Density and abundance estimate of Antillean manatees (Trichechus manatus) within a landlocked lake in Southeastern México

Abstract

Manatees are threatened along their range.  In México, this species is listed as endangered.  Manatee conservation strategies require density or occupancy estimates, especially in areas where species face survival risks.  On turbid waters, like those found in rivers and lakes of the Southern Gulf of México coast, visual methods used to detect and count manatees underestimate actual numbers.  Our goal was to estimate the density and abundance (N) of Antillean manatees in a small State Natural Protected Area, where a manatee population of unknown size inhabits.  We performed line transects using a side scan sonar to detect animals and mark-recapture in the isolated population of manatees within Laguna de las Ilusiones, México, a landlocked lake that excludes transit.  Using distance sampling from 14 boat trips, estimates of density and abundance were 15.5 manatees km-2 and 27 ± 5 manatees in the lake (CV ≈ 16.6 %).  With MARK, from six capture events and 19 individual encounter records, the abundance estimated was 24 manatees (CV ≈ 16 to 24 %).  Previous number of manatees were based only on visual surveys, which reported at least seven manatees.  Density is lower than other similar studies along narrow waterways in important areas in México and other Central and South America countries.  Studying this endangered subspecies is limited by cryptic habits, turbid waters, poor funding, and low densities, making density or abundance estimates difficult.  However, within particular areas and established monitoring areas, these methods could be useful to generate baselines for conservation strategies.