Invading Gizzard Shad Blamed For Plight Of Roosevelt Lake Bass



The slimy, invading, prolific, fast-growing gizzard shad have taken over Roosevelt Lake.

Woe be to the hungry bass and those who love to catch them — and maybe to the economy of Rim Country too.

The delicate, soft-bodied gizzard shad live mostly on plankton. A single female can produce 380,000 eggs a year in her prime. That ought to be good news for bass and the other predator fish except for one problem: The gizzard shad within three years grow to 16 inches — too large for most bass to swallow. Bass have a famously large mouth — but only a 3-foot-long bass could gobble a 16-inch shad.

Unfortunately, the prolific shad seem to be eating so much of the plankton that the bluegills and threadfin shad are apparently struggling to compete. Threadfin shad grow to more like 6 inches, making them much better bass food. Those fish in the past provided the bulk of the bass bait.

“The major issue is that gizzard shad are taking the biomass over,” said Arizona Game and Fish Fisheries Branch Manager Chris Cantrell. “They’ve seen this back east. They’ll start spawning and can produce 380,000 young in their second year. They just take over. Tie up a lot of biomass. Eat plankton, algae, then they’ll move into more micro invertebrates. By year three, they’re 16 inches long — too large for the average large-mouth bass.”

The gizzard shad reproduce in such numbers that their young compete directly with threadfin shad and the newly hatched bass, crappie and bluegill for plankton. As they grow, they out-compete the full grown but still small threadfin shad for the plankton — only to grow too big for any of the prized sport fish to prey on.

Fishing advocates say a dramatic decline in the catch rates for bass and crappie can have an impact on the regional economy. A 2001 survey estimated that bass fishermen at Roosevelt Lake pump $63 million annually into the regional economy. Fishermen spend about 500,000 hours annually on the lake and catch about 250,000 bass — most of which they return to the lake, according to Game and Fish surveys.

The non-native shad reportedly first showed up in the lake in 2005, but weren’t officially documented until 2007. No one knows where the gizzard shad came from, but Cantrell suspects a fisherman may have inadvertently introduced the shad as a bait fish. Their numbers have increased dramatically since. One recent electro shocking survey found that gizzard shad accounted for 28 percent of the fish sampled, compared to 18 percent for bass and 19 percent for threadfin shad. Other species like catfish don’t show up in the electro fishing surveys because they’re more resistant to the effects of the electric shock.

Fishermen have reported a sharp drop in catch rates for bass and crappie in the past several years as the gizzard shad population has grown. However, that period also corresponds with big fluctuations of the lake level, from brimming two winters ago to less than half its capacity now. Such big fluctuations also work against bass and crappie, while gizzard shad apparently do well in warm, shallow waters.

When the lake filled after a long drought several years ago, bass fishing boomed as nutrients from the newly flooded areas seeped into the underwater ecology and bass found plenty of submerged trees and brush tangles to hide in to await passing prey.

A recent meeting involving Tonto Basin anglers illustrated the apparent impact on the fishery. Fishermen who reported easily catching 20 or more crappie a day now say they’re lucky to catch five.

At the 2011 FLW Roosevelt Lake bass tournament, 92 percent of the anglers caught the maximum number of fish allowed and the weight of the fish caught by the top 10 finishers totaled 350 pounds. In the most recent FLW tournament, only 13 percent of the professional fishermen ended up with the maximum number of fish and the top finishers had just 200 pounds of fish. The tournament winner had just 10 fish after three days of fishing.

Moreover, the tenacious shad has also shown up in Apache Lake, which only recently recovered from a massive fish kill. Golden algae several years ago killed more than 95 percent of the fish in Apache Lake, said Cantrell.

The same algae caused a major fish kill in the lower reaches of the Salt River last summer — and a minor fish kill in Roosevelt Lake. Curiously enough, that die off in the lake late last summer mostly affected gizzard shad, which remain a “whimpy” fish that dies easily when handled or exposed to toxins, said Cantrell. Biologists still don’t know why the golden algae sometimes creates lethal blooms, usually in warm waters. One theory holds that when the algae run out of organic matter in the water to feed on, it creates a toxin that kills fish — which ends up filling the water with nutrients the algae can consume.

Game and Fish restocked Apache Lake, just downstream from Roosevelt and fishing was booming this spring. Now, the gizzard shad pose a potential threat to the fishing in the whole chain of Salt River lakes.

Cantrell said the department has set aside $50,000 to try to figure out exactly what’s happening to the complex interactions between fish species in Roosevelt.

Despite the alarming decline in angler success in Roosevelt, Cantrell said it’s still not clear how the arrival of the gizzard shad will affect the lake’s fish ecology in the long run.

For instance, some experts maintain that bass may grow far larger on a diet of bigger fish, rather than relying so heavily on crayfish and the 3-inch threadfin shad.

Some people who manage fishing ponds that charge anglers for a chance to hook a monster bass deliberately stock gizzard shad into those ponds to beef up the bass.

Moreover, the arrival of large numbers of 12- and 16-inch gizzard shad may prove a boon to the lurking channel and flathead catfish in the Roosevelt Lake, since they can grow to 50 pounds and easily handle larger prey.

But the gizzard shad has posed a challenge to a growing number of lakes and streams.

For instance, the gizzard shad has spread throughout Lake Powell in the past decade. But in Lake Powell, some experts think that might prove more of a blessing than a curse — since the lake is dominated by schools of voracious striped bass. The striped bass have outstripped the food supply offered by the threadfin shad, causing repeated population crashes. Grown gizzard shad are too large for the striped bass to swallow, but the sheer numbers of younger shad may help sustain the sports fishery there, according to an online discussion of the issue by Wayne Gustaveson, a Utah Game and Fish fisheries expert.

Gustaveson said it appears the gizzard shad in Lake Powell were introduced before 2000 from fish transported to Morgan Lake near Shiprock, N.M. from a fish hatchery in Texas. From Morgan Lake, they somehow spread to Lake Powell, where they rapidly colonized the whole of the 200-mile-long lake.

Cantrell said the lake’s fish populations may stabilize once the gizzard shad finds its place in the system. That seems to have happened in some eastern lakes, where the gizzard shad has long dominated with a boom-bust population dynamic.

Some anglers are clamoring for the stocking of bass into the lake. But Cantrell said that might just prove a waste of money until experts know how many fish live in the lake — which would require an expensive sonar survey.

“If we’ve got 5 million fish and 18 percent of those are bass — then stocking isn’t going to do us any good. There are things we want to learn first.”

In the meantime, Game and Fish hopes to find the money to start dropping new, concrete fish structures into the lake — which will not only concentrate the fish but boost the hunting success of the bass by providing cover for these peerless ambush predators.

“We’re hoping over time it will return back to its previous glory days,” said Cantrell of Roosevelt Lake. “There are still memorable and trophy fish in that lake. There’s nothing terrible with that lake. It’s a great fishery still.”


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