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

Abstract

I examined the effects of turbidity, discharge, and temperature on hoop-net catch rates of native humpback chub Gila cypha, speckled dace Rhinichthys osculus, flannelmouth suckers Catostomus latipinnis, and bluehead suckers C. discobolus in the Little Colorado River, Arizona. Results indicated that native fish catch rates were primarily influenced by whether turbidity levels were below or above species-specific thresholds of approximately 545 nephelometric turbidity units (NTU) for humpback chub, 221 NTU for speckled dace, 846 NTU for flannelmouth suckers, and 70 NTU for bluehead suckers. The effects of discharge were negligible, but discharge did dictate the turbidity level. Turbidity levels at hoop-net deployment relative to the turbidity thresholds predetermined much of the catchability of fish. Catch rates were highest in a high-catch zone consisting of the lowermost turbidities, which ranged up to the start of a transition zone for each species (humpback chub, 54 NTU; speckled dace, 29 NTU; flannelmouth sucker, 81 NTU; bluehead sucker, 26 NTU), 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 the 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 using turbidity as cover to reduce the predation risk as turbidity increases from below to above the thresholds. Moreover, turbidities within the low-catch zone could be totally incapacitating the fishes' visual capabilities such that fish perceive these turbidity levels as unlimited visual cover. The turbidity threshold concept probably applies to the hoop-net catch rates of other fishes in other riverine systems.*

Extending the turbidity record: making additional use of continuous data from turbidity, acoustic-Doppler, and laser diffraction instruments and suspended-sediment samples in the Colorado River in Grand Canyon

“…Turbidity has also been related to the behavior of sightfeeding fishes and their prey (Gregory, 1993 Stone, 2010;Yard and others, 2011). A trout piscivory study conducted on the Colorado River in Grand Canyon, near the confluence with the Little Colorado River ( fig.…”

“…Thus, because the grain-size distribution of suspended sediment varies substantially in the study area (see, for example, Topping and others, 2000aTopping and others, , 2000bTopping and others, , 2010 and different grain-size distributions give rise to very different levels of turbidity at identical suspendedsediment concentrations, it is preferable to relate fish feeding behavior directly to an optical measurement, such as turbidity, than just to suspended-sediment concentration. Stone (2010) related fish behavior on the Little Colorado River ( fig. 2) directly to turbidity.…”

“…He established species-specific turbidity thresholds, above which particular species of fish are much less likely to enter a hoop net. Stone hypothesized that the effects of changes in visual clarity reflect a behavioral switch by many fishes, from relying primarily on physical cover, such as hoop nets, in clear water conditions, to using conditions of low visual clarity as cover to reduce the predation risks (Stone, 2010;Gregory, 1993). A drawback of optical turbidity instruments is that the upper limit of recordable turbidity is determined by the instrument's maximum recording level.…”

Characterizing Growth and Condition of Endangered Humpback Chub in the Lower Colorado River

“…It enters the Colorado River 126 km below Glen Canyon Dam and 100 km below Lee's Ferry. The lower 21 km is spring fed, with perennial base flows of~229 ft 3 /s (6.5 m 3 /s; Gorman and Stone 1999;Stone 2010). The carbonated spring water forms large travertine dam complexes as it degasses.…”

“…Turbidity is generally low during periods of base flow, but can reach very high levels (. 10,000 formazin nephelometric units) during periods of flooding (Stone 2010). Water temperatures in the Little Colorado River are also seasonally variable and, compared to the mainstem Colorado River, are warmer during spring, summer, and fall, and cooler in winter.…”