The rise and fall of water net (Hydrodictyon reticulatum) in New Zealand

During the late 1980's to early 1990's a range of aquatic habitats in the c entral North Island of New Zealand were invaded by the filamentous green al ga, water net Hydrodictyon reticulatum (Linn. Lagerheim). The alga caused s ignificant economic and recreational impacts at major sites of infestation, but it was also associated with enhanced invertebrate numbers and was the likely cause of an improvement in the tr out fishery: The causes of prolifi c growth of water net and the range of control options pursued are reviewed . The possible causes of its sudden decline in 1995 are considered, includi ng physical factors, increase in grazer pressure, disease, and loss of gene tic vigour.