Most of us have seen a raccoon or deer carcass along a busy highway. But terrestrial species are not the only ones threatened by collisions with speeding vehicles.
In Florida, being killed by a boat is the most common way for endangered manatees to meet their end. Though Florida officials have no verified estimate for the total number of manatees currently roaming the state’s waters, around 41 percent of recovered manatee bodies are the victims of watercraft collisions. These numbers, however, most likely underestimate the problem since not all boat-slain manatees turn up.
In order to best protect a manatee’s contact with boats, wildlife managers need to know where both the animals and the boaters spend most of their time. In the past, researchers have undertaken such studies, but they often use uncorrected animal counts, or those based on simple observations scientists make in the field. The problem with this strategy, however, is that biologists can easily miss an animal—even one as big as a manatee—resulting in potentially skewed data that may lead protective measures astray.
To get around this issue, a team of researchers in Florida set out to map manatees and boats with a variety of methods. They conducted nine aerial surveys of the waters surrounding Collier Country, Florida, noting both boats and manatees. Though the 12-foot long, 1,800-pound animals aren’t too difficult to spot floating about in Florida’s clear waters, stormy weather, murky water or plants can sometimes obscure the animals from view. To make up for possible flaws in the observers’ manatee-spotting skills, the researchers applied occupancy models, a way to statistically estimate the probability of an animal’s presence versus the chance of someone actually spotting it.
Observations and stats in hand, the scientists used their findings to build a computer model that predicts the distribution of both manatees and boats, highlighting the areas at highest risk for potential collisions. To make their model as robust as possible, they included known manatee-preferred factors, determined from previous studies, such as warmer water temperatures and proximity to sea grass beds—manatee food. They also accounted for seasonality, weather and proximity to human developments.
“We modeled the distribution of Florida manatees and boats as a function of environmental variables,” explained Julien Martin, a research scientist with the Florida Fish and Wildlife Research Institute in St. Petersburg, Florida. “For instance, our model can estimate the probability that manatees are present as a function of distance to sea grass.”
The results revealed that sea grass did indeed serve as one of the most important predictive factors for where manatees turn up. The warmth-loving creatures also strongly avoided cold waters. Some findings, however, were less obvious. Although the researchers originally suspected that manatees would avoid human developments, the opposite turned out to be true. Manatees tended to approach developments, perhaps because sea grass beds often grow nearby.
With this data, Martin and his colleagues produced a map of the Florida coastline with a gradient of overlaid risk zones and published these results in the journal Biological Conservation.
Of course, no model perfectly reflects the real world, and the researchers acknowledge that their data only estimate risk. The more aerial survey counts of manatees that are loaded into the model, however, the more accurate it will become. Martin also hopes to create updated maps based upon manatee density rather than simple individual animal presence.
Identifying risk zones gives wildlife managers the information they need to implement manatee speed zones, or protected areas where strictly enforced speed limits aim to allot both boaters and manatees more time to spot one another and get out of the way. The model can also make predictions about areas not yet surveyed, giving wildlife managers a heads-up on possible danger zones that may require a closer look. The researchers think their new method could be applied to other species too, whether on land or in the sea.
“Quantifying and better understanding factors that affect manatees and boat distribution can help us identify areas where manatees and boats are most likely to co-occur, and may ultimately be useful for designing protection zones,” Martin said.