Gizzard Shad in Lake Powell

Not open for further replies.

wayne gustaveson

Staff member


In 2007, we find gizzard shad have occupied all of Lake Powell. It really only took 3 years for the migration of shad to reach from the upper San Juan to the dam all the way to the headwaters of the Colorado River. In netting samples during November 2006, gizzard shad accounted for almost as much fish flesh as striped bass. The largest shad caught to date came from Wahweap bay. The huge shad was 19.25 inches long and weighed 2.8 pounds. Gizzard shad grew to larger size than normal in the fertile waters found during 20003-2006.

While threadfin numbers crashed in 2006 in response to heavy predation from massive numbers of adult game fish, gizzard shad adults continued to become more numerous. There are no natural predators for big gizzard shad. They just get big and eat algae and detritus off the bottom. Large numbers of adults are needed to provide the new crop of shad each spring that all game fish depend on for growth and survival. With threadfin presently low in numbers, the progeny of gizzard shad will provide needed food fish during the critical spring months. There may not be enough shad to go around in 2007 but the presence of gizzard shad will provide at least some food to keep the fisheries on track for the near term.


Gizzard Shad- Background Information

One single adult gizzard shad was collected from Lake Powell in 2000 near the San Juan inflow.

While the origin of the new species in Powell is unknown it has been reported by US Fish and Wildlife Service that gizzard shad were accidentally introduced into Morgan Lake near Shiprock, NM 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 9 different species besides largemouth bass (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 sport fishery on the Navajo Reservation. The 1200 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.

No gizzard shad were found in Lake Powell during 2001.


The San Juan inflow (pictured here) was the site of gizzard shad access to Lake Powell.

During routine fish sampling in August 2002 on Lake Powell's upper San Juan arm six gizzard shad were collected. Shad averaged 4 inches and were suspected to be naturally reproduced within the lake.

Gill nets set from the mouth of the San Juan (Jacks Arch) to Neskahi Canyon in July 2003 collected 78 adult gizzard shad. Downlake movement of the new species is confirmed as they are present and commonly found in the entire San Juan arm. Sampling was expanded both up and down lake from the mouth of San Juan to find out how far shad have moved.

During August 2003 gill nets set in Gunsight Canyon (Padre Bay) captured one adult gizzard shad. This fish confirmed the presence of gizzard shad in the main stem from Padre Bay to the mouth of the San Juan.

Gillnets set in November 2003 confirmed the presence of gizzard shad in Good Hope Bay.

The new shad species is now established Lake-wide during Fall 2003. We were surprised at the large size of the shad. Average length was 11 inches and they weighed one-half pound. The largest shad collected was 15 inches and weighed over a pound-and-a-half. While the shad are not overly long their robust girth makes them too large to fit a 2-pound stripers' mouth. These adult fish will be able to roam freely through Lake Powell without fear of predation. This free movement will, perhaps, quicken establishment of the gizzard shad in the entire lake.


Gizzard shad grow quickly and attain a much larger size than threadfin which, to this point, were the only shad in Lake Powell. By 2005, some adults were now 18 inches long and weighed 2-pounds. 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 shad numbers. Some years threadfin have been totally eliminated from the open water where stripers prefer to feed. In other years, 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.



Characteristics: GIZZARD SHAD

Coloration: Back silvery blue, somewhat iridescent; sides silvery above, whitish below; abdomen white. Fins darkened. Dark purplish spot about the size of the eye present immediately behind the upper end of the gill opening in y-o-y. Spot becomes obsolete and disappears with age.

Mouth: small subterminal, slightly overhung by the rounded snout. Centrally notched upper jaw protrudes slightly beyond lower jaw. Maxillary reaching below the anterior margin of the eye. Gill rakers long, slender


Body: Deep strongly compressed laterally. Average TL 225-350 mm. Scales large, cycloid , deciduous. Lateral line lacking. Median lateral series of scales 61 (52-70). Ridge of sawlike ventral scutes on abdomen.

