New Fish Species Found at Lake Powell
GIZZARD SHAD STATUS During routine fish sampling in August on Lake Powell's upper San Juan arm six gizzard shad were collected. This forage species is new to Lake Powell and the main-stem Colorado River drainage. Shad averaged 4 inches and were suspected to be naturally reproduced within the lake. Ramifications of a new species of fish are wide and varied.
Gizzard shad grow quickly and attain a much larger size than threadfin 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 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. The unknown element is how fast gizzard shad will colonize Lake Powell and where they will reside. Gizzard shad prefer 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 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 due to their larger body size. The outcome is unknown and will be the subject of close scientific scrutiny by the Utah Division of Wildlife Resources during the 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 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.
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 sufficient adult gizzard shad have taken up residence in Lake Powell and have produced a year-class of young in the huge reservoir. The development of the gizzard shad population may take only 2 years or may be delayed much longer.
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: GS differs from TFS 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 filer 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
CHARACTERISTICS: THREADFIN SHAD
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 decreses 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
Longevity: Seldom older than 2-3 years.
MANAGEMENT IMPLICATIONS 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 has slowed 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 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 forage fish that is larger and contains more food value.
Gizzard shad population development is unsure. Their 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. ---------------- Fish and Wildlife Blunders in Lake Powell BY SKIP KNOWLES THE SALT LAKE TRIBUNE After years of telling Utah biologists to forget about stocking gizzard shad in Lake Powell because of concern for sensitive species, the U.S. Fish and Wildlife Service accidentally did just that. "We considered it years ago and Fish and Wildlife said absolutely not," said southern region biologist Dale Hepworth. "Now they did it by mistake. That's kind of comical." Gizzard shad, and as many as eight other unwanted species, were accidentally stocked several years ago in Morgan Lake near Shiprock, N.M., along with a load of largemouth bass intended for the lake. The lake periodically overflows down Chaco Wash into the San Juan River, a major tributary to Powell. Biologists, though, are unsure when the first gizzards made it to Powell. Powell biologist Wayne Gustaveson calls the accident bleak for downriver species but a great thing for Powell. Called "stink shad" in their native southeastern United States, gizzards are not a catchable game fish. But as a forage fish, they could bring back the days of screaming fishing rod reels and huge striped bass.
Gustaveson estimates that at least 2,000 gizzards exist in Powell based on the six specimens netted in the San Juan River arm of Powell this month. They indicate a breeding population Gustaveson predicts will spread throughout the lake in two to five years. The intruders are bad news for endangered humpback chubs downstream from Powell. Gizzards will not eat the chubs if they spread downriver, but they could out-compete them for plankton and biomass.
The Little Colorado River, 100 miles downstream from Glen Canyon Dam, which forms Lake Powell, is home to the largest known population of endangered humpback chubs. Non-native threadfin shad currently live in Powell but have never spread far upriver or downriver. But gizzards are a much larger, more robust and faster breeding variety that love muddy water. Threadfins take two years to reach 3-4 inches in length. Gizzards grow that large in two months, Gustaveson said. Powell's once-famous "striper" fishery collapsed in the mid-'80s when striped bass wiped out the threadfin shad planted there as forage.
Periodically, the threadfins bounce back, but it is a short-lived boom. Don't count your trophy stripers before they hatch, says Gordon Mueller, an ecologist with U.S. Geological Survey. He will be surprised if stripers can wrap their lips around dinner-plate shaped adult shad. He caught the first gizzard shad in the Powell system in June of 2000 during a project that sampled 40,000 fish. In a "finny" foreshadowing, the 14-inch gizzard showed up in the net with three endangered razorback suckers, caught in the muddy mouth of the San Juan. "It's not good news. It may have repercussions for not only the endangered fish but for recreation fishery," Mueller said.
The greedy gizzards will gobble up plankton, a food source for all young fish. "There's another chair at the table," Mueller said. And unlike the threadfin that is always small enough for predators to eat, the gizzard gets up to 18 inches. Too much of Powell's productivity is already tied up in large carp, Mueller says, and gizzards may just be another big fish that predators can't eat. "It's very unfortunate that they're there," Mueller said. "Our ability to create change is a lot better than our ability to direct it." Nothing can be done about the gizzards without harming other aquatic life, said USFWS spokesman Tom Bauer, in Albuquerque. And what of the USFWS hand in the accidental introduction? "I'm not going to get into that name calling game," Bauer said. And what of those other mystery fished dumped in that New Mexico lake? Nobody knows, but scientists have not caught them in Powell yet. The largemouth bass initially came from Inks Dam National Fish Hatchery in south-central Texas in 1998, where gizzard shad are abundant. Subsequent loads of bass transported to Morgan Lake from the hatchery were found to have as many as nine different species besides largemouth bass. Guadalupe bass, logperch, gizzard shad, white bass, bluegill and dollar sunfish, to name a few.
Gizzard shad exist in Utah in shallow Willard Bay as a boon to the walleye and wiper fishery there, but are periodically killed off in droves by cold temperatures.
Article from Waynes Words http://www.wayneswords.com/gshad.htm The internet link has photographs.