Unreliable population inferences from common trapping practices for freshwater turtles

Abstract

a b s t r a c tFundamental questions in ecology and conservation require reliable data about population size and structure. For freshwater turtles, such data are often obtained via mark-recapture trapping, but commonly used trap types are biased in the sex and age classes they sample although these biases are seldom quantified. We present data from 11 populations of Western painted turtles (Chrysemys picta bellii; 1107 turtles total, n caught per pond 6-322) captured in hoop traps, dip-nets, and basking traps to examine bias in captures and the impact on estimates of population size and sex/age ratios. Hatchlings and juveniles were primarily captured in dip-nets, while hoop nets had the lowest capture rates for adults. Most turtles were caught only once; among recaptures, the majority were recaptured in the same trap type. Estimates of population size and sex/age ratios varied strongly when we calculated results from each trap type separately versus combining all captures. These results show clearly that turtle sampling that uses only one trap type will almost certainly mis-estimate population size and sex/age ratios. These results are troubling in the light of current practice: of population studies of North American turtles published during 2009-2014, 45% used only one trap type, and 49% of studies did not even mention possible sampling biases. The conservation implications are serious, as current trapping efforts probably result in erroneous population estimates and sex/age ratios, which may encourage management actions that are not needed or may obscure actions that are in fact necessary for viable populations.

Assessing the Impacts of Urbanization on Sex Ratios of Painted Turtles (Chrysemys picta)

“…We captured more males in the spring than later summer, and more males in smaller hoop nets. This was not unexpected as differential capture rates by trap type and season are well documented [47,49,50,[65][66][67][68]. We acknowledge that seasonal and trap differences in sex-specific capture rates could be obscuring any impacts of urbanization on sex-ratio, but we do not think these differences were large enough to do so, given our extreme male skew.…”

Rethinking biogeographic patterns: high local variation in relation to latitudinal clines for a widely distributed species

“…We surveyed turtles at ten ponds throughout the central and southern valley. The elevations of ponds ranged from 385 m on the valley bottom to 924 m and ponds were selected to represent the range of ruralurban settings in which turtle populations are found in this region (Tesche and Hodges 2015). Each pond was trapped for a single capture-mark-recapture session of 3-8 sunny days.…”

“…Each pond was trapped for a single capture-mark-recapture session of 3-8 sunny days. The length of each trapping session was determined by the recapture rate, as we trapped for a longer period at sites with low recapture rates in an attempt to improve population estimates (Tesche and Hodges 2015). Three hoop nets (76.2 cm diameter, 3.81 cm 2 mesh; Memphis Net and Twine, TN) were set at each pond.…”

“…We also discount sampling variance as an explanation: all ponds were sampled by the same team of researchers, in the same year, and classified using the same size thresholds. Our research was partly designed to obtain rigorous population estimates for these sites (Tesche and Hodges 2015), so we are also confident we obtained representative samples from each pond. Phenotypic plasticity is often high for turtle populations.…”

“…If local populations are highly variable in body sizes of either or both sexes, latitudinal or other large geographical patterns could be falsely inferred if local variation is insufficiently sampled. Biases can arise if traps preferentially catch one sex or certain sizes of animal (Tesche and Hodges 2015), or if museums prefer large specimens. Maturity threshold values are particularly important in species where it is difficult to distinguish between the sexes or to identify sexual maturity, both of which may occur for reptile species with temperature-dependent sex determination.…”