Flexible life history strategies in a New Zealand diadromous galaxiid, giant kokopu (Galaxias argenteus)

Bruno O. David^1*, Lindsay Chadderton², Gerard P. Closs¹, Bernard Barry³, Andreas Markwitz³.

¹Department of Zoology, University of Otago, P.O. Box 56, Dunedin, New Zealand.
^*Present address- Northland Conservancy, Department of Conservation, P.O. Box 842, Whangarei, New Zealand
²Northern Regional Office, Department of Conservation, P.O. Box 112, Hamilton, New Zealand.
³Institute for Geological and Nuclear Sciences, Wellington, New Zealand.
Email: bdavid@doc.govt.nz

Diadromy (migration between marine and freshwater) is a common strategy in many species of coastal freshwater fish. In New Zealand, nearly half the fish are considered diadromous, including 5 Galaxiidae. The largest galaxiid, giant kokopu (Galaxias argenteus) is believed to exhibit an amphidromous life history, typical of many New Zealand fish. However, the occurrence of isolated coastal populations, often associated with either coastal freshwater wetlands or estuaries, suggests diadromy may be facultative, even where access to marine habitats is continuously available. Otolith microchemistry of 35 giant kokopu was examined using particle induced X-ray emission (PIXE). Fish were collected from five coastal streams with varying geomorphologies in Southern New Zealand. Most fish exhibited relatively uniform and low Sr:Ca ratios throughout the otoliths, indicative of life-cycles completed wholly in freshwater. Exceptions were all 10 fish collected from a stream draining into a estuarine lagoon, all of which reared in estuarine conditions, and one of eight fish from a coastal lake that had had a marine larval phase. Within the coastal region studied, non-diadromous freshwater or estuarine recruitment appears to dominate in giant kokopu. Diadromy does not appear to drive population dynamics, although it may be an important dispersal mechanism.

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