Sampling nesting sea turtles: optimizing survey design to minimize error

Abstract

Many sea turtle studies globally use counts of nesting activities as a proxy for population abundance estimates and as an indicator of trends within the population. Often these populations are sampled temporally and spatially, but few previous studies have examined the impact of different sampling techniques on the accuracy of these estimates. We investigated temporal sampling errors using a multi-species approach, examining 10 populations comprising green, loggerhead and leatherback sea turtles. Sampling errors were investigated from random, regular and continuous sampling regimes spanning 5-80% coverage. A count approach was used rather than an individual-based capture-mark-recapture approach to broaden the scope and application of the research. Modelling showed that even low survey coverage of 5% gave reasonably accurate estimates of annual nesting activity, with estimated errors of ca. 20% (mean + 1 SD equalling 84.1% of surveys). Survey error is low relative to changes in abundance from the inter-annual variations in nesting activity that occur in sea turtle populations. Thus, annual studies are important to estimate sea turtle abundance, even if these studies have low survey coverage. An increase in survey effort may be more cost effective if spent combining estimates of total nesting activity with sampling turtles as part of a capture-mark-recapture study. This approach will provide a second estimate of annual abundance as well as an estimate of demographic parameters including clutch frequencies, remigration intervals, survivorship, immigration and emigration. This estimation of sampling errors may be useful in the design of monitoring programmes and can be used to guide management and policy decisions.