Biological assessment of Kahana Stream, Island of O'ahu, Hawai'i: An application of PABITRA survey methods

Abstract

Aquatic biologists surveyed Kahana Stream on O'ahu, Hawai'i, during December 2001 and January, March, and May 2002 to provide a background of information before restoring water diverted from the headwaters of the stream since the mid-1920s. Kahana Stream has all but one species of macrofauna common in unaltered Hawaiian streams, but abundance and distribution of amphidromous species differ conspicuously. A single specimen of 'o'opu 'alamo'o (Lentipes concolor) was found near the headwaters; until recently, this species was regarded as extinct on O'ahu. Only two individuals of the freshwater limpet (hihiwai, Neritina granosa) were found, and the brackish-water limpet (hapawai, Neritina vespertina) was not observed. Construction of the Waiahole Ditch Tunnel about 80 yr ago reduced the amount of water entering Kahana headwaters, and unimpeded growth of hau (Hibiscus tiliaceus) from the shore into the stream has slowed water movement in the middle and lower sections of the stream and estuary. Reduced flow has resulted in an extension farther inland of certain estuarine and lower-reach species (the prawn Macrobrachium grandimanus and fishes Eleotris sandwicensis and Stenogobius hawaiiensis). Alien fishes and larger invertebrates occur throughout Kahana Stream. Catches of newly hatched fish (S. hawaiiensis) and invertebrates (limited to crustaceans) moving downstream toward the ocean were meager. Recruitment of animals moving from the sea into the stream included only crustaceans and a single individual fish (S. hawaiiensis). Benthic algae were considerably more diverse than recorded for other O'ahu streams. Hau removal and extensive trimming at key locations along Kahana Stream should precede the addition of water to the basin to avoid flooding and to enhance beneficial biological effects.