MANATEES IN FLORIDA: 2001

by THOMAS H. FRASER, Ph.D

Prepared under a grant from:
CCA FLORIDA
905 EAST PARK AVENUE
TALLAHASSEE, FLORIDA 32301

March 2001

TABLE OF CONTENTS

Executive Summary ............................................................................................ ii
Introduction ........................................................................................................1
Methods ............................................................................................................ 2
Population Estimates and Trends ........................................................................ 2
Mortality Issues ................................................................................................. 5
Discussion ......................................................................................................... 8
Conclusions ....................................................................................................... 9
Literature Cited .................................................................................................10
Figures 1-16 .....................................................................................................12
Appendix 1 Methods of Analyses ......................................................................28
Appendix 2 Results of Non-Linear Fitted Minimum Count Model .......................30
Appendix 3 Correlations, General Linear and Standard Regression Analyses ......32
Appendix 4 F.A.C. 68A-27.0012 and Biological Criteria ...................................50

EXECUTIVE SUMMARY
The status of the Florida manatee population and the impacts of various types of mortality on manatees have been the subject of much recent debate. This report reviews and evaluates those issues using sources of information from state and federal agencies and from scientific literature. Some of the major conclusions and observations are as follows:

• All of the biological data indicates that the West Indian Manatee in Florida has sustained an increasing population for the last 25 years. The minimum count increase has been at a rate of 6-7% per year. Much of the early increase occurred in the absence of wide-spread speed zone regulations and later (~1991-present) large-area speed zones in some counties.
• A population estimate model was developed using a nonlinear (curve) equation fitted to the "minimum" aerial and other estimates for numbers of manatees by year for 1976 through 2001. The raw data and the model clearly show an increasing manatee population.
• Other site specific evidence supports the general observation that the manatee population is increasing. Research in the Crystal River and Blue Springs areas estimated about a 8.2-9.7% increase per year in counts for 20+ years of records. Tampa Bay, even though it has lost more than 80% of its seagrasses, has also seen a large increase in manatees.
• Increases in the manatee population should be expected to coincide with increasing numbers of dead manatees from all causes including boats over time.
• Mortality data are consistent with an increasing population of manatees. If the natural mortality rate has remained relatively unchanged over the past several decades, then relative change should reflect trends in the general population. If the perinatal mortality (natural death at or near birth) rate per capita has remained relatively unchanged over the past several decades, then relative changes should indicate trends in the population, particularly female
• An examination of per capita manatee deaths more accurately determines relative mortality trend issues than the use of annual numerical totals. Analysis shows that while the annual deaths are increasing, the annual deaths per 100 manatees are not trending up. Additionally, annual perinatal deaths are increasing over time, but the annual deaths per 100 manatees are not trending up.
• Manatees showed a spectacular recovery in Lee County from all causes of mortality, especially the 149 red-tide related deaths in 1996, based on comparative synoptic aerial counts in 1996 and 2001. This suggests that manatees are, and will be, resilient to effects of cold mortality, hurricanes and red-tide events in Southwest Florida. This year's record high statewide count implies that all areas of the State with sustainable populations are also resilient to known causes of episodic mortality.
• No statistical models are needed to show the positive changes with for time minimum manatee population and boater registration, boating related manatee deaths per 1000 registered boats and minimum manatee population per 1000 registered boats. Collinearity between population and boats is clear, even though these two variables are independent of one another. The rate of boating deaths is not changing faster than the number of registrations with respect to the population. The manatee population is growing much faster than the number of boating deaths. When boat-related mortality is adjusted per 100 manatees there is no apparent change with time.
• Increasing boat related deaths of manatees is a complex issue deserving of further analysis. However, the manatee per capita analysis shows a stable, but variable trend, without increasing. Increases in the population have a significant effect on the annual number of deaths. Unfortunately, Florida should expect the increasing annual numeric trend to some extent. This is not a sign of failure to adequately protect manatees, but a positive indication of successful population growth as long as there is no sign of increasing per capita mortality rate. The perceived failure (measured only by total body count) causes a repeating loop for more regulation which must be restructured.
• One serious threat to Florida's manatees which has not been addressed is a risk assessment for a highly infectious disease being acquired as the result of overcrowding at artificial warm water discharges.
• The Florida Fish and Wildlife Conservation Commission has a process (Florida Administrative Code, 68A-27) which lays out the requirements for listing, downlisting or delisting a species along with the biological characteristics for each category: endangered, threatened, and species of special concern. Based on examination of these State criteria, manatees appear to exceed the conditions necessary for reclassification to a species of special concern, or perhaps, to be listed as recovered. Similar action on the federal level is appropriate. Reasons for such consideration include an increasing population for more than 20 years, full occupation of its habitat without significant restriction on forage, water, or reproductive areas, and virtually no real probability of the manatee becoming extinct in the next 100 years due to boat interactions.

