Production of fish in two small streams in the north island of New Zealand

Data are presented on biomass, mortality, growth, and production of fish populations inhabiting two small streams in the Wairarapa district of the North Island of New Zealand. The main water of both streams contained a mixed population of bullies (Philypnodon breviceps Stokell), brown trout (Salmo trutta Linn.), and eels (Anguilla dieffenbachii Gray, and A. australis schmidtii Phillipps). The headwater tributaries of one of the streams contained a large population of Galaxias divergens Stokell and a few eels, but other fish species were absent. All species of fish appeared to be using the same food supply of small benthic invertebrates, mainly ephemeropteran and dipteran larvae. Biomass of benthos per unit area was appreciably higher in the headwater tributary examined than in the main waters downstream. Mean annual stock of benthos in the tributary (Hinau north branch) was 19.75 g/m², in the Hinau main water 10.71 g/m² and in the Hinaki main water 13.47 g/m² Nevertheless, production per unit area from the nearly monospecific fish population in the Hinau north branch (14.28 g/m²) was less than that of the mixed populations in the main waters (24.17 g/m² in the Hinau, 73.54 g/m² in the Hinaki). Possibly fish production in the headwaters might be greater if the species composition of fish was more diverse.