The snag bag: a new method for sampling macroinvertebrate communities on large woody debris

La Trobe University Faculty of Science, Technology and Engineering Murray Darling Freshwater Research Centre

MDFRC item.

Sampling macroinvertebrate communities from natural snags (large woody debris) is difficult, particularly in southeastern Australia where the predominant river red gum snags are very large and heavy. This makes removal of natural snags impractical. The Snag Bag is a new method for sampling macroinvertebrates from natural snags. It consists of a 250 m mesh bag with the sides closed by Velcro. When opened out, one side of the Snag Bag can be passed under the snag and the bag is then closed around the snag, before brushing the surface down to remove invertebrates. The Snag Bag is removed from the snag by loosening the Velcro above the snag and closing it below, until the bag has almost been removed. The bag is then quickly pulled from the snag and out of the water. Snags with diameters of 5–30 cm at depths of up to about 50 cm can be sampled. The Snag Bag was trialed in two lowland rivers in south-eastern Australia. Repeated sampling showed that its efficiency was about 96%, comparing very favourably with other methods of collecting macroinvertebrates in streams. Its efficiency was not affected by current speed or the surface area sampled. A subsampling procedure was developed and tested, as whole samples were often extremely large. A proportion of the sample equivalent to a set surface area (0.05 m2) was sorted. If fewer than 300 animals were obtained, further known percentages of the original sample were processed fully, until 300 animals had been removed. The Snag Bag provided quantitative data that allowed discrimination between densities of invertebrates on snags at two different sites on the same river.