No abstract

We quantified tag retention, survival, and growth in juvenile, captive‐reared Humpback Chub Gila cypha marked with three different tag types: (1) Biomark 12.5‐mm, 134.2‐kHz, full duplex PIT tags injected into the body cavity with a 12‐gauge needle; (2) Biomark 8.4‐mm, 134.2‐kHz, full duplex PIT tags injected with a 16‐gauge needle; and (3) Northwest Marine Technology visible implant elastomer (VIE) tags injected under the skin with a 29‐gauge needle. Estimates of tag loss, tagging‐induced mortality, and growth were evaluated for 60 d with each tag type for four different size‐groups of fish: 40–49 mm, 50–59 mm, 60–69 mm, and 70–79 mm TL. Total length was a significant predictor of the probability of PIT tag retention and mortality for both 8‐mm and 12‐mm PIT tags, and the smallest fish had the highest rates of tag loss (12.5–30.0%) and mortality (7.5–20.0%). Humpback Chub of sizes 40–49 mm TL and tagged with VIE tags had no mortality but did have a 17.5% tag loss. Growth rates of all tagged fish were similar to controls. Our data indicate Humpback Chub can be effectively tagged using either 8‐mm or 12‐mm PIT tags with little tag loss or mortality at sizes as low as 65 mm TL. Received July 24, 2014; accepted October 31, 2014

We used an integrated assessment model to examine effects of flow from Glen Canyon Dam, Arizona, USA, on recruitment of nonnative rainbow trout ( Oncorhynchus mykiss ) in the Colorado River and to estimate downstream migration from Glen Canyon to Marble Canyon, a reach used by endangered native fish. Over a 20-year period, recruitment of rainbow trout in Glen Canyon increased with the annual flow volume and when hourly flow variation was reduced and after two of three controlled floods. The model predicted that approximately 16 000 trout·year–1 emigrated to Marble Canyon and that the majority of trout in this reach originate from Glen Canyon. For most models that were examined, over 70% of the variation in emigration rates was explained by variation in recruitment in Glen Canyon, suggesting that flow from the dam controls in large part the extent of potential negative interactions between rainbow trout and native fish. Controlled floods and steadier flows, which were originally aimed at partially restoring conditions before the dam (greater native fish abundance and larger sand bars), appear to have been more beneficial to nonnative rainbow trout than to native fish.

No abstract

No abstract

Negative interactions of Rainbow Trout Oncorhynchus mykiss with endangered Humpback Chub Gila cypha pose challenges to the operation of Glen Canyon Dam (GCD) to manage for both species in the Colorado River. Operations to enhance the Rainbow Trout tailwater fishery may lead to an increase in downstream movement of the trout to areas where they are likely to interact with Humpback Chub. We evaluated the effects of dam operations on age‐0 Rainbow Trout in the tailwater fishery to inform managers about how GCD operations could benefit a tailwater fishery for Rainbow Trout; although this could affect a Humpback Chub population farther downstream. A near year‐long increase in discharge at GCD in 2011 enabled us to evaluate whether high and stable flows led to increased spawning and production of age‐0 Rainbow Trout compared with other years. Rainbow Trout spawning was monitored by fitting a model to observed redd counts to estimate the number of redds created over a spawning season. Data collected during electrofishing trips in July–September and November were used to acquire age‐0 trout population and mortality rate estimates. We found that high and stable flows in 2011 resulted in 3,062 redds (1.7 times the mean of all survey years) and a population estimate of 686,000 age‐0 Rainbow Trout (second highest on record). Despite high initial abundance, mortality remained low through the year (0.0043%/d) resulting in significant recruitment with a record high November population estimate of 214,000 age‐0 Rainbow Trout. Recent monitoring indicates this recruitment event was followed by an increase in downstream migration, which may lead to increased interactions with downstream populations of Humpback Chub. Consequently, while our results indicate that manipulating flow at GCD can be used to manage Rainbow Trout spawning and recruitment, fisheries managers should use flow manipulation in moderation to minimize downstream migration in order to reduce negative interactions with other species in the Colorado River. Received September 8, 2014; accepted March 28, 2015

We examined influences of 3 d of reduced flows on biotic assemblages in the Colorado River below Glen Canyon Dam, Arizona. Exposure in the varial zone reduced standing crops of periphyton, pondweed Potamogeton pectinatus, and benthic macroinvertebrates, and losses following the drawdown were not explained by expected seasonal trends. Losses in macroinvertebrate densities and mass and in periphyton mass and chlorophyll a suggest that effects of a sudden reduction in dam discharge of moderate duration and magnitude are comparable with losses associated with the varial zone of fluctuating daily flows. Compared with expected seasonal changes, abundance of rainbow trout Oncorhynchus mykiss increased during and following reduced flows. Proportional composition of fish less than 152 mm total length (TL) was greater after the drawdown, whereas the percentage of trout 406–558 mm declined. Relative condition factor before, during, and after the drawdown did not differ significantly for all fish combined, following a long‐term pattern. However, among length‐classes, condition of fish 305–405 mm TL declined following reduced flows. Feeding and composition of dietary items generally followed expected patterns of seasonal change and failed to correspond with changes in periphyton, macrophyte, and macroinvertebrate benthic densities after the drawdown. However, fewer rainbow trout ate Cladophora glomerata, and they ate less of this alga. In comparison, fish ate more gastropods following the drawdown than accounted for by long‐term patterns, but frequency of occurrence was unchanged. Sudden reduction in flows of moderate duration and magnitude, although significantly reducing benthic assemblages, had little apparent negative short‐term consequence for rainbow trout.

Long-term fish monitoring in the Colorado River below Glen Canyon Dam is an essential component of the Glen Canyon Dam Adaptive Management Program (GCDAMP). The GCDAMP is a federally authorized initiative to ensure that the primary mandate of the Grand Canyon Protection Act of 1992 to protect resources downstream from Glen Canyon Dam is met. The U.S. Geological Survey's Grand Canyon Monitoring and Research Center is responsible for the program's long-term fish monitoring, which is implemented in cooperation with the Arizona Game and Fish Department, U.S. Fish and Wildlife Service, SWCA Environmental Consultants, and others. Electrofishing and tagging protocols have been developed and implemented for standardized annual monitoring of Colorado River fishes since 2000. In 2009, sampling occurred throughout the river between Lees Ferry and Lake Mead for 38 nights over two trips. During the two trips, scientists captured 6,826 fish representing 11 species. Based on catch-per-unit-effort, salmonids (for example, rainbow trout (Oncorhynchus mykiss) and brown trout (Salmo trutta)) increased eightfold between 2006 and 2009. Flannelmouth sucker (Catostomus latipinnis) catch rates were twice as high in 2009 as in 2006. Humpback chub (Gila cypha) catches were low throughout the 10-year sampling period.