This paper is concerned with comparing the relative performance of small (0.1 m2) and large (0.25 m2) Hamon grabs for sampling the benthic macro-invertebrates from coarse offshore marine sediments. We present results based on an actual survey near the Isle of Wight, UK, but we also summarize and develop a statistical theory as well as demonstrate the theory using a small simulation study. Our results show that: the larger grabs are more difficult to handle, especially from small vessels; for a given sampling area the small grab can provide a more precise estimate of abundance than the larger grab if the individuals are clustered; the smaller grab can detect more species than the larger grab if individuals from the same species are clustered together (in our study, approximately 5% more species); and the smaller grab may undersample larger macrofaunal species and also species that live on larger cobbles.

We present theoretical equations to demonstrate the two samplers abilities to capture cobbles of different sizes. Multivariate techniques suggest that differences in the composition of the fauna collected using the different sized grabs are statistically significant, but small. It is more expensive to process samples from the smaller grab per unit area. However, per grab, the smaller grab is cheaper. We also present a model to demonstrate the effect on species occurrence and abundance.