Scientists Design Systematic Framework to Quantify Pacific Leatherback Sea Turtle Bycatch

Marine researchers have established a standardized systematic review protocol designed to aggregate fragmented fisheries interaction data and quantify total incidental bycatch for critically endangered leatherback sea turtles across the entire Pacific Ocean basin. The Pacific Ocean supports two distinct populations of leatherback sea turtles, Dermochelys coriacea, identified respectively across eastern and western nesting ranges, with both populations classified as Critically Endangered by the International Union for Conservation of Nature. Historically, nesting beach disturbances and the human harvesting of turtle eggs and adult turtles drove substantial population collapses, but extensive international conservation interventions have largely mitigated direct threats on terrestrial nesting habitats. Currently, incidental capture during commercial and artisanal fishing operations represents the most pervasive and unquantified hazard preventing population recovery across both ocean divisions. Because leatherbacks undertake extensive trans-oceanic migrations and have lengthy generation times, incidental captures in diverse fishing gears cause ongoing population decline. This systematic review protocol establishes a comprehensive framework to compile existing academic and grey literature to estimate basin-wide mortality rates for future conservation planning.

Incidental fisheries bycatch causes two primary outcomes for sea turtles: immediate mortality that directly removes individuals from the population, or morbidity involving sublethal injuries. Leatherback turtles routinely interact with multiple gear types, including pelagic longlines, gillnets, and industrial trawls, operating across both small-scale community fleets and large industrial operations targeting species such as tuna and billfish. Under the classification framework of the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services, small-scale fisheries exhibit low capital investment, family or community labor structures, small vessels operating nearshore on single-day trips, and local catch consumption or direct commercial sale. Conversely, industrial fleets utilize high capital expenditures, motorized onboard processing mechanisms, large offshore vessels, and multi-day voyages servicing global seafood commodity supply chains. Regulating migratory bycatch across these distinct sectors remains extremely difficult because operational policies demand consensus and consistent enforcement among separate nations, regional fisheries management organizations, and international conservation conventions, leaving the true Pacific-wide removal rate unresolved.

Conservation decision-making for Pacific leatherbacks has historically relied on population viability analysis, a quantitative modeling approach that integrates demographic and life history parameters to simulate future extinction probabilities under different management actions. Reliable population viability models require accurate population-specific inputs, but parameter uncertainty frequently sparks concern regarding spurious predictions when derived from incomplete observations. For the East Pacific leatherback stock, existing population viability analyses based on available regional data have projected population extinction by the year 2080, prompting urgent international conservation interventions. For the West Pacific stock, population viability modeling has informed established bycatch limit regulations within the United States longline fleet alongside nesting beach assessments, though existing frameworks do not integrate all Western subpopulations into a single cohesive model. In both regions, mathematical projections are heavily driven by mortality and morbidity inputs derived from fisheries with unverified observer coverage, which frequently falls below one percent in artisanal and commercial fleets.

To overcome past analytical fragmentation, the systematic review protocol synthesizes methodologies from earlier literature surveys while addressing the constraints that historically limited cross-fishery comparisons. Prior systematic reviews of marine bycatch varied substantially in focal species scope, either evaluating multi-taxa assemblages of seabirds, marine mammals, and sea turtles, assessing all seven marine turtle species simultaneously, or examining single species. Historical geographic scopes ranged from local single-fishery assessments and national reviews to broader regional, basin-scale, and global compilations. Furthermore, previous studies often restricted evaluations to solitary fishing gears, such as pelagic longlines, or examined net and trawl interactions using disparate bycatch per unit effort standardizations. Data sources also varied, drawing selectively from fisher logbooks, onboard observer reports, or published scientific literature to reconstruct past catches, build risk maps, or generate extrapolated indices. The authors adapt these foundational methods by focusing exclusively on a single species across an entire ocean basin while evaluating all active fishing gear types.

To ensure reliable synthesis despite missing bycatch reports in poorly monitored sectors, the investigators adapted catch reconstruction principles established in broader fisheries research to a single-species assessment for the first time. Catch reconstruction methodologies systematically locate at least one baseline data source for every operational characteristic of interest, maintaining strict transparency regarding source provenance while explicitly scoring the analytical confidence of every estimate. The overarching objective of the systematic review is to assemble all documented information and quantitative estimates of Pacific leatherback bycatch into a standardized, accessible repository to inform conservation management. This synthesis involves three sequential steps: classifying bycatch records across specific fishery characteristics including gear type, fishing nation, operating region, and target species; categorizing numerical estimates by publication format across academic journals and non-peer-reviewed grey literature; and executing meta-analyses to identify underlying data discrepancies and pinpoint critical monitoring deficiencies requiring targeted scientific observation.

