Chain of Grub
Brimming with tasty snacks such as water fleas, a healthy lake offers fish many dining options.
By Dyanne Fry Cortez
O.H. Ivie Reservoir is an oasis at the edge of arid West Texas, built where the Concho River turns north to join the Colorado. It holds 554,000 acre-feet at conservation pool level. In this decade, it has seldom been more than half full. Still, it’s an impressive body of water, and a fine place to live if you happen to be an aquatic organism.
In 2000 and 2002, anglers at O.H. Ivie donated five largemouth bass to the Budweiser ShareLunker Program. Each fish weighed over 13 pounds. The largest, a lake record caught by Butch Gayle, tipped the scales at 14.58.
Craig Bonds, a biologist in the Texas Parks and Wildlife Department’s San Angelo fisheries office, figures those lunkers were roughly the same age as the lake. Built in 1990, O.H. Ivie filled to storage capacity in 1992. “It takes nine to 11 years on average to grow a fish to 13-plus pounds,” says Bonds. “Those fish were young when the lake first filled and was full of nutrients.”
When Bonds says “nutrients,” he means fertilizer. New lakes are full of it. Rising waters flood land where grasses and trees once grew and animals left their droppings. This organic matter decomposes, releasing nitrogen, phosphorus and other elements that stimulate the growth of aquatic plants. Green plants are the first link in a lake’s food chain, using chlorophyll to grab the sun’s energy and store it in a form that other living things can use.
O.H. Ivie has stands of hydrilla and pondweed in the shallows. These rooted plants provide food for some organisms and protective cover for many others. However, most of the food production in a lake is done by phytoplankton — a catch-all term for single-celled algae and other free-floating plants too small for the human eye to see.
The next level is occupied by zooplankton, an assortment of microscopic animals that includes rotifers and water fleas. Zooplankton graze on phytoplankton “just like a bunch of cows in a pasture,” says Bonds.
Fat zooplankton taste fine to insect larvae and other small animals — including baby fish. Nearly every species of fish eats zooplankton at some stage of development. As young ones grow, dietary preferences diverge. Threadfin shad stick to a plankton diet for most of their lives. Bluegill favor insects and their larvae. Redear sunfish, also known as “shellcrackers,” have specialized pharyngeal teeth for crunching up snails and mussels. Young warmouth and green sunfish start with insects, but eventually move on to crayfish and small fish.
Sunfishes tend to congregate in the rooted vegetation. It’s where their favorite foods hang out, and also a good place to hide from bigger fish that want to eat them. Largemouth bass cruise the same underwater jungles, seeking cover when they’re small and hunting larger prey as they increase in size. White bass tend toward open water, feasting on schools of shad. Flathead and blue catfish eat whatever fish they can catch, and the omnivorous channel cat, “like the raccoon of the terrestrial world, will pretty much eat anything,” says Bonds. Adult gizzard shad and smallmouth buffalo are detritovores, consuming the leftover parts of dead plants and animals.
Several factors can change the balance of a food chain. A shortage of submerged vegetation means no nursery for sunfish and the invertebrates that feed them. Fingerlings that can’t find cover may be wiped out before they have a chance to grow, which leads to food shortages farther up the line. Excess vegetation isn’t good either: if prey fish have too many places to hide, predators go hungry. Bonds says the hydrilla at O.H. Ivie is approaching the nuisance stage, but so far, fluctuating lake levels have kept it under control.
Availability of nutrients is an important factor. It determines the carrying capacity, or how much life a lake can support. Older lakes are less fertile than new ones. Nutrients flow in with runoff from the surrounding watershed, but West Texas has lived through several years of drought. Descendants of O.H. Ivie’s ShareLunker bass don’t have it as easy as their great-grandparents did. There are more fish in the lake, competing for fewer resources. In 2001, TPWD changed the lake regulations to encourage thinning of the largemouth bass population. There’s no minimum length limit; however, only two of the five-per-day bag limit can be under 18 inches.
One law of nature must always be observed: approximately 90 percent of the food energy is lost at each step of the chain. Only a few individuals will make it to the top.
Predator fish that survive to trophy size are kings and queens of their domain. They can go where they want, eat what they like, and nothing is big or mean enough to eat them — until some air-breathing biped fools one into biting on a hook, hoists it out of the lake and fries it up for lunch.