Fish and Game field officer Stuart Sutherland holds up boulders from the bed of the Mararoa River in Southland. The stones are covered in mats of the diatom Didymosphenia geminata, a single-celled alga native to temperate rivers in the northern hemisphere. The invasive alga was first noted in the Waiau River in Southland in 2004. By 2006 it had spread to some other South Island rivers. As it smothers the riverbed it reduces the number of mayflies and other aquatic insects, and this in turn is likely to reduce fish populations as their food supply drops.
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