Diel habitat use of two sympatric galaxiid fishes (Galaxias brevipinnis and G. postvectis) at two spatial scales in a small upland stream in Manawatu, New Zealand

Abstract

Two native New Zealand galaxiid fishes [the kōaro (Galaxias brevipinnis) and the shortjaw kōkopu (Galaxias postvectis)] are declining throughout the country. More knowledge regarding aspects of their ecology is needed in order for conservation actions to be optimally designed. A 100-m reach of small upland stream in Manawatu, New Zealand was mapped to scale and inventoried for microhabitat variables. Flow velocity, depth, surface turbulence, substrate size and interstitial refuge space availability were measured in 0.25-m2 quadrats throughout the reach. Twenty-nine kōaro and 22 shortjaw kōkopu were PIT-tagged within the reach and monitored during the day (N = 26) and night (N = 15) in 2008 with a portable PIT monitoring system. Of these, 72 % of kōaro and 68 % of shortjaw kōkopu were detected at least once and 62 % of kōaro and 46 % of shortjaw kōkopu were detected five times or more within the study reach. During both the day and night kōaro used microhabitats that were faster-flowing, deeper, more turbulent, with larger substrate particles and more interstitial refuge spaces when compared with all microhabitats available in the reach. Shortjaw kōkopu used microhabitats with larger substrate particles and more interstitial refuge spaces when compared to all microhabitats available within the reach. During the day, shortjaw kōkopu were found in areas with larger substrates and higher surface turbulence than areas where they were found at night. Four mesohabitat types were identified in the study area using cluster analyses. Kōaro used the same flow channel-type habitats during the day and at night. Shortjaw kōkopu used flow channel and pool habitats during the day, and used pool habitats at night. These results suggest that biotic factors such as competition from shortjaw kōkopu may have been influencing kōaro habitat use patterns. In addition, these findings have implications for river managers attempting to implement conservation measures to protect these species because increasing anthropogenic impacts such as sedimentation and water abstraction can have negative impacts on instream habitat.