Characterizing Manatee habitat use and seagrass grazing in Florida and Puerto Rico: implications for conservation and management

Abstract

The Indian River Lagoon on the Atlantic coast of Florida, USA, and the east coast of Puerto Rico provide contrasting environments in which the endangered West Indian Manatee Trichechus manatus experiences different thermal regimes and seagrass communities. We compare Manatee feeding behaviour in these two regions, examine the ecological effects of Manatee grazing on a seagrass community in the Indian River Lagoon, describe the utility of aerial surveys, radio tracking, and seagrass mapping to study Manatee feeding patterns, and develop hypotheses on sirenian feeding strategies in temperate and tropical seagrass communities. In both the Indian River Lagoon and Puerto Rico, Manatees were typically observed grazing in water depths = 2.0 m and more frequently on the most abundant seagrasses present in the community: Halodule wrightii in the Indian River Lagoon and Thalassia testudinum in eastern Puerto Rico. Where both H. wrightii and Syringodium filiforme were consumed in the Indian River Lagoon, Manatees tended to remove more S. filiforme than H. wrightii rhizome + root biomass. Even though 80 to 95% of the short-shoot biomass and 50 to 67% of the rhizome + root biomass were removed, grazed patches of H. wrightii and S. filiforme recovered significantly between February and August. H. wrightii may be both more resistant and resilient than S. filiforme to the impacts of Manatee grazing. Despite the significantly greater abundance of T. testudinum in Puerto Rico, Manatees exhibited selective feeding by returning to specific sites with abundant H. wrightii. They also appeared to feed selectively on T. testudinum shoots associated with clumps of the calcareous alga Halimeda opuntia. We hypothesize that Florida Manatees are less specialized seagrass grazers than Manatees in tropical regions like Puerto Rico. Continued research on Manatee grazing ecology in temperate to tropical seagrass communities will enable better protection and management of these vital and unique marine resources.

Feeding preferences of West Indian manatees in Florida, Belize, and Puerto Rico as indicated by stable isotope analysis

“…As in the Belize analysis, it is possible that differences in coastal development, pollution, and agriculture had an effect on nitrogen input in the surrounding waters. The stable isotope results for freeranging manatees agree with the hypothesis of Lefebvre et al (2000) that due to thermally driven seasonal migrations, Florida manatees are less specialized grazers than manatees in Puerto Rico, for which seagrasses are the main diet component available.…”

“…Alves-Stanley & Worthy (2009) calculated a mean half-life for 13 C turnover of 53 d, while mean half-life for 15 N turnover ranged from 65 to 72 d. Free-ranging manatees are known to switch diet sources (Best 1981, Lefebvre et al 2000, and it is likely that as a result of these long turnover times, an intermediate isotope signature may be measured rather than one strictly representing a more recent diet. Rather than attempting to predict manatee diet composition involving all 3 aquatic plant types, it was thought to be more useful and more accurate to focus on the use of freshwater plants versus seagrasses.…”

Carbon and nitrogen stable isotope turnover rates and diet–tissue discrimination in Florida manatees (Trichechus manatus latirostris)

“…While this possibility is true for all stable isotope analyses, the impact is minimized in tissues with high turnover rates because the time frame is short. Free-ranging manatees are known to switch diet sources (Best, 1981;Lefebvre et al, 2000), and the very slow turnover rates for carbon and nitrogen stable isotopes in epidermis tissue mean that unless the manatee has been feeding on the same diet for an extended period of time, the skin signature will always be in some transitional state. Slow turnover rates in manatee skin especially complicate estimation of the proportions of freshwater, estuarine and marine sources in the diet because δ Computing a precise diet-tissue discrimination value is essential when interpreting isotopic results.…”

Modeling habitat and bycatch risk for dugongs in Sabah, Malaysia

“…Defining the overlap between key habitats and fisheries threats has been one of the most important topics of marine conservation research (Lefebvre et al 2000). While dugongs frequently occur in shallow coastal waters, they have also been observed in deeper waters further offshore, where the continental shelf is wide and remains relatively shallow and protected (Rajamani 2009).…”