In the past 500 years humans have been an immense driver of change capable of causing what is recognized as the sixth mass extinction (Ceballos et al., 2020). The extinction of species and populations, and the decline of populations numbers have been documented in terrestrial, freshwater and marine systems with alarming consequences for the coexisting species, and for the provision of ecosystem services crucial for humans (Dirzo et al., 2014; McCauley et al., 2015). Hawksbill sea turtle (Eretmochelys imbricata) is one in thousands of species threatened with extinction (IUCN, 2021). It is considered Critically Endangered on the IUCN Red List of Threatened Species and is estimated to have a global decline of nesting females of over 80% in just 3 generations (approx. 100 years; Mortimer and Donnelly, 2008). The main factors influencing its decrease are the direct harvest of individuals to sustain a global shell trade, the transformation of nesting and foraging habitats, bycatch mortality of juveniles and adults in industrial and small-scale fisheries, and a vulnerable life history with maturity at 20-40 years (Miller et al., 2019; Wallace et al., 2011b). This is particularly relevant for the Eastern Pacific hawksbill population, believed to be the most endangered of all hawksbill populations (Gaos et al., 2010; Wallace et al., 2010).
Marine protected areas are an important tool to protect and manage marine species. In particular, marine reserves or no-take zones have proved to be beneficial due to the full protection from all extractive activities when they are well designed and both socially and policy supported (Gill et al., 2017). Nevertheless, spatial conservation planning for hawksbill sea turtles is hard due to their vast distribution ranges which can include several jurisdictional borders, and their dependence to two of the most productive coastal systems, reefs and mangrove estuaries, which are also important for small-scale fisheries (Aburto-Oropeza et al., 2008; Carrión-Cortez et al., 2013; Gaos et al., 2012b; Liles et al., 2011, 2017; Llamas et al., 2017). In response, strategies for the establishment of coastal marine protected areas have been expanded to prioritize restricted regions where species concentrate at vulnerable life history stages (i.e., nesting sites and foraging grounds) and to explicitly include local users in their design and enforcement (Rocliffe et al., 2014). Hawksbill turtles may particularly benefit from these strategies, but a deep understanding of the environmental conditions and habitat requirements that influence hawksbill’ occurrence in space and time is needed to evaluate this potential conservation tool (Guisan and Thuiller, 2005; Guisan et al., 2013).
Northwest Mexico represents the northernmost limit for hawksbill distribution in the Eastern Pacific and is an important foraging region for the species (Martinez-Estevez et al., 2021; SEMARNAT, 2012). Also, it is the most productive fishing region with around 700,000 tons produced annually (Paez-Osuna et al., 2016), and a highly biodiverse system with an extensive variety of habitats, nearly 5000 marine invertebrates, and more than 1100 marine vertebrates described, including many endemic species (Brusca et al., 2005; Lluch-Cota et al., 2007; Munguia-Vega et al., 2018). Descriptions of the high abundance of organisms in this region were common for travelers and locals until the first half of the 1900’s (Saenz-Arroyo et al., 2005). For instance, Steinbeck et al. (1951) mentioned in the book The log from the Sea of Cortez: “The abundance of life here gives one an exuberance, a feeling of fullness and richness. The playing porpoises, the turtles, the great schools of fish which ruffle the water surface like a quick breeze, make for excitement. The sea here swarms with life, and probably the ocean bed is equally rich”.
Sea turtles were a highly exploited group in northwest Mexico. For instance, hawksbills sustained an international shell trade and a regional trade of meat, eggs, and stuffed individuals until the 1960’s, and green turtles (Chelonia mydas) were taken by thousands per year in the late 1960’s and early 1970’s (e.g., 5,220 turtles/year at Laguna Ojo de Liebre, BCS; Early-Capistran et al., 2017; Saenz-Arroyo et al., 2005). Although hawksbill population declining made the trade not profitable, individuals have always been a valuable source in the region and illegal captures still occur. In Mexico, conservation strategies for hawksbills and other sea turtle species have been in place since 1979; however, lack of law enforcement and gaps of information on those important areas for the species have jeopardize the recovery of the population in the Mexican Pacific Ocean.
In the following doctoral dissertation, comprised of three data chapters, I study the spatial ecology and habitat use of hawksbill turtles in northwest Mexico, and identify highly-used areas and actions for the species recovery and the conservation of its habitats.
In Chapter 1, I surveyed 8 locations in the southwestern Gulf of California searching for hawksbill turtles. I measured and tagged individuals with unique ID tags and deployed satellite transmitters on twelve of them to analyze their movement patterns. By using a kernel density estimation analysis, I calculated home range areas and core areas of use of the individuals and overlapped them with current spatial conservation strategies. I demonstrated that hawksbills had highly restricted movements for periods of months to years. The areas they use are smaller than 5 km2, highlighting the need to protect them to increase the opportunities of population recovery.
In Chapter 2, I studied the foraging ecology of hawksbill turtles in a foraging ground at the south of Isla San Jose, off the Baja California Peninsula, which is known to have high densities of the species. The foraging ground included three benthic habitat types: a sandy reef, a rocky reef, and a mangrove estuary. I used four complementary approaches to have the most complete information on how this species use the habitats: acoustic telemetry to track individual movements and determine the use of space, behavioral video recordings to identify behaviors and preferred food items, feces analysis to determine ingested food items, and benthic habitat surveys to determine availability of preferred food items. I demonstrated that tagged hawksbill turtles spent up to four years in the location and benthic habitat where individuals were initially caught. Sponges, algae, tunicates, and mangroves were the main available food categories for the species, and the mean percent cover of hawksbill benthic food was significantly greater in the mangrove estuary than in rocky and sandy reef habitats. These findings highlight the relevance of mangrove estuaries for Eastern Pacific hawksbills and reinforce the need to improve their management for hawksbill conservation and fisheries enhancement.
Chapter 3 was materialized after a twenty day trip along the Gulf of California looking for hawksbills in mangrove estuaries and talking to local fishers about the current status of the species. That trip helped understand the differences between sites in the Baja California Peninsula, the islands, and mainland coast and provided the opportunity to use a collaborative database with hawksbill records from 1996 to 2019. I used the position data of hawksbill records to identify habitat type, and I developed a conservation index to determine the conservation status of hawksbill sites based on isolation, designated spatial protection status, presence of local engagement, and fishing pressure as factors influencing hawksbill presence and permanence. I demonstrated the relevance of the region for juvenile individuals and of reefs and mangroves as the main habitats used by the species. I identified highly used areas in northwest Mexico and I gave recommendations for the conservation of the species and its habitats.
Together, these three chapters are the first regional analysis on the movement, habitat use and conservation of hawksbill turtles in the Gulf of California and northwest Mexico, with particular emphasis on foraging habitats and in-water monitoring. This regional approach brings important baseline information on those conditions that favor hawksbill presence and identifies highly used areas that require immediate protection. These findings also provide a framework to develop a regional hawksbill conservation program from which the species and local fishing communities can benefit.