Estimating density of intertidal ghost shrimps using counts of burrow openings. Is the method reliable?

This study investigated whether surface hole counts could be used as a reliable estimate of density of the ghost shrimps Trypaea australiensis Dana 1852 and Biffarius arenosus Poore 1975 (Decapoda, Thalassinidea) in south eastern Australia. The relationship between the number of holes and the number of ghost shrimps was explored in two ways. Resin casts were used to document any changes in the number of burrow openings per shrimp burrow over time. Manual suction pumping (bait pumping) within a given mudflat area was used to directly compare the number of holes on the sediment surface with the number of ghost shrimps occupying the corresponding volume of sediment. Resin casting showed that throughout the year, the burrows of T. australiensis consistently had an average of two openings, whereas the burrows of B. arenosus showed much greater variability over time with two to four openings per burrow. Overall, a significant relationship between the number of holes and the number of ghost shrimps (mixed species populations) was found, with 2.1 burrow openings for each ghost shrimp. However, some temporal and spatial variation was seen in this relationship. We suggest that the hole count method may be reliable in estimating ghost shrimp densities with restricted use and site specific validation based on some limitations found in this study.