Preliminary Results of the LSSF on
Native Fishes in the Grand Canyon

Early in the year 2000, projected inflows to Lake Powell were determined to be low enough to trigger test flows as called for in the 1995 Fish and Wildlife Service (usfws) Biological Opinion (bo) on operation of Glen Canyon Dam. The bo calls for experimental test flows during water years in which the inflow approaches 8.23 million acre feet (maf), which is the minimum allowed by law to be delivered to the Lower Colorado River Basin. The Low Steady Summer Flows (lssf) test included a 31,000 cfs spike in early May, followed by steady flows of 8,000 cfs through the end of September, except for another 30,000 cfs spike from September 5–9. The 8,000 cfs flow was dictated by the necessity of delivering the minimum 8.23 maf to the Lower Basin. Without the lssf experiment, the outflow would have fluctuated between 6,000–13,000 daily from May through September.
The present population of humpback chub is limited in the Grand Canyon. The highly fluctuating and cold year-round releases from Glen Canyon Dam in the past contributed to this situation by reducing reproduction and survival of the young, although the cold, nutrient-rich output has produced a remarkable rainbow trout fishery below the dam. Summer releases from Glen Canyon Dam still fluctuated as much as 8,000 per day even under the Modified Low Fluctuating Flow regime which was adopted in 1996.
The rationale for this summer's test flows was that low steady flows would increase the stability and allow greater warming of shoreline habitats, including backwaters, which would in turn enhance the growth and survival of young fish, including the endangered humpback chub. Steven W. Carothers Associates (swca), Inc. was awarded a contract to study the effects of the lssf experiment on native and non-native fishes, particularly young of the year native fish, and small-bodied non-native minnows. We collected fish by shoreline electrofishing, setting shoreline hoop nets and minnow traps, and by seining backwater habitats. The vast majority of the fish were collected by seining in backwaters.

What we expected
From this experimental flow regime, we expected to see a longitudinal increase in main channel temperatures, and increases in nearshore temperatures. We did not expect to see any immediate effect on adult native fishes, other than enhanced spawning. We hoped to see increased spawning of native fishes resulting in greater abundance of young, higher growth rates and survival, and eventually greater recruitment into the adult age classes. Increased main channel temperatures would reduce the temperature shock of young humpback chub coming out of the Little Colorado River (lcr) into the main channel.
Possible negative effects of the flow regime include enhanced reproduction, growth, and survival of warm-water species of non-native fish that may be predators or competitors with the native species. The stable habitats and warm temperatures could expand the upstream distribution of both small- and large-bodied warm water fishes. The fall flow spike was intended to negatively impact the non-native species by flushing them out of habitats occupied by native fishes, while native fishes were expected to better withstand the increased flows.

Results

There was a substantial linear increase in main channel temperatures over recent years. The dam outflow is usually about 48 degrees fahrenheit and increases about 1.8 degrees for every 30 miles downstream in June. This year, temperatures increased about 1.8 degrees for every 22 miles, reaching 66.5 degrees fahrenheit near Diamond Creek, and 61 degrees fahrenheit near the lcr confluence. Previously, temperatures at Diamond Creek reached only 60 degrees fahrenheit. The 61 degrees fahrenheit mark is an important threshold, as it is the minimum temperature in which humpback chub can spawn, and eggs will hatch successfully.

Nearshore temperatures within the main channel did not increase more than main channel temperatures. Daily minimum and maximum temperatures for nearshore and main channel were nearly identical. However, the more protected backwater habitats held temperatures of up to 80 degrees fahrenheit and averaged three to four degrees warmer than the main channel.
The increase in main channel temperature may have actually induced some spawning of humpback chub in the main channel. Part of the bo calls for establishing a second reproducing population of humpback chub in the Grand Canyon (Valdez et al. 1999 in review). Currently, the vast majority humpback chub in the Grand Canyon are in a population centered around the lcr, with reproduction taking place in the lcr. Other humpback chub in the canyon are collected sporadically and no reproduction seems to occur outside the lcr, except occasionally near other tributary mouths, or near warm springs. This year, our sampling produced several larval humpback chub near river mile 197, approximately 130 miles below the lcr, and not close to any other tributary or spring. These fish were estimated to be between two and three weeks old. humpback chub were collected near this location on each of our three trips to date. This is strong, though circumstantial, evidence that these humpback chub were spawned in the main channel. We have not yet compared the growth of humpback chub to growth rates in previous years.
There were large numbers of juvenile suckers present in backwater habitats this year as well, with larval suckers present throughout the summer. Personal observations by boatmen and biologists familiar with the canyon and its denizens indicate higher than normal densities of these young suckers. However, we still need to compare our data with previous years' data to be sure.
The total numbers and density of all fish in backwaters increased downstream. The numbers of non-native fishes increased faster than natives, so that by river mile 200, the relative abundance of non-native fish was greater than that of natives. After the fall spike, numbers of natives and non-natives decreased substantially, down 65 percent for natives and 70 percent for non-natives.
One effect of the lssf not directly anticipated was its effect on the number of backwater habitats. The 30,000 cfs spring peak did rearrange some beaches, frequently resulting in a backwater habitat forming at the upstream end of eddy-formed beaches. The subsequent steady flows allowed these backwaters to persist all summer, providing cover and food for native and non-native species alike.
This effect may have contributed to the anticipated effect of increased numbers of small non-native fish, particularly the fathead minnow. Our sampling in backwaters produced large numbers of fathead minnows increasing downstream, particularly during the August trip. There did not appear to be substantial upstream expansion of non-native fishes. The majority of fathead minnows were collected below river mile 160.
Based on these preliminary results, the lssf appears to have been beneficial to native fishes, while not benefitting non-native fishes to an equal extent. However, repeated years of low steady flows could have a cumulative effect of increasing numbers of non-natives. We still have a lot of analysis to do, to compare the catch rate and growth rates seen this summer to previous years' data to determine if growth and abundance were significantly better than in previous years. The true success of this experiment will not be known for three or four years, until this year class of young fish matures, or recruits into the adult population.
Melissa Trammell