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Have Antillean manatees ( Trichechus manatus manatus ) entered the Eastern Pacific Ocean?

2022; Wiley; Volume: 39; Issue: 1 Linguagem: Inglês

10.1111/mms.12950

1748-7692

Héctor M. Guzmán, Candy K. Real,

Identification and Quantification in Food

The feeding ecology of sirenians determines their role in aquatic ecosystems (Aragones et al., 2012). Dugongs and manatees are aquatic herbivorous mammals that comprise the two distinct living genera, Dugong and Trichechus. Dugongs are restricted in their distribution to the South Pacific and Indian oceans, while manatees are restricted to shallow tropical and subtropical waters of the North and South Atlantic Oceans (Odell, 2009), with a preference for coastal habitats and brackish riverine habitats. All three currently known species of manatees, the West African, Trichechus senegalensis, (distributed in riverine and coastal waters along the tropical to subtropical eastern Atlantic Ocean), the Amazonian, Trichechus inunguis, (distributed exclusively in riverine habitats of the Amazon River and tributaries), and the West Indian, Trichechus manatus, (distributed in riverine and coastal waters along the tropical to subtropical western Atlantic Ocean), are listed as vulnerable species that are threatened with global extinction (IUCN, 2020). The Florida, T. manatus latirostris, and the Antillean, T. manatus manatus, manatee subspecies are listed as threatened and with stable and decreasing populations, respectively (Deutsch, 2008; Deutsch et al., 2008; Self-Sullivan & Mignucci-Giannoni, 2008). While the Antillean manatee is widely distributed from northern Mexico to central Brazil in northern South America, including the Caribbean islands, the Pacific Ocean is not part of its natural habitat (Jefferson et al., 2015). In northwestern Panama, the Bocas del Toro area (Figure 1) sustains the largest manatee population in the country (Guzman & Condit, 2017; Merchan et al., 2019), genetically related to the Belize/Mexico populations (Diaz-Ferguson et al., 2017). Here, we describe the 57-year human intervention chronicle leading up to the first confirmed report of an Antillean manatee sighting in the waters of the eastern North Pacific Ocean off Panama, and echo previously reported concerns (sensu Montgomery et al., 1982). The chronicle starts in the Gatun Lake, a 436 km2 water reservoir (Figure 1) created in 1913 for the operations of the 80-km long Panama Canal, which connects the North Atlantic to the North Pacific Oceans (McCullough, 1978). The abundance of water hyacinth (Pontederia crassipes) and other aquatic plants in the lake prompted the suggestion of biocontrol methods to curb invasion (Carse, 2014). In April 1962, the Health Bureau of the Panama Canal Company (PCC), which is now the Panama Canal Authority (ACP), envisioned the experimental idea of introducing manatees into the Gatun Lake for the control and eradication of nonendemic aquatic plants as previously tested in Guyana (MacLaren, 1967). Aquatic plants were breeding grounds for malaria-vector mosquitoes, which posed a significant public health problem, particularly near Gamboa (MacLaren, 1967; Panama Canal Company [PCC], 1963). In October 1963, a local hunter signed a contract with the PCC to capture 10 manatees in the Changuinola area, Bocas del Toro Province (Figure 1), northwestern Panama, (Maclaren, 1967; Panama Canal Spillway [PCS], 1964b; Stephens, 2002). In March 1964, the transport of manatees to the Panama Canal area began using C-47 cargo planes from the Howard Air Force Base, Pacific Panama. However, in April 1964, the first manatee to be flown to the Panama Canal was, unwisely, a 68 kg male nonnative Amazonian manatee (T. inunguis) introduced from Peru (PCC, 1964; PCS, 1964a) and not from Bocas del Toro as initially planned. T. inunguis is restricted to the Amazon Basin and is not endemic to the western Caribbean (Jefferson et al., 2015; Marmontel et al., 2016). In June 1964, the PCC started the transportation of nine Antillean manatees (T. manatus) from Bocas Del Toro to the Gatun Lake as follows. First, two females weighing 360 kg and 136 kg, respectively (Figure 2a–c), were brought by plane (4 hr of travel) from Changuinola (PCS, 1964b). Both females were placed in the same 2.8 hectares fenced area (MacLaren, 1967) as the Amazonian male, with the expectation of breeding (interbreeding) as “the fact that the new additions are female gave the Health Bureau manatee experts hope for a future population explosion in the manatee lagoon…” (PCS, 1964b, p. 1). Two additional males arrived later, on a separate flight. In July 1964, those five individuals, two females and three males, were already reported to be moving freely in Gatun Lake because of an unfortunate event that resulted in breaking the Manatee Lagoon's fence, and the manatees escaping (MacLaren, 1967; Montgomery et al., 1982; PCS, 1964d; Schad