Bias in sea turtle productivity estimates: error and factors involved

Abstract

The conservation and management of endangered species, including sea turtles, require consistent long-term monitoring of productivity (e.g., number of hatchlings emerged per nest, per female, per nesting site, per population). In sea turtle species, some of the relevant data are obtained by estimating the number of hatched eggs from fragments found in the nest after hatching. Yet, no formal assessment of the associated error has been published. Here we provide an estimation of the error associated with the most widespread method used to determine sea turtle productivity (count of shell fragments > 50% of the egg size) using a large dataset (n = 8539) of actual and estimated clutch sizes of four sea turtle species (Caretta caretta, Chelonia mydas, Dermochelys coriacea, Eretmochelys imbricata). The data are analyzed through linear mixed models with several explanatory variables. Results show that the error can be large in certain cases and, when the associated error rate is not adequately considered, may represent a serious problem in studies on reproductive parameters such as clutch size. Some significant explanatory variables suggest that some sources of error are linked to species-specific biological traits (e.g., clutch size, egg size, nest depth), and others imply human error. Other biotic and abiotic factors may also be involved. We recommend that—whenever possible—errors be assessed and adequately reported by studies that estimate clutch size, hatching and emergence success, or hatchling production.