Methods

Between May and June 2012, we developed a participatory research program with La Reforma shark fishermen who agreed to report turtle sightings during their offshore fishing efforts in the region. La Reforma, Sinaloa, Mexico, is a coastal community with a population of 6743 whose primary economic activity is fishing (Fig. 1). This cooperative program was designed to build capacity and train fishers to record basic data: sea turtle size, sex, and species. During routine permitted shark fishing trips from 6–8-m open-hull skiffs, known locally as pangas, fishers used entanglement nets (1000 × 50 m, mesh size = 25 cm stretched, 24 hrs/d) to catch a variety of shark species. Fishing efforts occurred in the offshore area between the Navachiste lagoon system and the Santa Maria–La Reforma lagoon system near the municipalities of Guasave and Angostura in northern Sinaloa (Fig. 1). All turtles that were incidentally captured during these efforts were removed unharmed immediately upon capture and pulled aboard for examination, measurements, and flipper tagging.

View Figure

• Download as Powerpoint
• Download Figure

To compare our data with previous studies, we calculated catch per unit effort (CPUE) adjusting for a net length of 100 m for 24 hrs. The straight carapace length (SCL) in centimeters (mean ± SE) of each turtle was recorded (Bolten 1999). Prior to release, each turtle was tagged with Inconel tags (Style 681, National Band and Tag Company, Newport, KY) in the rear flipper. All turtles were released near their initial capture site within 1 hr.

Results and Discussion

A total of 18 loggerhead turtles with a mean SCL of 61.8 ± 6.7 cm (range, 49.6–69.6 cm SCL) were captured during 15 fishing trips along the Sinaloa coastline (lat 24°43′430″N, long 108°25′026″W; Fig. 1). CPUE was 0.24 turtles each 24 hrs. All turtles were classified as subadults based on size (Seminoff et al. 2004). Capture date, SCL, and capture locations are presented in Table 1.

Table 1. Capture date, straight carapace length (SCL), and capture coordinates for loggerhead turtles (Caretta caretta) from Sinaloa, Gulf of California, Mexico.

View Fullscreen

Previously, 4 sea turtle species were reported in the state of Sinaloa, including olive ridley (Lepidochelys olivacea), green (a.k.a. black) (Chelonia mydas agassizii), hawksbill (Eretmochelys imbricata), and leatherback (Dermochelys coriacea) (Lemus and López 2002). To our knowledge, this is the first report of the presence and distribution of loggerhead turtles in Sinaloa.

The observed capture rates of loggerhead turtles in Sinaloa coast were surprisingly high for a species considered to be endangered (Convention of International Trade in Endangered Species of Wild Fauna and Flora [CITES] 2015). Whereas Seminoff et al. (2004) reported a loggerhead CPUE of 0.014 turtles/24 hrs in Bahia de los Angeles, the loggerhead CPUE in our study was 0.24 turtles/24 hrs, with a total of 18 turtles captured during 15 fishing days in May and June 2012. Although the capture rates are considerably lower than those documented on the Baja Californian Pacific coast loggerhead hotspot (Ramírez-Cruz 1991; Peckham et al. 2007), the observed capture rate in this study suggests a high abundance of loggerhead turtles in our study area.

Previously, the occurrence of juvenile and subadult loggerheads has been reported along the Pacific coast of BCP (Ramírez-Cruz et al. 1991; Resendiz et al. 1998; Nichols 2002). Loggerhead turtles from northern Sinaloa, Mexico, were also classified as juveniles and subadults, further confirming that adult loggerheads are rarely present in the eastern Pacific (Bowen 1995). Our study area in Sinaloa is considered to be a neritic zone with high levels of primary productivity and abundant marine wildlife (Ruelas-Inzunza et al. 2008). Presumably, the semienclosed ecosystem within the Gulf of California, coupled with characteristics distinct from those found in other foraging areas (Storelli et al. 2008; Ley-Quiñónez et al. 2013), provide ideal conditions that create a developmental habitat for juvenile and subadult loggerheads in the region.

Loggerheads in the Gulf of California are within the size range of individuals studied along the Pacific coast of the BCP. Whereas Seminoff et al. (2004) reported a mean SCL of 61.9 ± 2.1 cm, Gardner and Nichols (2001) found a mean size of 58.5 ± 11.1 cm SCL among stranded loggerheads along the Pacific coast of the BCP, and Peckham et al. (2011) reported a mean curved carapace length that is equivalent to 66.9 ± 6.7 cm SCL (Table 2). The loggerheads we encountered in Sinaloa (Table 2) are consistent with the sizes reported by Seminoff et al. (2004), suggesting a uniform size distribution among loggerhead foraging populations in the Gulf of California.

Table 2. Comparison of size (mean ± SE) data for loggerhead turtles (Caretta caretta) from the Gulf of California and Pacific coast of Baja California (BCP).

View Fullscreen

This report confirms the presence of loggerhead turtles in the Gulf of California within an area of high fishing activity. This area has been previously identified as an area with high risk for sea turtle bycatch and direct take (Aguilar-Gonzalez et al. 2014; Parra-Gaxiola 2012). Indeed, the fact that so many loggerheads were captured incidentally during shark-fishing efforts underscores the potential for problematic levels of bycatch in the coastal fisheries of Sinaloa.

Increased monitoring and tracking is needed to define loggerhead distribution and abundance within the Gulf of California. Social studies are also urgently needed to assess and mitigate environmental effects and human impacts including bycatch in coastal fisheries of the area. We recommend that the Gulf of California should be included as a target area for efforts to protect the loggerhead turtle.