Characterization of a Green Turtle (Chelonia Mydas) Foraging Aggregation Along the Pacific Coast of Southern Mexico
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Herpetological Conservation and Biology 12(2):477–487. Submitted: 29 October 2015; Accepted: 23 May 2017; Published: 31 August 2017. Characterization of a Green Turtle (Chelonia mydas) Foraging Aggregation along the Pacific Coast of Southern Mexico Samantha G. Karam-Martínez1,3, Isabel Raymundo-González1,2, J. Alberto Montoya-Márquez1, Francisco Villegas-Zurita1, and Francisco Becerril-Bobadilla1 1Universidad del Mar, Campus Puerto Ángel, Ciudad Universitaria, Puerto Ángel, Oaxaca, México 2Universidad Autónoma de Baja California, Facultad de Ciencias, km 103 Carretera Tijuana-Ensenada, Ensenada, Baja California, México 3Corresponding author, e-mail: [email protected] Abstract.—The primary foraging areas of Green Turtle (Chelonia mydas) in the Mexican Pacific are located near the Baja California Peninsula, although foraging areas also have been documented along the southern coast of the country. The goal of this study was to determine demographic characteristics, catch per unit of effort (CPUE), condition index (CI), and food preferences of Green Turtles in Chacahua Lagoon, Oaxaca, Mexico. This is the first study of its kind involving a foraging aggregation along the Pacific Coast of southern Mexico. Between June 2009 and May 2010, we captured 16 Green Turtles with entanglement nets and had 25 total captures. Individuals that we captured more than once spent up to six months in the lagoon. Based on size, we classified 14 turtles as adults and two as juveniles. We could not determine the sex of all turtles. The mean monthly CPUE was 0.095 turtles/100 m net/12 h, which was one to two orders of magnitude lower than those reported for Baja California foraging areas. The mean CI was 1.38, similar to that reported for other Eastern Pacific foraging sites. Eight turtles exhibited injuries, mainly on the carapace and likely from outboard engine boats. Samples of esophageal and oral cavity contents that we collected from some turtles revealed that they consumed Gracillariopsis lemaneiformis, a macroalgae species distributed in several patches throughout the lagoon. Our study provides data on a foraging aggregation of Green Turtles that may be used to guide conservation efforts and research in an under-studied region. Key Words.—Condition index; conservation; entanglement net; macroalgae; Oaxaca; size distribution. Introduction lagoons where they feed on sea grasses, macroalgae, and invertebrates (Seminoff et al. 2002a; Amorocho and The Green Turtle (Chelonia mydas) is widely Reina 2007; Lemons et al. 2011). distributed in tropical and subtropical areas of the The primary foraging areas of Green Turtles in the world. The species is currently listed as endangered Mexican Pacific are located on the Baja California by the International Union for Conservation of Nature Peninsula (BCP), where several studies have examined (IUCN), and many of its populations continue to decline population structure, abundance, diet, growth rate, (International Union for Conservation of Nature. 2014. mortality, and other population characteristics The IUCN Red List of Threatened Species. Version (Seminoff et al. 2002a, 2002b; Koch et al. 2006, 2007; 2014.2. Available from http://www.iucnredlist.org Labrada-Martagón 2010). In addition, several foraging [Accessed 24 April 2017]). In the Eastern Pacific, Green aggregations have been reported in southern Mexico in Turtles inhabit coastal waters from the U.S. West Coast the Superior, Inferior, and Mar Muerto lagoons on the (Eguchi et al. 2012) to South America (Denkinger et Isthmus of Tehuantepec and in the Pastoría Lagoon, al. 2013); their primary nesting beaches are located in Oaxaca (Márquez et al. 1982; Márquez 1996; Karam- Michoacán and the Revillagigedo Archipelago, Mexico, Martínez et al. 2014), but no population studies have and on the Galapagos Islands, Ecuador. In Mexico, been carried out on any of these aggregations. Thus, the Green Turtles nest sporadically in other areas along the goal of the present study was to document demographic Pacific Coast, including the state of Oaxaca (Márquez characteristics, catch per unit of effort (CPUE), 1996). After the breeding season ends, East Pacific condition index (CI), and food preferences of Green Green Turtles typically migrate to shallow bays and Turtles from the foraging aggregation in the Chacahua Copyright © 2017. Samantha G. Karam-Martínez 477 All Rights Reserved. Karam-Martínez et al.—Green Turtle foraging aggregation in Mexico’s South Pacific. Figure 1. Location of the study site, Chacahua Lagoon, Oaxaca, on the Pacific Coast of southern Mexico. Lagoon, Oaxaca, Mexico, to increase our knowledge of We left the entanglement nets in place for 34–66 this species along the Pacific coast of southern Mexico. continuous hours. The field crew took 8-h shifts to guard the nets constantly from outboard motor boats. Materials and Methods To prevent injuries to sea turtles or other organisms, the field crew navigated along the nets checking them Study site.