How to detect an elusive aquatic mammal in complex environments? A study of the Endangered Antillean manateeTrichechus manatus manatusin French Guiana

Abstract

The Antillean manatee Trichechus manatus manatus is a cryptic mammal that inhabits, among other areas, murky rivers and estuaries of Central and South America. The difficulty in detecting and counting manatees is an obstacle to monitoring their population status, as traditional count sampling is unsuitable. We conducted a quantitative analysis of the distribution and abundance of the manatee across its known range in the rivers and estuaries of French Guiana, based on data from a range-wide line transect visual survey combined with a side-scan sonar survey, identification of feeding sites, and silent observations at fixed points. A total effective effort of 248 hours and 1,129 km of boat survey was completed. We used two relative abundance indices: the combined encounter rate, which combines encounters per km from either visual or side-scan sonar surveys, and the global detection index, which is defined as the sum of all evidence per unit time. Manatees were detected in all nine study units. Niche modelling was used to analyse the space selection by manatees, and helped to detect differential use of habitats according to the season. The model predicted that coastal areas are used more extensively during the dry season. In the absence of better techniques to detect wild manatees in complex habitats this monitoring protocol may be relevant and replicable in hydrological systems where manatee detectability is constrained by biogeographical characteristics.

Abundance of manatees in Panama estimated from side‐scan sonar

“…Like any counting estimate, it depends on detectability, but we found the large, slow‐moving animals easily visible in the sonar image. By calculating density only in the 80‐m‐wide core of the line‐transects, we corrected for reduced detectability beyond 40 m. Still, we have not made independent counts to assess detection rate, so we cannot be precise about how many were missed within 40 m. Prior work compared sonar detections of manatees with visual observations (Rodas‐Trejo et al , Brice , Castelblanco‐Martínez et al ), and Gonzalez‐Socoloske and Olivera‐Gómez () found that sonar identified >80% of animals that were seen. None of those observations, however, led to abundance estimates.…”

“…The sonar device is costly—in the range of US$20,000—but once purchased, surveys can be finished for the cost of gasoline and thus repeated across seasons and years, allowing tests of population trends and seasonality. On the other hand, the unknown detection rate and difficulty with floating vegetation must be recognized as limitations to sonar (Gonzalez‐Socoloske et al , Brice , Castelblanco‐Martínez et al ), and aerial surveys are more efficient at covering large areas. Long‐term conservation planning would thus benefit by wider counts from the air to establish manatee distribution across Panamanian wetlands, combined with more detailed local studies with active sonar, satellite tags, and perhaps new methods based on individual recognition of vocalizations (Castro et al ), a method we are currently testing in the Changuinola River.…”

“…Scarcity of such estimates can be largely attributed to turbid water in much of its habitat in Central America. Considering the conservation imperative of knowing the abundance of rare species, alternative estimation methods are needed, and sonar provides a novel technique for observing manatees through murky water (Rodas‐Trejo et al , Gonzalez‐Socoloske et al , Gonzalez‐Socoloske and Olivera‐Gómez , Brice , Castelblanco‐Martínez et al ). We therefore tested high‐resolution dual‐channel side‐scan sonar to survey manatees in a protected Panamanian wetland over 12 months and developed Bayesian tools to create rigorous statistical estimators of abundance.…”

Manatee habitat characterization using side-scan sonar

“…Gonzalez-Socoloske and colleagues (Gonzalez-Socoloske, 2007, 2013; Gonzalez-Socoloske et al ., 2009; Gonzalez-Socoloske & Olivera-Gomez, 2012) demonstrated that side-scan sonar could be successfully used to detect manatees in both freshwater and marine habitats. Subsequent studies have confirmed this ability in other locations (Arévalo-González et al ., 2014; Guzman & Condit, 2017; Puc-Carrasco et al ., 2017; Castelblanco-Martínez et al , 2018). McCarty (2014) demonstrated the use of side-scan sonar to detect alligator gar and Flowers & Hightower (2013) demonstrated the use of this technology to identify Atlantic sturgeon.…”

Manatee genomics supports a special conservation area in the Guianas coastline under the influence of the Amazon River plume

“…Sighting records are widespread in FG, both in dense and well-preserved mangroves and on the lower parts of all river systems. Important seasonality in habitat preference has been noticed in relation to strong seasonal fluctuations in the relative contributions of marine and continental waters to the estuarine waters (Castelblanco-Martínez, dos Reis, & de Thoisy, 2017). However, FG manatees also make use of large freshwater streams, with records up to 100 km inland (UICN France et al, 2017).…”