Accuracy of pitfall traps for monitoring populations of the amphipod Orchestia gammarella (Pallas 1766) in saltmarshes

The intertidal amphipod Orchestia gammarella displays a key role in the functioning of sand and salt marsh habitats, mainly due to its role in decomposition processes and as a main prey item for fish and birds. This study evaluates the accuracy of two sampling methods for estimating population size, biomass and cohort composition. Pitfall trapping, a passive and easily displayed method, was compared with the depletion of quadrats, an active and time-consuming method, considered here as the reference method. Sampling took place in North-West European saltmarshes (Mont Saint-Michel Bay, France) during the summer, 2003 (64 replicates per method). Linear regression revealed that activity-densities (number of individuals per trap) were strongly and positively correlated with densities (number of individuals per quadrat). Dry biomasses obtained from the two sampling methods were also significantly and positively correlated, but with a low coefficient of correlation. Cohort composition differed between the methods, with proportionally less juveniles and more intermediates in pitfall traps. We conclude that pitfall traps can be used for easily monitoring changes in the size, and to a lesser extent for measuring the biomass, of an amphipod population, but not for estimating its cohort composition.