Environmental factors associated with the decline of dwarf inanga Galaxias gracilis McDowall in New Zealand dune lakes

1. The dwarf inanga Galaxias gracilis is restricted to 13 dune lakes along an 80 km length of coastline on the west coast of the North Island, New Zealand, and its abundance has declined in at least eight of these lakes. It is therefore a rare and endangered endemic species. 2. A range of environmental variables were compared between lakes where it is still common and lakes where it is now rare to identify variables associated with its low abundance. 3. Lakes where dwarf inanga were rare all had a lower pH, lower concentrations of calcium, magnesium and silica, and higher concentrations of chloride than lakes where dwarf inanga were common. In addition, they contained different charophyte plant species, as well as stocked populations of eels and/or introduced exotic fish species capable of preying on one or more of the life stages of dwarf inanga. 4. Factors not associated with the low abundance of dwarf inanga included lake size and depth, land use in the catchment, lake level fluctuations, the extent of emergent vegetation, littoral zone substrate composition, and water quality parameters including maximum water temperature, water transparency, and minimum oxygen levels in the hypolimnion. 5. The identification of factors associated with the low abundance of dwarf inanga has allowed testable hypotheses to be developed to account for its decline in certain lakes. However, biological data, particularly on the location of spawning grounds and on the feeding requirements of larvae, are required before all these hypotheses can be tested. 6. The results have formed the basis for a management plan for the conservation of this species, in which whole-lake experiments are being carried out to identify the effects of introduced plants and fish on its population size. ©1997 John Wiley & Sons, Ltd.