Quantifying local ecological knowledge to model historical abundance of long-lived, heavily-exploited fauna

Abstract

Deriving robust historical population trends for long-lived species subject to human exploitation is challenging in scenarios where long-term scientific data are scarce or unavailable, as often occurs for species affected by small-scale fisheries and subsistence hunting. The importance of Local Ecological Knowledge (LEK) in data-poor scenarios is increasingly recognized in conservation, both in terms of uncovering historical trends and for engaging community stewardship of historic information. Building on previous work in marine historical ecology and local ecological knowledge, we propose a mixed socio-ecological framework to reliably document and quantify LEK to reconstruct historical population trends. Our method can be adapted by interdisciplinary teams to study various long-lived taxa with a history of human use. We demonstrate the validity of our approach by reconstructing long-term abundance data for the heavily-exploited East Pacific green turtle (Chelonia mydas) in Baja California, Mexico, which was driven to near extinction by a largely unregulated fishery from the early 1950s to the 1980s. No scientific baseline abundance data were available for this time-frame because recent biological surveys started in 1995 after all green turtle fisheries in the area were closed. To fill this data gap, we documented LEK among local fishers using ethnographic methods and obtained verified, qualitative data to understand the socio-environmental complexity of the green turtle fishery. We then established an iterative framework to synthesize and quantify LEK using generalized linear models (GLMs) and nonlinear regression (NLR) to generate a standardized, LEK-derived catch-per-unit-effort (CPUE) time-series. CPUE is an index of abundance that is compatible with contemporary scientific survey data. We confirmed the accuracy of LEK-derived CPUE estimates via comparisons with fisheries statistics available for 1962–1982. We then modeled LEK-derived abundance trends prior to 1995 using NLR. Our model established baseline abundance and described historical declines, revealing that the most critical (exponential) decline occurred between 1960 and 1980. This robust integration of LEK data with ecological science is of critical value for conservation and management, as it contributes to a holistic view of a species’ historic and contemporary conservation status.

Integrating ecological monitoring and local ecological knowledge to evaluate conservation outcomes

“…Nonetheless, current local abundance remains significantly below baseline levels. This result is consistent with the consensus among senior fishers that green turtles are abundant but below numbers observed when they were young harpooners (Early-Capistrán et al 2020a). Thus, continued conservation measures are required for the population to return to historical levels.…”

“…Bahía de los Ángeles (BLA), Baja California, Mexico (28.951917°, −113.562433°) is a warm-temperate green turtle foraging area in the Gulf of California (Figure 1). This index site hosts significant in-water aggregations of turtles and systematic long-term scientific monitoring (Early-Capistrán et al 2020a), initiated in 1979 through government-sponsored efforts (Seminoff et al 2008). In-water monitoring began in 1995 and has continued as a joint effort between government, academic, and non-governmental institutions (Figure 2).…”

“…We used an existing dataset of LEK-derived catch-per-unit-effort (CPUE) estimates to establish baseline abundance and analyze long-term change before scientific monitoring (Early-Capistrán et al 2020b) (Figure 2). We compiled data in an on-going transdisciplinary research process with the BLA community initiated in 2012.…”

“…Successful conservation of highly migratory, long-lived species such as sea turtles requires efforts and policies implemented across large spatio-temporal scales, along with locally-grounded data and practices (Mazaris et al 2017; Vierros et al 2020). Local Ecological Knowledge (LEK) can contribute to sound conservation and management practices by establishing local baselines and recovery targets; evaluating population status; integrating the cultural dimensions of human-environment interactions; and supporting equitable and inclusive practices (Early-Capistrán et al 2020a; Lee et al 2018; Poe et al 2014). LEK-based approaches are particularly important for taxa impacted by small-scale fisheries or subsistence hunting, for which technical or baseline abundance data may be scarce or un available (cf.…”

“…This process was coupled with intense egg collection in the populations’ tropical reproductive habitats (Delgado-Trejo & Alvarado Díaz 2012; Seminoff et al 2015) (Figure 1). Long-term conservation and research began in the late 1970s as green turtles diminished, and in the early 1980s the fishery collapsed (Early-Capistrán et al 2020a; Seminoff et al 2008) (Figure 2).…”