Biologists discover gizzard shad explosion at Roosevelt
Dec 2, 2008

PHOENIX – Arizona Game and Fish Department biologists conducting an autumn fish survey at Roosevelt Lake discovered that a relatively new invader, the American gizzard shad, has experienced a population explosion here at Arizona’s largest inland lake.

 “This species looks like threadfin shad on steroids,” said Fisheries Chief Kirk Young. “These wide-bodied invaders from the eastern United States are shaped like footballs and can readily grow past the size where they are available to most sport-fish as forage.”

Young added that it is a wait-and-see proposition to determine if these invasive shad will have positive or negative impacts on Roosevelt or possibly the other popular fisheries along the Salt River.

During the recent fish survey at Roosevelt Lake, most gizzard shad sampled were in the 9- to 14-inch range and the largest two shad measured 17.7 inches long and weighed 2.3 pounds.

“We are still entering all the survey data, but based on what we saw during the sampling process, it appeared that gizzard shad were almost as numerous as the largemouth bass,” said Natalie Robb, the Mesa regional fisheries program manager.

At Roosevelt Lake, gizzard shad were first discovered during water quality sampling during January of 2007. Department biologists at the time recognized that gizzard shad are capable of rapid reproduction – a single female can produce up to 400,000 eggs. But the biologists were not expecting these newcomers to experience such a rapid population expansion.

Gizzard shad, which are native to the eastern United States, will likely compete for space and food with threadfin shad, another nonnative that has become the primary forage fish for sport-fish in the state’s larger impoundments. Immature gizzard shad will also compete for food sources with the larval stages of popular game fish.

However, at about 1-inch in length gizzard shad become more specialized, lose their teeth, exhibit deeper bodies and become filter feeders that consume small invertebrates and phytoplankton (free-floating algae).

“In Texas, they have found that bass can’t generally eat gizzard shad larger than 7 inches long,” said Robb, adding that on the plus side, these large shad can provide a significant meal for bass that do eat them.

Gizzard shad are seldom caught by hook and line and their pungent odor and soft flesh generally render them unsuitable as table fare, but in some parts of the country anglers use them as cut bait for catfish.

Robb explained that in most Arizona lakes, the predominant forage fish is the threadfin shad, which even as an adult is readily fed upon by sport-fish species such as largemouth bass, smallmouth bass and crappie.

However, gizzard shad grow to about 4 inches in length during their first year and readily grow to sizes of 9 to 14 inches, yet can exceed 20 inches in length. In Texas, a gizzard shad harvested with a spear gun measured 18.25 inches and weighed in at 2.97 pounds.

Based on the experiences in the Colorado River Basin and other locations where gizzard shad have invaded, biologists said these quick-growing, rapidly-reproducing shad from the herring family will likely spread to the other lakes along the Salt River, such as Apache, Canyon and Saguaro.

Biologists at Lake Powell first noted gizzard shad in 2000 near the San Juan inflow. In netting surveys at Powell in 2006, gizzard shad accounted for almost as much fish flesh as striped bass. These large invasive shad have spread to Lake Mead as well as the headwaters of the Colorado River.

Gizzard shad life history
Gizzard shad feed on both phytoplankton and zooplankton, which are then ground in the gizzard section of the gut.

This species is most often found in large schools. Its common name “skipjack” is derived from the fact that schooling gizzard shad can sometimes be seen leaping out of the water or skipping along the surface on their sides.

Dense schools of young gizzard shad provide forage for predatory fish such as bass, but these wide-bodied shad quickly grow past the primary prey size.

Gizzard shad spawn in the spring, usually in shallow protected water. Eggs and milt are released in the school, seemingly without regard for individual mates. Adhesive eggs attach to submerged objects and hatch in about four days. A single female can produce around 400,000 eggs.

Young gizzard shad feed on microscopic animals and plants, as well as insect larvae. Adults feed by filtering small food items from the water.

Characteristics: Bright silvery blue-green on back, silvery sides and dull white belly. They have a wide body that is stockier than most herring.

Native Range: Extends from the Great Lakes and St. Lawrence River west to eastern South Dakota and central New Mexico.

How to tell gizzard shad from threadfin shad: Gizzard shad have an upper jaw that projects well beyond the lower jaw. If you run your finger underneath the mouth forward and if the fingernail catches on the upper jaw and opens the mouth, you have just become acquainted with a gizzard shad.