Predicting Sea Turtle Bycatch in Sinaloa, Mexico
ENV765: Term Project
The Gulf of California: Biodiversity Hotspot
The Gulf of California is rich in biodiversity, high in biological productivity, and endemism of marine life, and is home to some of the essential fisheries in Mexico (Paez-Osuna et al., 2016). Five of the seven sea turtle species are found in the Gulf of California: Olive Ridleys (Lepidochelys olivacea) (Abreu-Grobois & Plotkin, 2008), Green turtles (Chelonia mydas) (Seminoff, 2004), and Hawksbills (Eretmochelys imbricata) (Mortimer & Donnelly, 2008), Leatherback (Dermochelys coriacea) (Wallace, Tiwari & Girondot 2013) and Loggerhead sea turtles (Caretta caretta) (Seminoff et al., 2004).
Sea turtles have a cosmopolitan distribution, with a large abundance of juvenile sea turtles along the Pacific Coast of Baja, California (Martínez-Estévez et al., 2022; Seminoff et al., 2004). The Olive Ridley congregates around the large islands for feeding and in the coastal lagoons and bays around the gulf states before nesting (Lluch-Cota et al., 2007). The Loggerhead and Hawksbill sea turtles feed in the Gulf of California waters (Lluch-Cota et al., 2007). Leatherbacks and Olive Ridleys nest in Baja, California (Nichols, 2003).
Various human activities adversely impact sea turtles in the Pacific, including incidental capture in commercial fisheries, boat strikes, marine debris ingestion, and intentional harvest (Wallace et al., 2008). Marine megafauna species interact with various fishing gear because they occupy broad geographic ranges spanning geopolitical boundaries and oceanographic regions that support many different fisheries (Wallace et al., 2010). Bycatch is believed to be the leading cause of mortality for subadult and adult sea turtles at sea (Degenford et al., 2021; Lewison, Freeman, & Crowder, 2004).
Fishing in Sinaloa
Small-scale fisheries (SSF) are widely acknowledged for their global contributions to food security and livelihoods for coastal communities (Silva et al., 2022). They provide 50% of the global catch for direct human consumption and employ nearly 90% of the world's estimated 120 million fishers (Kolding et al., 2014). When coastal fishers use high bycatch gear such as gillnets, a more diverse catch can be taken to market, or fish catches can be consumed locally or used as bait in other fisheries (Senko & Nalovic, 2021). Sea turtle bycatch is prevalent in coastal fisheries of developing nations, where fishing communities have few livelihood alternatives and experience high rates of poverty and limited access to education (Senko & Nalovic, 2021; Béné, 2003).
Fishing techniques that pose the most significant risk to sea turtles are trawling, longlining, and gillnetting (Mustika et al., 2021). Sea turtle mortality from trawl fisheries is a function of the tow time; turtles captured in trawls are likely to be forcibly submerged for lengths of time greater than their average dive times, which may result in the turtle becoming comatose and eventually drowning (Sasso & Epperly, 2006; Lutcavage & Lutz, 1997; Henwood & Stuntz, 1987). Additionally, delayed mortality due to metabolic disturbance and decompression sickness upon release is expected to be high (García-Párraga et al., 2014). For Leatherbacks, bycatch is most frequently reported (over 50%) by gillnet fishers in Mexico (Ortiz-Alvarez et al., 2020). Loggerhead bycatch and stranding are among the highest reported rates in the world in Baja California, with observed bycatch rates for local small-scale longline (29 loggerheads 1000 hooks–1) and gillnet (1.0 loggerhead km–1 of the net) fisheries (Bojórquez-Tapia et al., 2021; Peckham et al., 2008). In 2012, stranded sea turtles had spiked, increasing from the previous decade by 210–600% (Morales-Zárate et al., 2021; Seminoff et al., 2014). This stranding brought federal fishing restrictive actions (Diario Oficial de la Federación 2018, 2016, 2015) because the high mortality was assumed to be an effect that derived from bycatch (Morales-Zárate et al., 2021).