et al., 1981). One was reported around the smaller 2 km Miraflores Lake, approximately 12 km south of Gamboa Town and just 700 m from the North Pacific Ocean (Figure 1). This manatee passed the two-step Pedro Miguel Lock (16 m above sea level), one lock away (Miraflores) and one step closer to reaching the North Pacific Ocean. In October 1964, two males, 317 and 195 kg, respectively, were brought from Changuinola to the Summit Zoo, and were later placed in the fenced Manatee Lagoon, Chagres River (PCS, 1964c). Finally, in December 1964, two additional manatees were airlifted from the Rio Changuinola to the Manatee Lagoon near Gamboa and placed alongside the two others already in the semienclosed area. Both individuals were approximately 3 m long and 362 kg in weight, a pregnant female, and a male (PCS, 1964d). By the end of 1964, eight adult and one juvenile (MacLaren, 1967) manatees had been introduced into the Panama Canal from Bocas del Toro; five were moving freely, presumed in the lake area or the Rio Chagres (MacLaren, 1967; PCS, 1964d). A tenth and last manatee to be captured was not transported from Changuinola due to military logistical problems and was euthanized (Stephens, 2002). In July 1965, the first manatee birth was recorded in the waters of the Canal within the lagoon in Gamboa (PCS, 1965a), possibly born from the pregnant female introduced in December 1964. In October 1965, one of the previously escaped manatees was sighted near Frijoles town (PCS, 1965b), 18 km further north of Gamboa from the Gatun locks (Figure 1). Consequently, by the end of 1965, eleven manatees (three females, seven males, and a newborn were freely feeding throughout the Panama Canal (MacLaren, 1967). In 1966, the manatee program to control aquatic plant growth was abandoned as implausible, realizing that thousands of manatees would be needed to achieve substantial control (PCC, 1965). “It was concluded that the size and scope of the Canal Zone waters will not be effectively controlled by the use of manatees for removal of aquatic vegetation in mosquito control” (PCC, 1965, p. 118), and a herd of up to 2,000 individuals would be required (MacLaren, 1967; PCS, 1982, p. 1). In April 1982, a dead manatee measuring 3.2 m and 735 kg was found inside the one-step Pedro Miguel west lock (PCS, 1982; Montgomery et al., 1982), thought to have died due to a collision with a ship or pressure from the lock gates (Figure 2D). Four deaths had been reported previously in 1982, in addition to two females found dead in 1980. One was in the Mandinga River near Gamboa in 1980, affected by an explosion with dynamite during a dredging operation and another in Lake Miraflores was mutilated by boat propellers (PCS, 1982). Illegal hunting of manatees was reported for the first time in Gatun Lake at this time. On September 4, 2020, the 3,503 t oil tanker LARA (IMO No. 9320623), approached the west Miraflores Locks from the Pacific side of the Isthmus (see Figure 1). Two ACP pilots sighted a manatee on the vessel's starboard side. Two aerial surveys of the locks using a drone searched unsuccessfully for the animal, and an unsuccessful search by boat was attempted for approximately 2 hr, on both sides of the Pacific entrance to the Panama Canal, from the Miraflores Locks to the Balboa Port and nearby marinas. The manatee was not found but had presumably remained in the Pacific. Manatee reports from within the Panama Canal have varied widely. Since 1977, more than 50 sightings have been reported between the Gatun and Miraflores Locks, including at Contractor Hill, Gamboa, Barro Colorado Island, Paraiso, and Pedro Miguel (PCS, 1982). Shad et al. (1981) estimated 37 individuals by the end of 1980 and no more than 25 individuals for 1981 based on observations and questionnaires. Montgomery et al. (1982) estimated 25 individuals and suggested that “introduction into the eastern Pacific Ocean may already have happened, or could soon occur, particularly as the manatee population in Panama Canal waters continues to increase.” Mou et al. (1990) reported only one individual in Gatun Lake in 1987 based on aerial surveys. Muschett and Vianna (2015) reported the most recent estimate of 20–25 manatees after the highest number of manatees sighted during a single aerial survey was 16 individuals. Recently, Muschett and Morales (2020) modeled the effect of isolation and mortality on the survival of manatees in the Panama Canal, suggesting that the population has increased through the addition of new individuals. Although possible, this would require wild manatees to move from the Caribbean up and against the waterflow of the three-step Gatun Lock to 26 m above sea level and into Gatun Lake. Manatees have been observed at port facilities about 5 km from the Caribbean entrance to the Canal. However, there is no comprehensive and reliable population estimate to confirm or refute a population increase in the lake. On the other