Assessing the Population Level Impacts of North Pacific Loggerhead and Western Pacific Leatherback Interactions
February 11, 2020
A population assessment for the NP loggerhead turtle, DPS and the WP leatherback turtle nesting population, with the sole purpose of evaluating the population- level impacts of a single U.S. commercial fishery, on these two populations.
This population assessment is for the North Pacific (NP) loggerhead turtle (Caretta caretta) Distinct Population Segment (DPS) and the western Pacific (WP) leatherback turtle (Dermochelys coriacea) nesting population, with the sole purpose of evaluating the population- level impacts of a single U.S. commercial fishery, the Hawaii-based shallow-set longline (SSLL) fishery on these two populations.
Both populations are listed as Endangered under the Endangered Species Act (ESA), NP loggerheads as a DPS and WP leatherbacks as a global species.
This assessment was performed in a Bayesian framework with four main components:
- nesting data imputation (leatherbacks only)
- nesting trend analysis
- population viability analysis (PVA)
- incorporation of direct SSLL fishery take into the PVA (i.e., adding a “take model” component)
Conducting PVAs under scenarios with and without future takes by the SSLL fishery allowed for evaluation of the impact of the fishery on the population status (e.g., abundance relative to pre-determined thresholds) and trends for the two populations.
Trends and abundance for nesting females were estimated using a Bayesian state-space model (Boyd et al. 2017) with exponential population growth as the underlying process. Time series data used in the model originated from long-term nesting beach monitoring programs in Japan (loggerheads) and Indonesia (leatherbacks).
The loggerhead data from Japan were provided as annual nest counts from three index beaches (Maehama, Inakahama, and Yotsusehama) from 1985 to 2015. The leatherback data came from two index beaches in Indonesia (Jamursba Medi and Wermon) from 2001 to 2017.
The leatherback data set contained months with no monitoring effort; thus, we developed a model to impute the missing data in order to produce a time series of annual nest counts. The imputation model was autoregressive with a lag of one month (AR1 model) where the relationship between the numbers of nests in two months was modeled by the Fourier series.
Martin SL, Siders Z, Eguchi T, Langseth B, Yau A, Baker J, Ahrens R, Jones TT. 2020. Assessing the population level impacts of North Pacific Loggerhead and western Pacific Leatherback interactions in the Hawaii-based shallow set fishery. U.S. Dept. of Commerce, NOAA Technical Memorandum NOAA-TM-NMFS-PIFSC-95, 183 p. https://doi.org/10.25923/ydp1-f891.