A Comparison of Methods for Estimating Relative Abundance of Grapsid Crabs

Abstract

Grapsid crabs are one of the most common, and potentially important, elements of the mangrove fauna but relatively little information is available on patterns in their distribution and abundance. In part, this may be due to difficulties in estimating the abundance of burrowing species. By not having reliable methods of estimating changes in distribution and abundance of crabs, ecological impacts of crabs may be greatly underestimated. We tested several methods for estimating the apparent abundance of eight species of grapsids in a north Australian mangrove forest. These methods included continuous and instantaneous visual counts at two distances, two types of pitfall traps and photography. We also excavated crabs to test the reliability of the best of these methods. Overall, pitfall traps equipped with funnels proved most useful, although these did preferentially capture larger crabs. An exception was the large crab, Neosarmatium meinerti, which was rarely captured and more reliably estimated by burrow counts. Traps proved to be most useful in this study and may be for long term studies of grapsid species, however, the selection of a method should be made after careful evaluation of the questions and relevant information required for any particular study. Finally, these methods may also prove to be useful in other environments such as salt marshes.

Site specific distribution of the mud shore crab Helograpsus haswellianus in temperate wetlands

“…Accurate assessment of crab abundance is not easy, but the surface activity of crabs reflected in pitfall traps can give a good estimate of proportional abundance (Mazumder and Saintilan 2003;Kent and McGuinness 2006). The results presented in this study reflect both the crab abundance and activity between habitats.…”

“…Accurate estimations of crab abundance in vegetated intertidal areas are difficult to obtain, as all common sampling methods have their biases (Kent and McGuinness 2006). Although pit traps are limited to catching crabs active on the surface and tend to catch larger individuals (Smith et al 1991), they were deemed the best choice out of all available nondestructive methods for frequent monitoring of crab populations.…”

“…Weekly surveys were carried out during the 4 main lunar phases. Trapping was conducted over a three-day period as a longer trapping period may increase accuracy of abundance estimates (Smith et al 1991;Frusher et al 1994;Kent and McGuinness 2006). On the first day, the traps were opened and fitted with a funnel.…”

“…This can be caused by sampling bias (Kent and McGuinness 2006), behavioural differences (Czerniejewski and Wawrzyniak 2006), sex specific habitat distribution (Johnson 2003), mortality rates ) and preferential predation (Bergey and Weis 2008).…”

Mangrove rehabilitation in high erosion areas: Assessment using bioindicators

“…This could be because the instantaneous count method used only allowed for identification to coarse taxonomic levels. Although Kent and McGuinness (2006) remark that instantaneous counts are suitable for crabs in situations where time is limited, in this study the use of this method limited the level of identification possible. Future studies should consider methods that allow for better identification, such as the enclosure removal method (Koch and Wolff, 2002;Geist et al, 2011) or timed capture method (Macintosh et al, 2002;Ashton et al, 2003).…”