Manatee Technical Advisory Council (MTAC) Recommendations Workshop on Manatee Population Status

Manatee Technical Advisory Council (MTAC) Recommendations Workshop
on Manatee Population Status
July 25-26, 2001

Background Information

Understanding the stability of the manatee population in Florida is fundamental to the development of sound management strategies. A stable or increasing population is also the key criterion for recovery under the requirements of the Endangered Species Act. "How is the manatee population doing?" "How many are there?" "Are the efforts to protect them appropriate?" Virtually all interest groups involved in manatee protection efforts, from scientists and policy makers to boating and wildlife enthusiasts, have asked these questions. Although the questions seem relatively simple and straightforward, the scientific process of finding the answers and the results of that process are not. Paradoxically, readily available statistics from aerial survey counts and carcass salvage have contributed to both general awareness and controversy about the status of manatees in Florida.

The Manatee Technical Advisory Council (MTAC) conducted a public workshop on July 25-26, 2001, to examine scientific information related to manatee population status. The purpose of the workshop was to provide an open forum for discussion of differing methods and measures of population status, identify potential areas of scientific consensus, and develop recommendations for further research and study. It is important to note that the MTAC workshop was not intended to determine the status of manatees or provide a comprehensive review of population biology. These issues are expected to be addressed in a scientific workshop on Population Ecology and Management in April 2002.

Summary of Information Presented

The MTAC workshop included several presentations involving analysis of winter aerial survey counts of living animals and carcass counts. Raw results of both types of counts are variable, but have generally increased statewide since the mid-1970’s. Four regional subpopulations have been defined: Northwest (including the Crystal River area), upper St. Johns (including the Blue Springs area), Atlantic Coast, and Southwest. Patterns for these subpopulations of manatees in Florida are more complex. However, most scientists agree that relying upon these uncorrected data for estimating population growth is problematic. Winter aerial counts, particularly statewide synoptic (or near synoptic) surveys conducted since 1990 in areas most likely used by manatees are commonly reported or understood by casual observers as a “census” of manatee population. However, these counts reflect a subset of the total population that is detectable to observers in the areas surveyed. Such surveys represent a minimum count of the number of individual animals visible in the survey area, but unfortunately it is not possible to determine in a reliable way what fraction of the true population has been observed or is present at the survey site. Efforts to validate or calibrate the counts show that the results are affected to an unpredictable degree by a host of uncontrolled and site-specific variables, including weather prior to and during the count, water temperature and clarity, date and time of day, survey techniques, observers, and manatee behavior and movements. Counts can vary dramatically over a period of days, due to factors unrelated to actual changes in population. This variability increases uncertainty associated with mathematical and statistical trend analysis of the aerial counts. Early aerial counts from the 1970’s and 1980’s focused upon known manatee aggregation areas, primarily power plant warm-water discharges and natural springs. During the past decade, statewide synoptic aerial survey methods have been refined in terms of coverage, technique, and selection of survey date in hope of optimizing counting conditions and detectability. The changes in survey method are also a confounding variable that may account for a portion of the apparent increasing “trend” in statewide manatee counts, particularly when uncorrected low results from nonsystematic surveys in the 1970’s are included with highest annual counts from the 1990’s. Long-term aerial surveys from selected index sites, such as natural springs and power plants, also exhibit variability but may be more amenable to validation or correction through experimental or statistical methods.

Annual carcass count is also an observational index, not an actual mortality rate (although it is often described as “mortality rate” in technical and popular media). Carcass reporting and recovery may have improved over the period of record or vary geographically, contributing in an unknown way to apparent patterns and trends. A great deal of knowledge on human-related and natural causes of death, as well as basic research on physiology and anatomy, is based upon assessment of recovered carcasses. Analysis of the rate of increase in numbers of carcasses associated with specific causes of death or geographic area may be a more useful measure for management decisions than raw carcass counts alone. However, using carcass counts in combination with aerial survey counts to calculate “mortality rate” compounds uncertainty associated with each data set and produces highly hypothetical results, at best.

