Trap characteristics and species morphology explain size-biased sampling of two salamander species

Demographic studies often depend on sampling
techniques providing representative samples from populations. However, the
sequence of events leading up to a successful capture or detection is
susceptible to biases introduced through individual-level behaviour or
physiology. Passive sampling techniques may be especially prone to sampling
bias caused by size-related phenomena (e.g., physical limitations on trap
entrance). We tested for size-biased sampling among five types of passive traps
using a 9-year data set for two species of aquatic salamanders that have a 20
and 61 fold change in length over their ontogeny (Amphiuma means, Siren
lacertina). Size-biased trapping was evident for both species, with body
size distributions (body length mean and SD) of captured individuals differing
among sampling techniques. Because our two species differed in girth at similar
lengths, we were able to show that size biases (in length) were most likely
caused by girth limitations on trap entry rates, and potentially by differences
in retention rates. Accounting for the biases of sampling techniques may be
critical when assessing current population status and demographic change.