Diagnosis: Gizzard Shad differ from Threadfin Shad by: subterminal mouth with a distinct notch in upper jaw; a much shorter dorsal fin filament; absence of black pigment on the chin and floor of mouth; more than 17 midventral scutes in the prepelvic series; more scales in the lateral series; more anal fin rays.

Biology: Schooling, juveniles in quiet surface waters, adults near bottom. Spawn at night in spring, in shallows. Eggs randomly scattered adhere to plants, rocks or firm substrate. Temps 10-22C, peaks at 19-22C. Six week spawning period. Most spawn at age II. Fecundity 22,000 to 350,000. Incubation 2-4 days. Buoyant larvae become plankton. Life span 4-6 years, few live beyond age III. Adult die-offs common. Adults bottom filter feeding detritivors principally from littoral zones. Open water fish have mostly phytoplankton and sand for digestion. Juveniles planktivorous.

Preferred temp is 22.8-23.9C


Color: back dark gray to bluish black; sides and abdomen silvery.

Eye spot: distinct spot smaller than eye behind gill. Always present.


Fins: yellowish, except the dorsal; caudal deeper yellow.

Mouth: terminal - bottom lip protruding; bottom of mouth cavity with black spots; maxillary in front of eye margin

Body: Deep, strongly compressed laterally. Average TL less than 110 mm. Scales cycloid, deciduous. Lateral line lacking. Median lateral series of scales 40-48. Ridge of sawlike ventral scutes on abdomen. Dorsal Fin filament: long, reaches almost to caudal, TL adults: <110 mm

Scale count: median series, 40-48

Fin ray counts: dorsal 14, anal 20-25,


Biology: usually congregating in large schools over deep water in daylight hours, moving to shallower water at night. Sensitive to cool temperature, swimming ability decreases below 12 C. Death occurs at 5C. Spawning occurs shortly after dawn along shore over weeds or driftwood with temperatures between 21-26 C. Adhesive eggs attach to first item touched. Fecundity is 900 to 21K

Feeding: Select limnetic particulate zooplankton and filter feed on smaller plankton.

Longevity: Seldom older than 2-3 years.


Lake Powell is forage limited with striped bass reproducing without limit and smallmouth being perfectly matched with the rocky habitat. Predators outnumber prey. Striped bass have suffered periods of malnutrition when threadfin have been eliminated from the open water by predation. Smallmouth growth slows when shad are not available to supplement their crayfish diet.

Gizzard shad have the potential to fit well with the existing threadfin population. Threadfin are open water plankton feeders as adults while gizzard shad feed mostly on plant life and debris on the bottom. There is an overlap between young threadfin, gizzards and young bass for the same plankton in the same shallow water in spring and summer. Once that hurdle is cleared surviving sport fish should grow much better with an additional large-bodied forage fish that is long lived.

Gizzard shad life history indicates a preference for stained, turbid water. Most of Lake Powell is clear open water better suited for threadfin dominance. The inflow areas are classic gizzard shad habitat. These productive areas are already the most important threadfin production spots. Now these zones must be shared with a new shad. The outcome of shad population development depends on how far and fast gizzard shad colonize open clear water after leaving the turbid zone to forage for plankton.

By 2005, gizzards shad were found to occupy shallow coves more often than threadfin which choose open deep water. There is more movement and mixing of species at night but during the day shad are often separated in different habitats.



We are just beginning to understand how the recently arrived gizzard shad will fit into the forage picture at Lake Powell. Gizzard shad have the potential to fit well with the existing threadfin shad population.

Threadfin shad may be the best forage fish ever invented. They never outgrown the mouth size of bass, walleye or stripers. Adult threadfin can be eaten just as quickly as young shad. The shad school provides a ready source of nutrition for all adult warmwater predators. Threadfin shad consume plankton and quickly convert that to shad flesh, which is then eaten by game fish. Threadfin are the main link in a very short, but extremely efficient food chain at Lake Powell.

All newly hatched fish are immediately introduced to the real world of big, bad, competition. Fish that get biggest soonest tend to win. Sometimes it is an environmental accident caused by an early spring thaw that allows bass and crappie to spawn sooner than shad. That leads to fast growth for bass as newly hatched shad can be directly consumed at the perfect size needed by bass fry.

