Biological Assessment of Kahana Stream, Island of O‘ahu, Hawai‘i: An Application of PABITRA Survey Methods

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 (hı̄hı̄wai, Neritina granosa) were found, and the brackish-water limpet (hapawai, Neritina vespertina) was not observed. Construction of the Waiāhole 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. Kahana Stream is a prominent feature of the recently renamed Ahupua‘a ‘O Kahana State Park, a designation that appropriately emphasizes the fact that the valley is the only publicly owned ahupua‘a in the state of Hawai‘i. Kahana Stream and Valley are located on windward O‘ahu about halfway between the towns of Kāne‘ohe and Lā‘ie. The watershed extends from the small village of Kahana near sea level inland to the crest of the Ko‘olau Mountains at Pu‘u Pauao with an elevation of 814 m. Orographic rain averaging about 760 cm each year at the back of the valley provides continuous flow in Kahana Stream and its tributaries. The flow of water is typically clear, cool, and brisk at inland sites but becomes less clear, warmer, and sluggish in the lower and deeper sections close to the coast. Kahana Stream forms a true estuary in its lower reaches where fresh water mixes with salt water before emptying into a partly Pacific Science (2005), vol. 59, no. 2:273–281 : 2005 by University of Hawai‘i Press All rights reserved 1 This research was supported by a grant from the Division of Aquatic Resources and Commission on Water Resource Management, Department of Land and Natural Resources, State of Hawai‘i, for research on Hawaiian stream fishes via the Research Corporation of the University of Hawai‘i and the U.S. Fish and Wildlife Service Sport Fish Restoration Program, Grant Agreement No. F-14-R. Manuscript accepted 16 April 2004. 2 Museum of Natural Science, Louisiana State University, Baton Rouge, Louisiana 70803 (e-mail: [email protected], [email protected]). 3 School of Natural Resource Sciences, University of Nebraska, Lincoln, Nebraska 68583-0759 (e-mail: [email protected]). 4 Department of Biology, University of Tampa, Tampa, Florida 33606-1490 (e-mail: [email protected]). 5 Division of Aquatic Resources, 76 Aupuni Street, Hilo, Hawai‘i 96720 (e-mail: Robert.T.Nishimoto@