What Will Gizzard Shad Do To Lake Powell? 

“My unworthy prayers have been answered,” one fisherman wrote in a message posted on an  internet site where people discuss fishing at Lake Powell. Indeed, there is considerable optimism since an established population of gizzard shad were found in the lake. Anglers hope the little fish will help break the boom and bust cycle that has plagued fishing there.

Biologists say nobody knows how the scenario will play out. There is potential for improvement but there could also be negative factors.

A DWR crew found the new shad while sampling fish on the upper San Jan arm. Wayne Gustaveson is the DWR project leader at the lake. His official stance is cautious. But on his web site (www.wayneswords.com), which includes the discussion board referenced above, Wayne often speaks his mind more freely. He posted this message:

“I am trying to be cautious with my predictions, but under this calm exterior there is some latent excitement about what I am seeing. We have shad in places we haven't had them for a long time. Striper numbers are down and condition is up. Gizzard shad are here and will provide more grub sometime in the future. Brush is growing around the edges of the lake with the recent rain. This will eventually be fish habitat.

“For a drought, low water, poor runoff and a long hot summer, we are doing quite well indeed. We will hold off on predicting 5-pounders (smallmouth) and stay with keeping the 3-pounders healthy for right now. That would be good enough for me. Five pound fish need so much food that we can only keep them that big for a short time. Let's see if gizzard shad do something special before counting big fish.”

In another message he posted:

“We went to San Juan/Neskahi and found large schools of large (threadfin) shad (5-6 inchers). There were no small shad but the big ones were good food for the larger predators. The mix of other fish was mostly green sunfish and smallmouth, like most other locations. This was as expected. We examined each shad carefuly so that we would be sure to detect a gizzard shad if present. We found none and handled hundreds.

“Then we went up to the inflow and found gizzard shad mixed in with the threadfin. The ratio is 100:1. We could not find them in any location away from the muddy water.”

This has been an unusual year at Powell. Fishing has been good and most fish are healthy, despite the severely low water level. It is far to early in the game to attribute this good fortune to gizzard shad. But, as Wayne said, changes are coming and there is reason for excitement.

Wayne’s official news release about the gizzard shad is given below:

Press Release

Page, Ariz.—Six gizzard shad were collected on Lake Powell's upper San Juan arm during routine fish sampling in August, the Utah Division of Wildlife Resources announced Aug. 22.

This forage species is new to Lake Powell and the main-stem Colorado River drainage. The shad that were collected averaged 4 inches in length. It's suspected that they were naturally reproduced within the lake.

Ramifications of a new species of fish range from good to bad.

Gizzard shad grow quickly and attain a much larger size than threadfin shad, which to this point were the only shad in Lake Powell. The rapid growth means that largemouth and smallmouth bass are able to eat shad for only a short time each spring. Then shad and young bass may actually compete for the same limited planktonic food.

Striped bass are the dominant predator in Lake Powell and have for decades decimated threadfin shad from the open water. In other years, threadfin shad numbers have been cropped as newly hatched shad are eaten almost as fast as they enter open water in search of food. Gizzard shad will grow large enough to provide a bigger ration of food for stripers for a longer period of time. It may be that striped bass size and condition will increase as the gizzard shad become widespread and fully established.

The unknown element is how fast gizzard shad will colonize Lake Powell and where they will reside. Gizzard shad prefer mud-stained water and have been shown to lose the competitive battle with threadfin for food in open, clear water. Gizzard shad are more adept at bottom-feeding on algae while threadfin are better adapted to feeding on free-swimming zooplankton. It may be that both shad species will be limited to productive inflow areas that now exclusively harbor threadfin. Or gizzard shad may populate the open water and proliferate there because of their larger body size and greater ability to produce offspring.

The outcome is unknown and will be the subject of close scientific scrutiny by the Utah Division of Wildlife Resources during the new species colonization period. The outcome will likely be determined by striped bass as they feed on both gizzard and threadfin shad.

While the origin of the new species in Lake Powell is unknown, it has been reported by the U.S. Fish and Wildlife Service that gizzard shad were accidentally introduced into Morgan Lake near Shiprock, New Mexico, with a shipment of largemouth bass in 1998. The bass came from Inks Dam National Fish Hatchery in south-central Texas in the Rio Colorado drainage where gizzard shad are abundant in the surface water used at the hatchery.

Subsequent loads of bass transported to Morgan Lake from the hatchery were found to have as many as nine different species besides largemouth bass (these fish species included Guadalupe bass, logperch, gizzard shad, white bass, bluegill and dollar sunfish). These shipments were refused but gizzard shad were already firmly established in Morgan Lake.

Logistics prevent shad from being chemically removed from Morgan Lake, which is an important largemouth bass sport fishery on the Navajo Reservation there. The 1,200 acre lake provides water to the APS power plant near Shiprock. Lowering the lake would require the power plant to be shut down for an extended period. Poisoning fish without lowering the lake would block intakes with dead fish and effectively shut down the power plant, as well.

One single adult gizzard shad was collected from Lake Powell in 2000 near the San Juan inflow. This fish was suspected to be a downstream migrant from Morgan Lake. No gizzard shad were found in Lake Powell during 2001. Now it appears that enough adult gizzard shad have taken up residence in Lake Powell to produce a year-class of young in the huge reservoir. The development of the gizzard shad population in all of Lake Powell may take only two years, or may be delayed for decades.