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Major volcanic eruptions in the central North Island of New Zealand and, in particular, an eruption dated at ad 186, spread ignimbrite and volcanic ashes (tephra) over a broad area, deposition of which caused major defaunation of affected habitats. However, riverine freshwater fish faunas were undoubtedly affected far beyond the zones of ignimbrite deposition and ash settlement as a result of ash-laden floods moving down rivers. In areas where entire river systems were covered with ash, fish faunas that recolonized rivers now consist almost entirely of diadromous species that have been able to recolonize the river systems by dispersing through the sea. However, some rivers, that were probably severely affected in their headwaters, have lower elevation and more distant tributaries whose catchments escaped ash showers; these provided refuges for non-diadromous species which could recolonize upstream once rivers became inhabitable. In such rivers the fish faunas are now a mix of diadromous and non-diadromous species.