—The Chacahua Lagoon is located in the visually each hour, and examining the mesh carefully by Lagunas de Chacahua National Park in the municipality pulling the upper line every 2 h. When the boat navigated of Villa de Tututepec de Melchor Ocampo on the to our land base for personnel changes, one person west coast of Oaxaca, Mexico (Fig. 1). The lagoon is guarded the nets from a wooden dinghy (approximately approximately 600 ha and is connected to the Pastoría 1 h each time). For some monthly sampling excursions, Lagoon by a narrow channel. Chacahua had an average we left the entanglement nets in one location for the depth of 1.80 m in 1997–1998 (Pantaleón-López et al. entire duration of sampling. During other excursions, 2005) and in 2007–2008 (J. Alberto Montoya-Márquez, we moved nets between nearby patches of macroalgae unpubl. report). (< 1.2 km distance between sites) during the course of the sampling period. Data collection and analysis.—We conducted multi- Upon capture, we kept turtles onboard the boat or in day sampling trips once a month between June 2009 a shaded area on the beach. We used a flexible tape to and May 2010 (n = 11). During each excursion, we set measure the curved carapace length notch to tip (CCL) entanglement nets composed of 0.80–0.95 mm diameter and the curved carapace width (CCW; Bolten 1999) monofilament with a length of 200–450 m, a height of 4 and we used a spring scale to weigh each individual m, and a mesh size of 60–70 cm (knot to knot). We tied to the nearest 0.1 kg. To facilitate comparison with anchors to the ends and stretched the net. We decided other studies, we transformed the CCL measurements to employ a larger mesh size than the one suggested into straight carapace lengths (SCL) using the equation (40–50 cm stretch) by Ehrhart and Ogren (1999) based SCL = (CCL˗2.2464)/1.0363, which Seminoff et al. on our previous experience in Pastoría Lagoon (Karam- (2003) developed based on measurements of Green Martínez et al. 2014) in which we employed nets with Turtles in Baja California (BC). We also tried another mesh size of 30–60 cm stretch, and found that several transformation equation, SCL = (0.919×CCL)+1.9645, large-sized Green Turtles swam parallel to the net, even from Velez-Zuazo et al. (2014) obtained from Green touched it, but did not become entangled. Turtles from El Ñuro, Perú. The transformed values 478 Herpetological Conservation and Biology from both equations were very similar (differences from (1999). Because of the difficulty of using this technique 0.1 to 0.5 cm). with East Pacific Green Turtles (Amorocho and Reina We used the mean size of nesting Green Turtles to 2007) and the stress caused to the animals, we decided distinguish maturity stages (Nichols 2003; Seminoff et to use the less invasive option of opportunistically al. 2003; Carrión-Cortez et al. 2010). As we lack the collecting fluid that was expelled from the esophagus genetic data required to identify the specific nesting and oral cavity when handling turtles. These samples beach used by the turtles captured in Chacahua, we used were fixed with 4% formaldehyde (i.e., 10% formalin) the smallest mean size documented among all potential for species identification (Seminoff et al. 2002a). We nesting beaches (Javier Alvarado and Alfredo Figueroa, released all turtles near their site of capture. unpubl. report; Juárez-Cerón et al. 2003; Zárate et al. Assisted by local residents on our field crew, we used 2003) to distinguish adults and juveniles. Thus, we direct observation to identify the macroalgae patches classified turtles with SCL < 77 cm as juveniles and frequented by the turtles. We collected samples of these those with SCL ≥ 77 cm as adults based on the mean patches, storing them in plastic bags and fixing them nesting size at Michoacán (Javier Alvarado and Alfredo with 4% formaldehyde for species identification. We Figueroa, unpubl. report). Sea turtles do not reach used the taxonomic keys published by Dawson (1954, maturity at a uniform or minimum size (Meylan et al. 1962), Abbot and Hollenberg (1992), and Fischer et al. 2011), so size is not a guaranteed indicator of maturity (1995), as well as the online database (Guiry and Guiry. (Miller 1997); however, in the absence of specialized 2011. Algaebase. Worldwide Electronic Publication, techniques like radioimmunoassay or laparoscopy, we National University of Ireland, Galway. Available from relied on external morphology to provide a general http://www.algaebase.org [Accessed 09 August 2011]) description of life stages present in the study area. to identify species of algae. We used one morphological feature to identify adult male turtles: the muscular tail extends well beyond the Results carapace, whereas females have short tails that project only slightly beyond the edge of the marginal scutes We did not capture any turtles during the first 20 h (Wibbels 1999). This method, however, is not reliable of our first sampling excursion (June 2009).