hand, potential inbreeding depression of the first introduced stock may have occurred during the past 50 years, possibly affecting population viability. Only a population genetic study can confirm whether inbreeding has been affected by the addition of new individuals (Kenney et al., 2014). Indeed, new genomic feasibility techniques could be used to measure inbreeding depression in natural populations and resolve this enigmatic question (Kardos et al., 2016). Based on the historical context and the evidence presented herein, we can conclude that the Antillean manatee observed by the oil tanker LARA may have passed into the North Pacific Ocean some decades ago, as previously suggested and as recently as September 2020. These events could have started months after the four individuals escaped from Gamboa, and one was spotted in the Miraflores Lock (Montgomery et al., 1982; PCS, 1964c). The current Panama Canal Authority inherited this wild population, possibly without realizing the dimensions of the invasive species problem. In fact, in addition to the possible disturbing interbreeding between two species outside their natural ranges, the Amazonian and Antillean manatees that were initially introduced have the potential to reproduce remarkably well with many offspring possible over the past 57 years. It is unknown whether other manatees might have passed into the Pacific. However, it is possible given the various estimates of the resident population, which have ranged from 20 to 25 individuals. In 2021, a project was begun to periodically survey for animals, using programmed drone flights, to estimate the population size, geographic distribution, and habitat preference. These drone surveys were coupled with a year-round acoustic monitoring program (sensu Merchan et al., 2019) using a hydrophone network installed along the waterway habitats (Figure 1). Additionally, manatee tissue samples collected years ago are being used to understand the genetic structure of this population (personal observation). The interest is to investigate possible interbreeding between the 1960s introduced Amazonian and Antillean manatees and inbreeding depression, assuming that new animals could move naturally into the population (Muschett & Morales, 2020). The combined ecological and genetic information would inform potential management options for this artificial manatee population. It is logical to try to prevent the passage of more manatees into the North Pacific Ocean, “where no manatees have lived for several million years” (Domning, 2009; Montgomery et al., 1982). This task may not be easy. In June 2016, the expanded Canal added the new Cocoli Lock parallel to Pedro Miguel and Miraflores Locks with a spillway of three 427 m long, 55 m wide chambers into the Pacific. The ACP may evaluate management options that include the capture, relocation, and permanent confinement of the individuals in specific areas of Gatún Lake or Bocas del, which seems to be the only viable solution without removing or sacrificing these animals (sensu Castelblanco-Martinez et al., 2021). Relocation and confinement within Gatun Lake can provide an opportunity for evaluating manatee adaptation to climate change (sensu Edwards, 2013), and would avoid potential collision threats from the more than 14,000 merchant vessels (e.g., cargo, tanker, and cruise ships) annually transiting the canal. Captive animals may provide a platform for wildlife education and opportunities for nature tourism in the Panama Canal. Hector Guzman: Conceptualization; data curation; investigation; methodology; project administration; supervision; validation; writing – original draft; writing – review and editing. Candy K. Real: Data curation; investigation; writing – review and editing. We thank the Panama Canal Authority for the consent to publish this first record and for providing the permit to investigate the population status and distribution. We thank ACP Pilot Captains Eric Hendrick and Ivo Quiroz, Jr. for providing the sighting information, Daniel Muschett, Angel Tribaldos, and Angel Ureña for providing authorization to enter the canal facilities and the immediate logistical support to search for the animal in the Pacific entrance of the Canal. We thank three anonymous reviewers for valuable comments to the manuscript. No specific funding was received for this study. However, the documentation research time was funded under the Smithsonian Tropical Research Institute and the Secretaria Nacional de Ciencia, Tecnología e Innovación de Panamá (SENACYT). Historical information is available on the internet, in public libraries, and at the Roberto F. Chiari Library at the Panama Canal Authority, Panama City, Panama. Not applicable. The use of historical photos was authorized by the Panama Canal Authority's Roberto F. Chiari Library. The manuscript was revised by the Office of Communication of ACP. The authors declare no competing interests.