The review framework applies the SPIDER search strategy tool, which structures analytical eligibility around Sample, Phenomenon of Interest, Design, Evaluation, and Research type to answer primary and secondary research questions. The primary question seeks to determine how many leatherback sea turtles are killed each year in Pacific fisheries through bycatch interactions, while the secondary question evaluates whether specific data sources or operational fishery characteristics generate systematically elevated mortality estimates. During protocol development, the authors executed a scoping exercise using ten selected reference articles reflecting diverse fishery sizes, gear types, target catches, and Pacific geographic sectors. Abstracts from these reference studies were processed through word cloud generation software to extract dominant terminology, which was combined with methodological SPIDER parameters to construct a validated Boolean search syntax.

The comprehensive literature retrieval architecture targets academic publications alongside grey literature across multiple bibliographic databases and organizational channels. Academic literature will be systematically harvested from SCOPUS, the Web of Science Core Collection, and Aquatic Sciences and Fisheries Abstracts, adapting the verified Boolean syntax for each engine interface. Because grey literature produced by commercial bodies, industry groups, and government agencies rarely appears in commercial academic databases, the authors developed supplementary Google Scholar queries using targeted search strings. These queries combine mandatory core terms with specific operational factors, encompassing sixty-three designated nations and territories possessing Pacific Exclusive Economic Zones or documented fishing presences, alongside thirty-three official languages. Non-English records retrieved through these language-specific queries will be sorted by relevance, with the top ten candidate publications translated into English using DeepL translator before undergoing initial eligibility screening based on titles, keywords, and abstract summaries.

Supplementary literature identification incorporates snowball sampling by screening bibliographies of eligible papers, contacting lead investigators, and querying specialized scientific networks, including the CTURTLE listserv, the Inter-American Tropical Tuna Commission Bycatch Working Group, the Inter-American Convention for the Protection and Conservation of Sea Turtles Scientific Committee, the Laúd OPO Network, and MigraMar. Researchers will also systematically review administrative bycatch filings from individual fishing nations and examine official websites of multilateral management authorities, such as the Western and Central Pacific Fisheries Management Commission, the Food and Agriculture Organization, the South Pacific Commission, and the Secretariat of the Pacific Regional Environment Programme. Articles satisfying initial abstract screening will undergo full-text appraisal by the lead author using the Collaboration for Environmental Evidence critical appraisal tool to identify and control methodological bias, with an additional reviewer independently resolving ambiguous classifications.

During formal data extraction, studies deemed eligible will have detailed characteristics systematically compiled, prioritizing wild leatherback observations by life stage, sex, and Pacific geographic region. Documented phenomena of interest include specific fishing gear configurations, capture conditions such as mouth-hooking, foul-hooking, or body entanglement, and metrics of fishing effort encompassing vessel numbers, active soak times, and operating areas. Methodological study design will record source type, noting whether data originated from case studies, controlled experiments, fisher interviews, logbooks, onboard observers, government filings, industry reports, or surveys. Data evaluation will extract counts of turtle interactions, clinical fate, recorded sublethal morbidities, and confirmed mortalities. Each data point will receive a numerical certainty score from one to four adapted from catch reconstruction frameworks, where multiple published values receive the highest score of four, single published values receive three, extrapolated values receive two, and unverified gear averages receive one.

The analytical framework standardizes reported interactions into quantitative metrics, calculating bycatch per unit effort and mortality per unit effort normalized to one thousand hooks, one net, one purse seine, or one trawl set. To incorporate cryptic post-release mortality, total mortality per unit effort will be derived by adding observed direct mortalities to the estimated proportion of released live turtles expected to die from trauma, using gear-specific rates from the Inter-American Tropical Tuna Commission. To scale normalized rates into an absolute annual Pacific mortality estimate, researchers will incorporate publicly accessible daily vessel tracking data from Global Fishing Watch. By matching documented hours of fishing activity with gear classifications and flag states, bycatch rates will be transformed into total mortality per fishing hour and multiplied by total annual fleet effort. Synthesized estimates will be tested for statistical assumptions, analyzed through contingency tables, chi-squared tests, and analysis of variance, and evaluated through expert surveys before presentation to an International Union for Conservation of Nature planning workshop, with all resulting datasets archived in the Dryad repository.

Subject of Research: Systematic review protocol for quantifying Pacific leatherback sea turtle bycatch mortality

Article Title: A systematic review protocol for quantifying bycatch of critically endangered leatherback sea turtles within the Pacific Ocean basin

Article References: Ortega, A. A., Mitchell, N. J., Marn, N., & Shillinger, G. L. (2024). A systematic review protocol for quantifying bycatch of critically endangered leatherback sea turtles within the Pacific Ocean basin. Environmental Evidence, 13(1), Article 27. https://doi.org/10.1186/s13750-024-00352-3

Image Credits: AI Generated

DOI: 10.1186/s13750-024-00352-3

Keywords: marine conservation, leatherback sea turtles, fisheries bycatch, Pacific Ocean, population viability analysis, systematic review protocol, Global Fishing Watch, endangered species, systematic, review, protocol, quantifying