There is general agreement among experts that carcass and aerial counts provide useful information about distribution of animals, habitat values, relative abundance of adults and calves, and human-related threats, but that other scientific approaches, such as “mark-recapture” techniques, should be applied to estimate population growth rate. Other vital measures, such as survival rates, age structure of the living population and reproduction rates are also required to assess the long-term viability or stability of manatees in Florida. Adult survival has been estimated based upon recognition and resighting of individuals uniquely “marked” in the wild by prop scars and other wounds. This approach does not provide an estimate of population, since the proportion of animals that is scarred and recognized is not known, and it does not produce instantaneous or even annual feedback to managers, since data must be accumulated and analyzed over an extended period. However, population growth models have been developed using survival and reproduction data from living, known animals, including a large number of animals that have been documented by photoidentification. Estimates derived from observation of known animals are assumed to apply to the entire population. This method seems most reliable in areas of Florida, such as Crystal River and Blue Springs, where a large portion of the animals are known, readily recognized, and have been observed over an extended period of time. Similar information is available for animals on the Atlantic Coast, although the portion of the population that is accessible for monitoring and photoidentification is lower and the habitat, as well as the animal movements, appear more complex. Photoidentification and calculation of critical vital measures is incomplete at this time for the southwest coast.

Conclusions

Each individual dataset or method of analysis has associated with it some degree of uncertainty. This is typical of most scientific data, and does not necessarily mean that some results or sources of information about manatee abundance and distribution should be ignored or rejected. The State of Florida is obligated to make resource management decisions on a daily basis, using the best scientific information available. Particularly when the resource that is being managed is a protected or listed species, it is not possible or ethical to wait for datasets or research to be perfected. Rather, it is important to understand the strengths, weaknesses, and limitations of available information, and use multiple lines of evidence to reduce uncertainty in conclusions. This understanding is also important for defining research priorities and data needs that could help to reduce uncertainty. In addition to this central conclusion, MTAC offers the following observations related to the scientific determination of manatee population status.

Using a combination of data types and models for assessing viability of manatees in Florida, it appears that populations in the northwest (Crystal River area) and Blue Spring have been increasing over the past decades. Estimated rate of increase of the population exceeds rate of increase in deaths, and aerial counts, adult survival, and reproduction measures also suggest a stable or growing population. However, these two regions represent less than 16% of the statewide count and are not representative of the rest of the state. This may be related to a long history of active protection and relatively fewer sources of human impact, which is reflected by a relatively lower number of manatee deaths than elsewhere in the state.
The status of manatees on the Atlantic Coast and Southwest Coast is less certain, in part because adequate information has not been developed to apply a “weight of evidence” approach. This is significant from a management perspective, since these two groups together account for approximately 84% of the statewide count.
Aerial counts of one index site on the Southwest Coast indicated little change in numbers of animals. Lack of adequate photoidentification prevents development of other vital measures at this time for this area, although large numbers of deaths from all causes is a potential concern.
On the Atlantic Coast, some models based upon aerial counts at index sites suggested population growth in the 80’s, with a leveling or even a decrease in the 90’s. Vital measures of adult survival and reproduction based upon photoidentification suggest a very low or no population growth on the Atlantic Coast. Also, the rate of increase of deaths exceeds the most optimistic estimates of population growth, a signal for concern about long-term stability.
Aerial counts are most useful for understanding relative distribution of manatees and habitat values. A great deal of progress has been made to optimize statewide counts and understand sources of variability. However, confounding variables unrelated to actual population size make it impossible to use these counts as an accurate statewide census. Although a statutory mandate requires an annual “scientific benchmark census” and report to the Legislature, the Governor and Cabinet, it should be made clear in this report that the uncorrected aerial count is a survey and not an accurate measure of the population.
Additional effort should be made to calibrate or validate aerial counting techniques, using experimental or statistical approaches, particularly for the Atlantic and Southwest regions where status of the populations is less certain.
Carcass recovery and analysis is critical for understanding causes of manatee death, basic manatee biology, and has provided information needed for some population models. Rate of increase of deaths or proportion of deaths, by cause of death or geographic region, may be more helpful for management decisions than numbers of carcasses alone.
Even if it becomes possible to determine the actual numbers of manatees in Florida, this figure alone will not be adequate to assess sustainability or stability of trends. Determination of vital statistics, such as survival rates, reproduction rates, age and structure of the living population are most important for evaluating the long-term stability of manatees in Florida. Research and monitoring efforts should be directed at refining such measures, in addition to aerial counts and carcass salvage.
The use of photoidentification as a surrogate for “mark-recapture” methods of estimating survival should be expanded and refined, especially in the southwest and Atlantic regions. Continued use of Passive Integrated Transponder (PIT) tags for rescued and released animals is also recommended. However, it is recognized that extensive tagging and recapture of wild animals, especially in areas where the population is dispersed over a large or remote geographic area, is subject to regulation and would present significant logistical, cost, and technological problems.
The scientific community will undertake a comprehensive review of these issues at its upcoming Population Ecology and Management Workshop in April 2002. This conference is expected to update the analysis of existing data, including vital measures described above, and provide perspective on the status of the manatee population. Significant management decisions should await the outcome of this scientific process.

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