VOLCANISM AND FRESH-WATER FISH BIOGEOGRAPHY IN THE NORTHEASTERN NORTH-ISLAND OF NEW-ZEALAND

Major volcanic eruptions in the central North Island of New Zealand an d, in particular, an eruption dated at AD 186, spread ignimbrite and v olcanic ashes (tephra) over a broad area, deposition of which caused m ajor defaunation of affected habitats. However, riverine freshwater fi sh 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 d iadromous species that have been able to recolonize the river systems by dispersing through the sea. However, some rivers, that were probabl y severely affected in their headwaters, have lower elevation and more distant tributaries whose catchments escaped ash showers; these provi ded refuges for non-diadromous species which could recolonize upstream once rivers became inhabitable. In such rivers the fish faunas are no w a mix of diadromous and non-diadromous species.