 2022, and 700 per year in 2052. At this time (Jan 2004) there is no action to change any factor in the death rate.
Deaths directly attributable to boats have also been increasing and has reached just under 100 per year. If the present rate continues, it will reach 125 in 2020, and over 200 in 2050. There has also been an increase in perinatal deaths, and from other human related factors, possibly hunting, and damage by locks in drainage canals.
The number of manatee alive is not precisely known. Surveys are made from aircraft, but not of all state waters. The manatee are difficult to spot, false counts may result from such items as beds of grass, and the count varies as much as 30% from year to year: observer training is a factor in accuracy, as is weather and probably water temperatures. Between 1991 and 2003 the January count appears to indicate an increase, from a low of 1269 in 1991 to a high of 3276 in 2003. Discarding the early period as resulting from less well trained observers, for the period 1995 to 2003 the count increased from 1443 to 3029, an apparent increase of 200 per year. However, the calculated probable error of the count for this last period is +/- 517 per year. Because of this large error and the large difference between successive counts, it does not seem to be safe to conclude that the manatee population is recovering.
However, there is another element to consider. The pattern of encounters between boats and manatee has parallels to encounters between two armies (except that manatees do not attack boats). One approach is known as Lanchester's equations: basically these assume two existing populations, each with a death rate and a birth rate, where the death rate can be the sum of natural and "enemy" processes. The model is simple to set up, but like all models must be calibrated against data from experience. Some typical assumptions lead to the following estimates:
- With existing populations, manatee birth rate and a continuation of the current increase in boat numbers, but no other change, the manatee eventually die or are killed off.
- A small reduction in manatee death rate will allow an initial recovery, but the increase in boats soon dominates, and the manatee die off.
- A major reduction in manatee death rate is needed to allow recovery of the manatee species.
There are possible approaches to preserving the manatee. One is to increase the birth rate, which would require converting the manatee from a wild species to a domestic animal by pens, artificial feeding, veterinary care, etc. However, there is no assurance that the birth rate would increase, although the death rate would probably be reduced.
If the manatee is to be left wild, the only possibility is to reduce the number of encounters, and reduction in the results of an encounter. Again the military experience offers guidance, in the form of "sweep rate" developed to analyze the factors relating search by ship and aircraft against submarines. Since sweep rate findings increase with the speed of the search device, reducing the speed of boats will reduce the number of encounters. At the same time the reduced speed will reduce the effect of an encounter, particularly those of the third kind.
An objection to lower speed is the difficulty of measuring speed on the water. Over a range of speeds, this objection is without foundation. Boats operate in two regimes: one is called displacement, where the both moves by pushing the water aside, where the speed is less than 1.5 times the square root of boat length. The second is called planing, riding on top of the water surface, which occurs above about 1.5 times the maximum displacement speed. Fuel consumption is markedly less in the displacement regime. Both the appearance of the boat on the water and the wake it forms clearly indicate the regime.
Accordingly, preserving the manatee will require one of two steps, or both:
- Reduce the number of boats,
-Reduce the speed of boats: the indication is that all boats should operate in the displacement regime, which was the situation until about 1920-1930.
Both would have other benefits, including reduction in the number of boat related human deaths, and in the importation of fuel.
Both should apply to areas used by manatee for residence or for traveling.