The ecology of Stewart Island freshwater communities
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
A comprehensive survey of the stream and river faunas of Stewart Island (invertebrates and fish) was undertaken between August 1987 and December 1989. In total, 113 invertebrate taxa and 12 fish species were recorded. The invertebrate fauna is characterised by a common core of taxa (Chiltonia rivertonensis, Oligochaeta, Chironomidae, Deleatidium, Zelandoperla, Zelandobius, Austroperla cyrene, Hydrobiosidae stenocerca, Hydrobiosidae, Simuliidae, and Scirtidae) and a paucity or absence of some common mainland families or genera, notably the Notonemouridae, Conoesucidae, and Archichauliodes. The fish fauna consists of diadromous species except for Galaxias vulgaris. A number of invertebrate and fish species are more widespread, or abundant, and occupy more diverse habitat types than reported in mainland studies. They include the isopod Austridotea benhami, and the fishes Galaxias argenteus, G. fasciatus and Gobiomorphus huttoni. These differences may reflect the more pristine conditions of Stewart Island streams and an absence of predatory salmonids on the island. Gut analysis of G. huttoni and G. fasciatus confirmed that they fed predominantly on benthic invertebrates and terrestrial prey items, respectively. Both species appeared to be opportunistic feeders and showed a low degree of prey selectivity. Experimental studies at six sites in the Rakeahua River system indicated that leaf litter breakdown (kamahi leaves) rates were rapid at headwater sites where feeding by shredders (principally Austroperla cyrene) was high. In contrast, breakdown rates were slower at mainstem sites even though the large detritivore/carnivore, Austridotea benhami was present. In summary, the Stewart Island freshwater fauna can be seen to possess a number of distinctive features, but the extent of these appear to be no greater than those found between the North and South Islands of New Zealand. Stewart Island is unique, however, because of the apparent absence of adventive species, including trout, that are common on the mainland. Because of this and the largely pristine nature of its catchments, I suggest that greater protection should be afforded to the island's freshwater systems than is imparted by Reserve and Conservation Land status.