Study area

The Maracaibo Lake is located in northwestern Venezuela (70°30′ - 73°24′ W and 8°22′ - 11°51′ N), and is connected to the Gulf of Venezuela by a natural channel (Laval et al., 2005), which was dredged during the 1930s. Nowadays, it consists of a 100.6- km- long and a 11- metredeep channel, navigable for oceangoing ships and tankers (Marin et al., 2022). The surface of the lake has an area of 12013 km², a maximum length of 155 km from Santa Rosa coast to Punta Palmas; and a maximum width of 12 km, from Guaimito to San Lorenzo (Rodriguez, 2000). It has a relatively flat bottom of an average depth of 26 m, and a maximum depth of 34 m (Laval et al., 2003). From the ecological perspective, the Maracaibo Lake basin is divided in eight regions with particular ecological features and diverse level of complexity (Medina and Barboza, 2006): (a) Gulf of Venezuela, (b) El Tablazo Bay, (c) Sinamaica Lagoon, (d) Maracaibo Strait, (e) Maracaibo Lake, (f) mouth of the rivers, (g) running waters, and (h) reservoirs. Regions a to f conform the Maracaibo system, while regions g and h include 135 permanent and intermittent rivers discharging about 40 km³ of water yearly to the lake.

Data Collection

To compile observations on manatee presence, the area of study was divided in two zones. Zone 1 (Z1) is placed at the municipality of Almirante Padilla, located north of El Tablazo Bay. The area comprises the lacustrine archipelago encompassing five islands and eight islets. Zone 2 (Z2) at the municipality of Miranda, on the eastern coast of Maracaibo Lake. The fieldwork was conducted during 2004 (February, April, July, and October) in Z1, and during 2009 (March, May, June, and July) and 2010 (May, July, August, and October) in Z2. A Relative Abundance Index (RIA) was estimated to compare among areas, expressed by the number of sightings per hour of survey effort (Martinez- Cedeira et al., 2003). We made informal and opportunistic conversations with local fishermen, as well as meetings with coastal communities, aiming to gather information about manatee presence, frequency of boat traffic, and people's perception on the vessels' effect on manatees. Finally, we reviewed published and unpublished documents to compile additional data about manatee presence and mortality in the study area.

With the goal of collecting evidences on the manatee presence, we conducted visual searches applying a variety of survey methods (Aragones et al., 2012). We conducted boatbased surveys following pre- designed routes (Fig. 1), using vessels equipped with outboard engines (25 hp or 400 hp at a speed of 15 to 20 km/h). Additional, observations were made from a drifting boat with the engine off. Fixed points of observation were selected either from an anchored boat in the middle of the survey area, or from high places located at shore. Regardless the type of survey, the observations were conducted from 0600 to 1800 h and consisted in scanning with binoculars the water surface in search of manatee evidence, such as the exposition of head, the back or the tail. Once an animal was found, its behavior (swimming, feeding, and breathing) was noted and if possible, photos or videos were taken.

We considered areas of manatee use, those informed by local inhabitants as traditional places for the species; or those in which we visually confirmed manatee presence. Those areas were described in terms of its proximity to human settlements, width of watercourses, water parameters and food availability. Water parameters obtained were pH, surface water temperature, and dissolved oxygen (with an YSI handheld multiparameter), salinity (with a refractometer), transparency (with a Secchi disk), and depth (with a portable Depth and Fish Finder, or with a graduated pole). Through boat surveys along the shore, we documented in situ the vernacular name of plants and algae that might be a potential food for manatees. When needed, a sample of some plants were collected in hermetic plastic bags and preserved in formalin solution at 10 % for further identification at the laboratory of the University of Zulia.

Occurrence and distribution

Ten hours of aerial surveys conducted by O'Shea et al. (1988) over Maracaibo Lake region resulted in only one tentative sighting of a manatee. Years later, Montiel- Villalobos and Barrios- Garrido (2005) reported other areas of use by the species. After those previous reports, we made active and passive searching of manatees by boat survey and obtained additional observations of manatees in the Maracaibo Lake region.

To avoid biases due to differences between the two sampling periods, we do not aim to perform any comparative analysis. Therefore, our analysis is only descriptive. The Maracaibo Lake system present interesting ecological characteristics, that makes it a suitable habitat for manatees. The lake offers sheltered areas, with small effect of waves and winds, characteristics preferred by manatees, especially for resting and calving (Aix- Arroyo et al., 1998). We evidenced the presence of abundant vegetation all year round, including mangroves and algae. The Maracaibo Lake is surrounded by an extensive coastal flat, with abundant sand beaches and channels bordered by mangrove islands composed by Rhizophora mangle, Avicennia germinans and Laguncularia racemosa. The observed aquatic and semiaquatic plants, have been reported as part of manatee diet in other localities of their distribution (Castellblanco- Martinez et al., 2009; Borges et al., 2008; Navarro- Martinez et al., 2014; Allen et al., 2017).

