| Summary: | The giant kokopu (Galaxias argenteus) is the largest of the galaxiid fishes, and is endemic to New Zealand. Some landlocked (non-migratory) populations exist, but giant kokopu are normally diadromous, and juveniles make up a small part of the annual whitebait run. The species is now regarded as threatened, and its rarity has led to controversy over proposed changes to whitebait fishing regulations. Although exploitation of the juveniles may be limiting recruitment to adult populations, the perceived decline of giant kokopu has been attributed mostly to the loss and degradation of its habitat. In order to manage and conserve the species, the critical features of giant kokopu habitat need to be determined.
Analysis of information from the New Zealand Freshwater Fisheries database, and from field surveys in Southland and along the western coast of the South Island, indicate that five habitat features are critical: the presence of instream cover, deep water, low water velocity, proximity to the sea, and overhead shade/riparian cover. These features were critical in both regions surveyed and for both juvenile and adult fish. The effects of different types of riparian and in stream cover were examined, but it appeared that the presence of some sort of cover was more important than its composition. Giant kokopu readily utilised artificially constructed habitat, which emphasised the importance of instream cover and low water velocity.
Diet was investigated from the examination of the gut contents of 105 fish, and results compared to other published information. Giant kokopu are probably best described as opportunistic feeders, as they utilise a wide range of foods of both aquatic and terrestrial origin. Terrestrial components of the diet appear to be significant for giant kokopu, which may partly explain the importance of overhead shade and riparian cover in giant kokopu habitat.
Giant kokopu have been found in a wide variety of water types around New Zealand, and are known to co-occur with 33 other native and introduced species of fish. It appears that they are more likely to occur in habitats where introduced brown trout (Salmo trutta) are absent, although the two species do sometimes co-occur and cannot be said to be mutually exclusive.
Migrations of juvenile (whitebait) giant kokopu into freshwater probably occur mostly after the end of the whitebaiting season, and their capture may no longer be a serious conservation concern. The impact of commercial eel fishing on giant kokopu populations is difficult to determine, and there may well be both detrimental and beneficial effects.
The conservation and management of giant kokopu will probably continue to be based upon management of their habitat, and these processes will be enhanced by the knowledge of the species' habitat requirements.
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