Abstract

Natural mortality events can lead to significant declines in the sizes of marine mammal populations.¹ It is critical to consider not only the effects that mortality events have upon entire populations, but also the role they play at subpopulation levels. In August 2001, the Florida Fish and Wildlife Conservation Commission was petitioned to evaluate the listing status of the Florida manatee (Trichechus manatus latirostris) to determine whether or not the manatee should be listed as endangered at the state level. A biological status review of the Florida manatee was conducted in light of the State of Florida's listing criteria.² To assess the possibility of a future decline in the population and the probability of extinction, we conducted a Population Viability Analysis using VORTEX, a readily available software package. A scenario was developed to model population dynamics with all of the life-history input parameters stable at currently known levels. This scenario was run a second time but included the occurrence of catastrophes. Catastrophes are defined as large, rare events that can greatly reduce the reproductive and age-specific survival rates of a population. Based on historical events for manatees and other marine mammal species, the potential catastrophes that we included in the scenario were disease outbreaks, red tide, and extremely cold winters. Input parameters for catastrophes included the estimated frequency of the event, an estimate of the reduction in survival rate caused by the event, and an estimate of the decline in reproduction due to the event. The effects of catastrophes were limited to the year of the event, with no residual effects on survival or reproduction. The four recognized subpopulations (Upper St. Johns, Atlantic, Northwest, and Southwest)³ were subjected to their own suite of potential catastrophes based on regional events (i.e., red tide mortality reported only on the west coast of Florida and cold stress primarily affecting juvenile animals in the Atlantic and Southwest subpopulations), with input parameters appropriate for each region (Table 1).

Table 1. Catastrophes and input parameters used in modeling manatee population dynamics.

For the population as a whole, adding catastrophes increased the probability of a >10% population decline in manatees over the next 45 years from 0 to 9.8% and increased the probability of >20% population decline over the next 45 years from 0 to 4.7%. However, when examining the subpopulations separately, the Southwest subpopulation appears to be particularly vulnerable to catastrophes. The probability of a >20% decline in the Southwest sub-population in the next 45 years increased from 61.2% to 94.4%; the probability of a >50% decline increased from 16.8 to 69.7%; and the probability of a >80% decline increased from 0.1 to 5.5% (Table 2). When evaluating the Atlantic, Upper St. Johns, and Northwest sub-populations, there was little effect of catastrophes on population decline compared to the same scenario without catastrophes. The Southwest subpopulation was the only one considered to be vulnerable to mortality related to both cold stress and red tides; it has suffered the highest mortality rates due to red tide. The probability of a 3.6% likelihood of massive mortality due to red tide was based on 30 years of monitoring and a single large mortality event (in 1996, when 149 animals are suspected to have died from red tide poisoning). Since then, elevated mortalities also occurred during 2002-2003, when 130 animals are suspected to have died due to brevetoxin exposure. These scenarios may have underestimated the probability of occurrence of red tide mortality events and hence, underestimated the impact on the Southwest subpopulation.

Had the population been evaluated only as a whole and not at the subpopulation level, the role and effect that catastrophes have on the Florida manatee Southwest subpopulation would not have been apparent. Modeling the possibility of unusual mortality events related to disease, red tide, and cold stress in the Florida manatee is important to understanding the factors that could potentially affect the survival of the species.

Table 2. Probability of a level of population decline over a specified time frame in the entire manatee population and in the Southwest subpopulation, with and without catastrophes.

Acknowledgments

Thanks to the many staff members involved in producing the FWC Biological Status Review and to Judy Leiby for improvements to this abstract. This work was entirely supported by the state of Florida's Save the Manatee Trust Fund.

References

1.  Harwood J, A Hall. 1990. Mass mortality in marine mammals: its implications for population dynamics and genetics. Trends in Ecology and Evolution 5: 254-257.

2.  Florida Fish and Wildlife Conservation Commission (FWC) 2003. Final Biological Status Review of the Florida Manatee (Trichechus manatus latirostris). Report by FWC Florida Marine Research Institute. St. Petersburg, FL.

3.  U.S. Fish and Wildlife Service. 2001. Florida Manatee Recovery Plan (Trichechus manatus latirostris), Third Revision. U.S. Fish and Wildlife Service. Atlanta, Georgia. 144 pp. + appendices.