Overriding Effects of Species-Specific Turbidity Thresholds on Hoop-Net Catch Rates of Native Fishes in the Little Colorado River, Arizona

Authors: Dennis M. Stone

Year: 2010 (xlii)

Abstract

I examined the effects of turbidity, discharge, and temperature on hoop-net catch rates of the four native fishes in the lower Little Colorado River, Arizona (see Stone, 2010, Trans. Am. Fish. Soc. 139: 1150-1170). My findings indicated that native fish catch rates were primarily influenced by whether turbidity levels were below or above the species-specific thresholds of approximately 545 nephelometric turbidity units (NTU) for humpback chub, Gila cypha, 221 NTU for speckled dace, Rhinichthys osculus, 846 NTU for flannelmouth sucker, Catostomus latipinnis, and 70 NTU for bluehead sucker, C. discobolus. The effects of discharge were negligible, but discharge did dictate the turbidity level, which predetermined much of the catchability of fish. Catch rates were highest in the high-catch zone consisting of the lowermost turbidities, which ranged up to the start of the transition zone for each species (<= 54 NTU for humpback chub, <= 29 NTU for speckled dace, <= 81 NTU for flannelmouth sucker, and <= 26 NTU for bluehead sucker), and the secondary effects of temperature were only detectable in this zone. Catch rates within the transition zone decreased at higher turbidities up to the thresholds and thereafter remained consistently low at all higher turbidities within this low-catch zone. The effects of turbidity thresholds on catch rates decreased for larger fish of all species. Theoretically, the effects of turbidity reflect a behavioral switch by native fishes from relying primarily on structural cover (e.g., hoop nets) to turbidity as cover to reduce the predation risk as turbidity levels increase from below to above the species-specific thresholds (sensu Gregory, 1993, Can. J. Fish. Aquat. Sci. 50: 241-246). Moreover, turbidities within the low-catch zone could be totally incapacitating the fishes’ visual capabilities, which fishes might perceive as unlimited visual cover. The turbidity threshold concept likely applies to the catch rates of other passive entrapment gears, fishes, and riverine systems.