The salinity fell within the range accepted by the Antillean manatees, which move between salty, estuarine, and freshwater environments. The salt water flux to the Gulf of Venezuela, however, is not constant along the year, being greater during the dry season (December - May) and lesser during the rainy season (May - November) (Laval et al., 2005), influenced by changes of precipitation and tide regime. Since manatees are required to drink freshwater every certain days (Ortiz et al., 1998), they prefer coastal areas because are associated to rivers (Aix- Arroyo et al., 1998). The sources of freshwater for manatees in the Maracaibo Lake are abundant, due to the large quantity of rivers feeding the lake system. One example of this is the Limon River, which provide an important input of freshwater to the system. Manatees are mainly found in warm water areas (>20 °C, Bossart et al., 2003), due to their incapacity to withstand low temperatures (Laist et al., 2013). Therefore, the temperature values of the study area fell within the range of ideal water temperature for the species.

Table 3.: Cases of manatees Trichechus manatus manatus stranded in the Maracaibo Lake between 1997 and 2009.

During our boat- surveys, we observed extensive meadows of subaquatic vegetation at less than 1 m deep, mainly at the margins of channels. However, few species were recorded probably due to the small survey effort. We recommend conducting a more comprehensive aquatic and semi- aquatic vegetation assessment. Local people claimed to see manatees foraging and feeding on these meadows, particularly in Toas Island during high tide, where the animals could approach them. It is interesting to note that in both Toas Island and San Carlos, there are artificial constructions made of concrete to support docks, which serve as substrate for the growing of micro and macro algae. These substrates offer nutrient for local manatees.

Threats to manatees and their habitat

The economy of the localities of Toas Island, San Carlos Island and Sabaneta de Palmas are based on small fishery activities. Additionally, several touristic developments have been built along the shore receiving an important affluence of tourists. Thus, boat traffic is constant during the year, with hundreds of boats transiting simultaneously. The traffic of vessels could be an important disturbance factor for manatees, as has been widely reported that the species prefers areas with low anthropic presence (e.g. Jimenez- Perez, 2005). Vocal communication seems to play a critical role for manatee survival, particularly in habitats where the vision is reduced due to high turbidity, in murky coastal or riverine waters (Sousa- Lima et al., 2002). Noise caused by boat engines can interfere with manatee acoustic communication by masking signals that contain biologically important information (Miksis- Olds et al., 2007). If the number of boats increases without control, a probable consequence could be a modification in habitat selection by manatees for such activities as resting or calving. Additionally, boat- related accidents are considered one of the most important threats to manatees in Florida (Bauduin et al., 2013), Belize (Galves et al., 2023), Puerto Rico (Mignucci- Giannoni et al., 2000), and Brazil (Borges et al., 2007). Our compilation of manatee mortality cases (Table 3) includes four events of manatees killed by boat collision in the region of Maracaibo Lake. It is expected that the boat- related mortality will increase if the traffic of boats is not controlled either by limiting the number of vessels, or by the implementation of slow speed, no- wake zones.

Another worrisome threat to manatees in the study area, was the vandalism. Although these events occurred more than a decade ago, it would be interesting to follow- up the perceptions and feelings of local inhabitants regarding manatee conservation. Besides direct threats to manatees, several environmental pressures jeopardize the habitat for the species in the region of the Maracaibo Lake, such as intense and long- term activities of agriculture, mining, and oil exploration that have produced important runoff of contaminants to the wetlands (Corona- Lisboa, 2013; Moronta- Riera and Riverón- Zaldívar, 2016). Also, dredging and sedimentation cause severe damages on the structural and environmental properties of the estuaries, leading to hyper- eutrophication of the waterbodies and the subsequent of high phytoplankton growth and low oxygenation levels (Rivas et al., 2011). As a consequence of the dredging navigation channels to facilitate the transit of large vessels, the salinity inside the system has increased significantly with time (Berghuis, 1995). This may cause critical variations in the benthic community composition of the system. (Hernández-Arana and Ameneyro-Angeles, 2011), including changes in the availability and composition of feeding resources for manatees (Medina et al., 2001).