Survival rate estimates of Florida manatees (Trichechus manatus latirostris) using carcass recovery data

Abstract

The Florida manatee (Trichechus manatus latirostris) is classified as an endangered species and is also protected under the Marine Mammal Protection Act. The manatees' coastal distribution coincides with areas of high human density, making manatees particularly vulnerable to human impacts. Important management decisions on both the state and federal level rely heavily on extinction models that require estimates of survival and reproductive rates. Current mark-recapture methods are unable to estimate survival rates for younger age classes because many young manatees lack the unique scarring used to identify individuals. Using an age-ratio technique, this research utilizes carcass recovery data to provide estimates of survival rates of Florida manatees < 4.5 years old while quantifying and incorporating all sources of uncertainty. The age-ratio technique requires counts of carcasses in each age class, carcass detection probabilities, and the proportion of animals alive in each age class. In order to determine the count of carcasses in each age class, several models were designed to estimate carcass age at death.

First, using physiological criteria, models were developed to determine the probability a carcass died within two weeks of birth (perinatal) based on carcass length and month of carcass recovery. Next, mathematical models were created to estimate the error in ear-bone aging techniques. Lastly, models were developed to estimate age from length for carcasses dying at ages older than two weeks old. With these models, age at death could be estimated for 98% of all collected carcasses. Using data from a region where all survival and reproductive rates are known (Upper St. Johns), carcass relative detection ratios by age class were calculated. Assuming relative detection ratios are the same for all regions, young survival rate estimates were then calculated for all other regions. Results show uncertainty in young survival rates is higher than what was previously assumed.