Datasets and Analysis
I wanted to understand the potential risk of fishing by incidentally capturing sea turtles in the waters surrounding Sinaloa as precursory research for my master's project. Fishers in Sinaloa primarily use gillnets to catch fish and have a bycatch rate of ~20%, and most fishers in the area report fishing as their primary occupation .
I downloaded data from Global Fishing Watch . The data from Global Fishing Watch has had the highest accuracy in recent years, so I decided to utilize 2019 data. The data is downloaded in CSVs for daily fishing hours for the entire globe. In Rstudio, I then subset the data so that it was only on Mexican-flagged vessels in my area of interest. I then merged my wrangled daily CSV files into monthly CSV files, grouping them by gear type and latitude & longitude. I could then convert these monthly CSVs into points in ARCPro. I then visualized the fishing hours for each month.
I downloaded tagged sea turtle data from OBIS-SEA map from 1997-2016. Many of the sea turtles were tagged by my collaborator for my master's project, Dr. Alan Zavala, at Instituto Politécnico Nacional.
Since there was no sea turtle tracking data for 2019, and fishing data from Global Fishing Watch was not accurate before 2017, I decided to overlay the sea turtle tracking data I had over the 2019 fishing data, as I am assuming fishing activity has not significantly changed in the past 10 years. As such, I turned all of the years of the sea turtle data to 2019. There are certainly issues with this assumption, but this project is meant to be an exploration into the possible seasonality of risk of fishing on sea turtles.
Data Catalog
Results
I found that spring and summer had greater fishing effort than the fall and winter months. Sea turtles were more likely to be found further offshore in the winter months when the fishing effort was lower overall. In the summer, most sea turtles are found closer to shore and in the lagoons of Sinaloa. Olive Ridley Sea Turtles (Lepidochelys olivacea) were the most common sea turtle species in the waters surrounding Sinaloa and were found the furthest offshore.
I intersected the fishing data with the sea turtle tracks to find if there were areas or months in which tracked sea turtles transversed areas with high fishing hours. I found that August was the only month in which tracked sea turtles crossed areas with > three hours of daily fishing effort. Olive Ridley Sea Turtles were the most common sea turtle species to occupy areas of high fishing effort and are likely at the greatest risk of bycatch in this region.
Future Research
Sinaloa's waters are considered critical developmental and foraging habitats for several sea turtle species. Previous studies have found high sea turtle bycatch rates in gillnetting, trawling, and longlining. However, further research is required to understand the regional spatial risks of artisanal fisheries on sea turtle populations. I am hoping to use the findings of my term project to provide insight into the seasonal risk of fishing on sea turtles.
For my master's project, I am investigating sea turtle bycatch on artisanal vessels in Sinaloa, Mexico, as a model for implementing camera technology to improve bycatch characterization in remote areas on small-scale fishery (SSF) vessels. In part 1, in collaboration with local fishers trained and permitted through Groupo Tortuguero de Las Californias (GTC), accidental capture of sea turtles during SSF fishing activities in Sinaloa I will remotely assess with Shellcatch vessel cameras (pictured to the left) to elucidate bycatch hotspots by month, species, and sea surface temperature. The hotspot analyses will be conducted using the generalized linear model in R and converted to a bycatch probability surface in ArcGIS. In part 2, I will resolve the frequency and mortality rate of total sea turtle bycatch in Sinaloa, Mexico, by small-scale fishery vessels and the critical fishing methodologies leading to the most lethal sea turtle bycatch rates. In part 3, I will compare bycatch data between fishery gears employed.
This project builds upon the strong relationship between two sea turtle conservation NGOs to bring cross-national collaboration, elevating both local voices and copious resources for effective environmental management. Grupo Tortuguero de Las Californias (GTC) is a prominent non-profit group based in Northwest Mexico that, for over twenty-two years, has collaborated with more than 50 communities and different countries worldwide. Upwell, founded in 2017, has partnered with many prominent universities and research programs to generate and mobilize scientific data to protect endangered turtles at sea.