INTRODUCTION
The West Indian manatee, Trichechus manatus, has special status as an endangered species in many countries throughout its wide range in the Caribbean islands and coastal zones of tropical and subtropical western Atlantic Ocean. In the United States the Marine Mammal Protection Act and the Endangered Species Act provide the Federal authority to control and restrict human activities that kill or harm manatees. Control of human related mortality has been a major focus for government agencies perhaps because of the uncertain interpretations of existing information relating to various kinds of mortality in the absence of definitive population information.

There is a poor information base concerning the historical status of manatee populations in Florida. Thus, there is no good basis for knowing the population base for the present growth curve. One can speculate that during the Little Ice Age in the northern hemisphere (~1350 to 1860) there were few manatees in Florida (generally too cold in winters, except at natural springs and along the lower southeastern corner for the State). More efficient forms of hunting manatees arrived with the Europeans beginning in the 1500s. Depending on winter temperatures and hunting pressure, this species may have been kept near the level where hunting was not very rewarding. We just do not know. During the Great Depression and World War II use of manatees as food could be expected to result in keeping population low. Unlawful manatee hunting may have declined significantly because of better economic status beginning in the 1950s, likely becoming rare by the mid 1980s.

If Florida's manatee population began a rebound because of lower hunting mortality, (new laws and general peer pressure about intentionally killing manatees by the 1970s), then this population could be expected to have a majority of animals under 30 years of age. Given the minimum counts reported by the State for 1976, about 29% theoretically (if all 738 are still living today) might be older than 25 years. However, years of continuous hunting should have eliminated most of the older age classes. The present age distribution should be weighted heavily to those animals younger than 25 years old. Boat-related manatee mortality has, in effect, supplanted, to some degree, hunting mortality.

The State of Florida has protected manatees since 1892. More recently, the State has embarked on rule making in thirteen Counties where the State deemed deaths of manatees resulting from collisions unacceptable. Rule making began more than 12 years ago and continues to the present by the State in an effort to reduce boat collisions with manatees.

The purpose of this report is to review and evaluate the status of manatees in Florida and how existing information might be used to have the State begin a complete re-assessment of its manatee policies, research and rule making process. The greatest human related mortality is the of result collisions between boats and manatees (Wright et al., 1995). Although the majority of manatee mortality is from natural causes, Florida's regulatory framework has become increasingly restrictive for boaters even as our manatee population has increased. Much of these regulations appears driven by the increasing number of boating deaths and boat use on the water each year.

METHODS
Various manatee data were provided by the Florida Fish and Wildlife Conservation Commission's Florida Marine Research Institute through their CDROM (2000), electronic files and written information. Other information has been taken from literature sources. Boating registration data was provided by the State for each County beginning in 1977. The most recent complete year was 1998. The 1999-2000 year data are not yet available. Relevant data were displayed in graphical format for inspection of possible relationships expected based on the generally perceived ideas of the environmental groups, boating groups, scientists and government regulators.

All data were analyzed using SAS data management, statistical and graphical software, version 8. Other methods are in Appendix 1 and 3.