In other years a late bass hatch may lead to shad enjoying a head start at the expense of bass fry. Shad with 2-weeks growth may be too large for bass fry to eat. Now bass must eat plankton instead of tiny shad and compete with larger shad that are much better equipped to eat plankton. Annual survival of a particular species may depend on a little luck with opportune timing of hatching and food availability.

Gizzard shad add complexity to the food chain. Young gizzards and threadfin require the same food after hatching as they drift with plankton in the warm surface layers. However, gizzard and threadfin have different spawning triggers that compartmentalize the shad spawn into distinct periods. The end result is more small shad produced over a longer period of time each spring. That is good news for the developing crop of predators. Bass may spawn as early as April 1st or as late as June 1st in Lake Powell. Abundance of small shad dictates the survival of each bass cohort over the spawning period.

The obvious forage drawback to gizzard shad is rapid growth. The average threadfin may never exceed 3 inches in its 3-year life spawn. The average gizzard shad often exceeds 4 inches in the first growing season. Beyond the 3rd year, gizzard shad will never be threatened with predation from anything but a trophy striper, a very large bird or otter. Bass can only consume gizzard shad for a limited window of time during the first year of shad life.

The bright side to large size is the constant presence of spawning adult gizzard shad. A mix of threadfin and gizzard provide small shad forage for consumption while banking the large gizzard shad adults for production of future shad generations. There should be some gizzard shad fry produced each year to provide bass forage, even if threadfin have been eliminated by striped bass predation.


With time gizzard and threadfin partition themselves in separate habitat.

Young of both species start life together feeding on plankton in open surface water and being consumed by larger fish. Threadfin remain in open water during their entire life, moving on shore only to spawn or after being forced into shallow cover by persistent predator attacks. The preferred escape mechanism for threadfin is schooling behavior where protection for individual shad comes from hiding within the large school. The habitat choice is open water as far away from any attack structure as possible.

Gizzard shad act much the same. They school as young fish and hide from predators within the school. But with time and growth, dietary needs change and gizzard shad seek the bottom. Gizzard shad mouthparts are best equipped for foraging and filter feeding on algae and detritus that is siphoned off the sandy substrate. Both young and adults commonly prowl along shore feeding on bottom but they avoid thick aquatic weeds. They move in and out of shallow coves in an unhurried peaceful search for food. In clear water, anglers commonly see cruising schools of adult gizzard shad. This behavior places adult gizzard shad in close quarters with shorebound bass while open water threadfin are more likely to live near striped bass. Proximity begets opportunity and it may be that gizzard shad will be a readily available bass food in Lake Powell. Historically, striped bass predation has periodically eliminated threadfin for the open water. In those incidences gizzards may continue to provide the shad meal necessary to maintain large and smallmouth bass at a decent size, even when threadfin are lacking.

Gizzard shad live at least 3 years with some surviving until 8 years old. Seasonal die-off of old adults is common in many waters. Threadfin rarely live longer than 3 years.

Threadfin are sensitive to cold water and may die in 40-degree water. Gizzards have a wide temperature tolerance.


Gizzard shad spawn on the surface at night in shallow water. Eggs stick to the first object touched after fertilization until hatching occurs in 2-4 days. Spawning may occur when water warms to the high 50’s but the peak happens from 66-72 F during a 6-week spawning period. Females may spawn from 22,000 to as many as 350,000 eggs.

Threadfin shad spawn on the surface shortly after dawn along a weedy shoreline or in open water around rafts of driftwood and debris. Adhesive eggs stick to leaves and branches of shoreline vegetation or floating debris until hatching occurs in 2-4 days. Spawning begins when surface temperature at dawn exceeds 62 F and may continue until surface temperature exceeds 83 F. Most spawning happens in May and June but a September spawn may occur when plankton forage conditions are ideal. Females carry 900 to 21,000 eggs for spawning annually.
Not open for further replies.