New aerial survey and hierarchical model to estimate manatee abundance

Abstract

A stochastic, stage-based population model was developed to describe the life history and forecast the population dynamics of the Florida manatee (Trichechus manatus latirostris) in four separate regions of Florida. This population model includes annual variability in survival and reproductive rates, demographic stochasticity, effects of changes in warm-water capacity, and catastrophes. Further, the model explicitly accounts for uncertainty in parameter estimates. This model is meant to serve as a flexible tool for use in assessments relevant to management decision making, and was used in the State of Florida's recent biological status review. The parameter estimates and model structure described herein reflect our understanding of manatee demography at the time that this status review was completed. In the Northwest and Upper St. Johns regions, the model predicts that the populations will increase over time until warm-water capacity is reached, at which point growth will taper off. In the Atlantic region, the model predicts a stable or slightly increasing population over the next decade or so, and then a decrease as industrial warm-water capacity is lost. In the Southwest region, the model predicts a decline over time, driven by high annual mortality in the short-term and exacerbated by loss of industrial warmwater winter refuges over the next 40 years. Statewide, the likelihood of a 50% or greater decline in three manatee generations was 12%; the likelihood of a 20% or greater decline in two generations was 56%. These declines are largely driven by the anticipated loss of warm-water capacity, especially in the Atlantic and Southwest regions. The estimates of probability of extinction within 100 years were 11.9% for the Southwest region, 0.6% for the Northwest, 0.04% for the Atlantic, and <0.02% for the Upper St. Johns. The estimated probability that the statewide population will fall below 1000 animals within 100 years was 2.3%. Thus, while the estimated probability of extinction is low, the model predicts that current and emerging threats are likely to result in a long-term decline in the statewide population and a change in the regional distribution of manatees. Analyses of sensitivity and variance contribution highlight the importance of reducing uncertainty in some life-history parameters, particularly adult survival, temporal variance of adult survival, and long-term warm-water capacity. This core biological model is expected to evolve over time, as better information becomes available about manatees and their habitat, and as new assessment needs arise. We anticipate that this core model will be customized for other state and federal assessments in the near future.

Pilot aerial study of the marine megafauna in northern coast Alagoas, Brazil

“…Antillean manatees are difficult to sight due to the fact that these animals spent a large amount of time under water (REYNOLDS; POWELL 2002) and are therefore more easily detected when in groups, although the precise count of individuals is more difficult (LANGTIMM et al, 2011). Aerial surveys were used in Florida to estimate the quantity of Trichechus manatus latirostris (HARLAN, 1824) in the region.…”

“…Aerial surveys within estuaries are inefficacious due to frequent turns and circuitous flight, path required to sample the irregular-shaped estuarine water bodies, and mainly to the turbidity of the water. This irregular-shaped turns difficult to consistently maintain a formal transect protocol (LANGTIMM et al, 2011). However, there is a considerable need to monitor species that use these ecosystems, such as T. m. manatus (LIMA et al, 2011).…”

“…In cases of dubious sightings or imprecise counts of individuals, recounts were performed (LANGTIMM et al, 2011) through circular flights over the location of the sight and imprecise records were discarded. A group was defined as two or more animals (MORALES-VELA et al, 2000).…”

“…Aerial surveys have been used throughout the world to estimate the distribution and abundance of aquatic animals, the most studied of which are sea turtles and marine mammals (MCCLELLAN, 1996;PREEN et al, 1997;ROOS et al, 2005;LANGTIMM et al, 2011). However, such surveys are rare in Brazil and mainly restricted to the southern region of the country (SECCHI et al, 2001;DANILEWICZ et al, 2010;ZERBINI et al, 2010;ZERBINI et al, 2011).…”

Structured decision making as a proactive approach to dealing with sea level rise in Florida

“…In the case of SLR in Florida it is easy to envision such examples. For instance, the increase in sea level may lead manatees to shift their distribution, which could in turn affect detectability (e.g., because the water in the new habitats may have different levels of turbidity, see Edwards et al 2007;Langtimm et al 2011). Therefore, monitoring programs should be revised appropriately to account for these two primary sources of variations in the monitoring data (Yoccoz et al 2001).…”