Developing methods for quantifying the apparent abundance of fiddler crabs(Ocypodidae : Uca) in mangrove habitats

Counting the number of individuals emerging from burrows is the most practi cal method for estimating the apparent abundance of Australian Uca species living in mangrove habitats. Experiments were conducted to investigate the effect on counts of quadrat design, distance of observer, quadrat size, rec overy time and observational technique. Significant differences in the appa rent abundance of one species were found when the subjects were within 2 m of the observer, and when a conspicuous quadrat was used. The largest quadr at tested provided the least variability in counts but an intermediate size (0.56 m(2)) was more practical. Most Uca active within a 30-min period eme rged during the first 10 min regardless of site, species, sex or season. Th ere was a linear correlation between scanning and continuous observation in dicating that the former method could be useful when sampling time was limi ted. Temporal changes in the apparent abundance of Uca suggest that long-te rm sampling and more detailed studies will be worthwhile.