Calculating reference points for anthropogenic mortality of marine turtles

Abstract

ABSTRACT1. Human-caused mortality threatens many marine turtle populations worldwide, with fisheries interactions being a primary cause for population declines. National and international management of fisheries interactions with marine turtles are rarely tied to turtle population biology. Quantitative tools tied to population-based objectives can provide insight into the effectiveness and urgency of bycatch mitigation.2. A management approach is proposed based on a bycatch control rule called Reproductive Value Loss Limit (RVLL), generalized from the Potential Biological Removal management model for marine mammal populations. For RVLL, population size is scaled by reproductive value to account for strongly age-structured population dynamics and age-dependent fisheries mortality rates in marine turtle populations.3. RVLL is an estimate of maximum sustainable mortality for a population, calculated from estimates of maximum population growth rate, total reproductive value in the population, and an uncertainty factor. RVLL estimates correspond to specified management goals and risk tolerances. For demonstration, simultaneous goals of maintaining populations above the maximum net productivity level (analogous to the population size that produces maximum sustainable yield) and preventing a decrease in adults are assumed, both with 95% probability. A management-strategy-evaluation-like process was used to explore parameterization of the RVLL equation for robust performance over a range of plausible life history characteristics and uncertainties in abundance and bycatch mortality estimates for marine turtle populations.4. The RVLL-based management approach presented here proved robust to several important sources of uncertainty and to violation of several key underlying assumptions, and can be adapted to account for important sources of bias. The architecture presented, including tailored management strategy evaluation, provides a useful basis for further development of reference-point-based management of human-added mortality in populations that experience large changes with age in reproductive value and human-caused mortality rates, as is the case for marine turtles.

A spatially explicit population model of simulated fisheries impact on loggerhead sea turtles (Caretta caretta) in the Northwest Atlantic Ocean

“…Annual mortality in terms of adult equivalents was robust to how removals of adult equivalents were distributed because using reproductive values accurately represented the contributions that turtles of any age class made to population productivity. Our results were consistent with those of Curtis and Moore (2013), who found that a PBR-type measure was a useful tool for making population assessments if it incorporated reproductive values, but it failed if implemented in terms of individual turtles. Reproductive values are sensitive to changes in reproductive parameters (Heppell, 2005), so it can be important to an adult-equivalent analysis to include uncertainty in remigration, fecundity, and age at first breeding.…”

“…We also investigate the role of spatial differences in mortality in affecting the simulated underlying population dynamics and consider region-specific changes in the magnitude of the incidental take on population dynamics. Several sea turtle population analyses have incorporated reproductive values for sea turtles (NMFS and USFWS, 2008;Wallace et al, 2008Wallace et al, , 2013Haas, 2010;Bolten et al, 2010;Curtis and Moore, 2013), although none also included a spatial component.…”

Improved estimation of intrinsic growthrmaxfor long‐lived species: integrating matrix models and allometry

“…This is especially important in conservation settings that use reference point (e.g., mortality limit) estimators based on r max . For example, PBR, which has also been adapted for seabirds and sea turtles (Dillingham and Fletcher , , Curtis and Moore , Richard and Abraham ), includes the parameter R max =exp( r max )−1, and is <0.10 for many of the long‐lived marine megafauna to which it is applied (Moore et al. ).…”

How much sea turtle bycatch is too much? A stationary age distribution model for simulating population abundance and potential biological removal in the Mediterranean

“…However, differently from marine mammals, the size and demographic structure of sea turtle populations are commonly unknown, and the numbers of nests or adult females are used as abundance indices. In order to overcome this problem and apply PBR to sea turtle populations where only the number of adult females is known, Curtis & Moore (2013) recently developed a maximum bycatch estimation approach which uses reproductive value equivalents in place of the number of individuals, an approach that includes additional uncertainty represented by the reproductive value equivalents. In this study, we addressed the demographic problem in a different way, by estimating potential distributions of the number of individuals in the age classes subjected to anthropogenic mortality and applying the original and simpler PBR approach.…”