POPULATION ESTIMATES AND TRENDS
Even though scientists have been studying manatees for more than 50 years (O'Shea, 1988), there is no critical mass of data or models which has been made available by the State that provides a status or general condition of the total manatee population in Florida with some reasonable confidence limits. Some have said that the population is increasing, others that its just better aerial counts of existing manatees that were not counted before. Both groups point toward the largely unknown, unquantifiable confidence limits that restricts the ability to interpret the existing data. Ackerman (1995) provided exponential regressions for each winter at two springs (Crystal River and Blue Springs) and estimated about a 8.2-9.7 % increase per year in counts for the 20+ years of records.

Aerial count and the more recent synoptic aerial counts, because of undercounts (coverage is not completely inclusive and deeper water, residential canal and marinas provide severe visual impediments), provide an understatement of the actual population at any one point in time. These numbers (relative abundance/unit effort) have been an attempt to add information that supports an analysis of population status. No one knows what fraction of the population is being missed. One interpretation given is that better conditions exist during colder winters resulting in higher counts than in years when winters are milder. There are other factors such as water color (transparency), wind speed, time of day that are known to affect counts. One serious impediment is that manatees will spend much of their time below the surface unless ‘sunning' at the surface in warmer, windless conditions. Thus Florida has a series of winter counts that, when taken as a whole, are impossible to begin to interpret in a reliable fashion with the occasional ‘better' count. These ‘better' winter counts (used here) are trending up and should be taken as positive information about the general population and known total mortality (Figure 1). Minimum population numbers are clearly rising faster than total mortality.

The population estimate model used is a nonlinear (curve) equation fitted to the minimum aerial estimate of numbers of manatees by year for 1976 through 2001 (Table 1). Earlier numbers are from Irvine and Campbell (1978) for 1976 and a revised (by State and Federal agreement) number up from Reynolds and Wilcox (1986) for 1985. Ackerman (1995, figs. 6 & 7) used exponential regressions to fit lines to data in Crystal River and Blue Springs which yield a single percentage rate of increase per year for each area. Here, the population growth model was used to generate predicted values for the years in which no survey was taken or for the milder winters during which many more manatees were known not to be counted than during the colder winters.

A non-linear equation was fitted to the observed data in Table 1 (Figure 2). The results of the iterations are given in Appendix 1. In the absence of more complex and dynamic models this equation is used to predict population levels between 1976 and 2001 (Table 2) for use in the boat mortality analysis. The raw data and the model clearly show an increasing manatee population, even if it has the qualification of "minimum population count". This model does not yield a single percentage increase, but rather a faster rate of change (7.2% per year) at the beginning (1976) of the curve, gradually slowing (5.9% per year) at the end of the curve (2001). If this empirical model has captured accurately the momentum of increasing minimum counts, then it may be helpful for predicting minimum population increases over the next 1-2 years while awaiting other model developments (Figure 2).

Other site specific evidence supports the general observation that the manatee population is increasing. Crystal River and Blue Springs have long records of population increases because of the clear water, early and continuous manatee research (see Ackerman, 1995, Figs. 6 &7). These special cases are not discussed here, but support the general trend. Tampa Bay, perhaps the best studied area by the Florida Marine Research Institute, has also seen a large increase in manatees (Figure 3). However, there has been only a small increase in seagrasses in Tampa Bay given that more than 80% has been lost (Lewis, 1986). Manatees generally graze the leaves rather then dig up the rhizomes when leaves are present and may not be limited by the enormous loss of seagrasses in Tampa Bay, except during the winter around the artificial warm water refuges.

Population estimates have problems whether State-wide synoptic counts occur or not. Scientists are unable to state with any confidence population levels or predicted future levels under various management conditions. Various reasons have been documented many times in scientific publications (O'Shea, 1988; Lefebrvre et al., 1995). A number of experiments have been suggested for trying to solve this dilemma.

Manatees in Florida cont'd