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Feature

Some of our Fish are Missing

The odd distribution pattern of freshwater fish found in the eastern North Island may be an indication of the after-effects of an 1,800-year-old volcanic eruption.

R. M. McDowall

The occurrence and distributions of species depend on a complex amalgam of historical and ecological influences -- a species occurs where it does because historically it has been able to reach those localities, but equally it is present because the conditions there have been, and remain, suitable, allowing it to survive there to the present day.

As a result of the huge Taupo eruption, a ring of ignimbrite spread to a radius of about 80 kilometres from the centre, while ash was blasted as much as 50 kilometres into the atmosphere. It was swept eastwards by strong westerly winds, covering the eastern North Island to a depth of 10 centimetres as far as the east coast. Virtually all life in the river systems in this area would have been exterminated.

Some contemporary events provide a perspective on the likely extent of the impacts of eruptions in river systems. As has been seen recently, from time to time, lahars (sudden discharges of water, mud, sand and boulders) break out from the crater lakes of the volcanoes of the central North Island. These relatively minor events cause major fish kills and sometimes have impacts that spread downstream for many kilometres. The continual discharge of water from the crater lake of Mt Ruapehu is enough to prevent establishment of fish and other fauna in the upper tributaries of the Whangaehu River.

The Mt St Helens eruption in the US had "immediate and catastrophic effects" over a "very large, massively disturbed area", with "extreme thermal effects" in rivers draining the mountain's catchments. There were massive debris flows in the rivers. If this happened when just 1 km3 was discharged, the chaos caused by the discharge of Taupo's 105 km3 is almost beyond imagination.

Apart from the direct deposition of ash onto the rivers themselves, erosion would have generated huge ash-laden flows of highly toxic water that probably swept down these rivers repeatedly, perhaps for decades. The floods were so great it is considered that they would have destroyed cities at the mouths of rivers like Wanganui, had they existed at the time. Nothing could have survived in Lake Taupo itself.

Some rivers that originate within the "ash zone" flow well beyond the zone, such as the Rangitikei and Wanganui Rivers to the south and south-west, the Waikato River to the north-west, and the Whakatane and Rangitaiki to the north. In these rivers, mainstems were probably defaunated -- lost their animal populations -- by floods. However, tributaries of these rivers that drain from areas beyond the ash zone will, in all probability, have acted as refuges and have retained their faunas.

Once conditions in the mainstem rivers and the upper tributaries were able to recover from the event, the faunas of these refuges would have provided a source for recolonising the whole river system. However, to the east of Lake Taupo, ash deposition occurred as far as the coast, and such refuges are much less likely to have been available. Total defaunation of the river systems seems probable.

If we look at the freshwater fishes that inhabit the rivers that flow eastwards from the central North Island, such as the Mohaka, we find that there are freshwater fish present, but they are, with only a few exceptions, species that customarily spend part of their lives in the sea, such as the longfinned eel.

Other species that are present in the rivers of Hawkes Bay to the south and Bay of Plenty to the north of the "ash zone", and which do not spend part of their lives in the sea, such as dwarf galaxias and Cran's bully, are in general absent from the rivers of the ash zone.

However, particularly to the south, where lower tributaries probably acted as refuges, the upper reaches of rivers like the Wanganui and Rangitikei do have populations of the non-sea migratory species.

What has been a perplexing question -- how to explain the odd distribution of these fish species -- can now be answered in historical and ecological terms.

In terms of history, probably all life, including freshwater fishes was exterminated from rivers where the ash was deposited following the eruption. In terms of ecology, and specifically in terms of behaviour and life histories, species that are able to migrate between river systems by entering the sea have been able to recolonise the rivers once conditions returned to "normal".

But species that do not have marine life stages have not been able to recolonise, and their absence is a question of history and not ecology. They are absent because they have been unable to get there, rather than because habitat conditions are unsuitable -- or at least that is my hypothesis, which would be refuted should these non-migratory species eventually be found to occur widely in the ash zone as additional localities are sampled.

There are occasional known localities for these fish in the ash zone, perhaps because there were some refuges that escaped the most damaging effects of the ash deposition. But they are few, and do not in my view refute the hypothetical scenario that I use to interpret fish distribution patterns.

Revitalizing Lake Taupo

One of the obvious questions to arise from this scenario is: How is that there are fish species in Lake Taupo, itself? The lake is thought to have existed prior to the last eruption, but there can be no question that any life survived there. Now, there are (apart from the exotic salmonids and others) populations of common bully, koaro and common smelt, as well as koura (freshwater crayfish) and kakahi (freshwater mussels). Some of these species are present also in Lake Waikaremoana, to the north-east, and well within the ash zone.

Several explanations are possible. It is known that there were introductions of at least freshwater shrimp from the lower Waikato River into Lake Taupo in the 1930s, and it is possible that common bully and/or common smelt were either accidentally or deliberately introduced at the same time. Smelt were also liberated into Lake Waikaremoana. However, there is somewhat tenuous evidence that common smelt were present in Taupo before the 1930s. And it is certain that koaro were there, as they are widely reported as an important food for Maori around the lake from the earliest times of European settlement. There is Maori tradition of species transfers and introductions, and quite possibly Maori releases of koaro, koura and kakahi into the lakes occurred at some time.

Alternatively, it is possible that one or more of these species found its way into Lake Taupo via headwater capture of streams in the upper Wanganui, Rangitikei or Whangaehu Rivers. Koaro (which is a sea-migratory species and part of the whitebait fishery), at least, could easily have reinvaded Lake Waikaremoana by migration upstream into the headwaters of the Waikaretaheke River prior to the landslide that formed Lake Waikaremoana.

Thus, what appear to be relatively static distributions of fish species around the central and eastern North Island have distributions that relate to a series of events beginning with the Taupo eruption and its extirpation of life in surrounding rivers and lakes through to human intervention to re-establish fish populations.

Bob McDowall is with NIWA (the National Institute of Water and Atmospheric Research) in Christchurch.