Spring 2003

Rock Bass:
Underwater Tree Hugger

This fish story begins with cottonwoods. Leaning forward over the water as if to seek a better view upstream, these fat, furrowed trees soak their roots in the life-giving upland streams of Chicago Wilderness. And their roots in turn give life to fish.

Tree roots under a stream bank, be they cottonwoods or box elders or silver maples, are a favored haunt of the rock bass, Ambloplites rupestris. This brassy-colored bass dwells in clear, cool, rocky-bottomed lakes and streams with weeds, tucking itself away in the shadows afforded by boulders, downed trees, and undercut banks. It's often called a "goggle-eye" for eyes that seem too big for its head. But like most things in nature, those enlarged orbs serve a purpose — they lend a predatory advantage to the rock bass, allowing it to feed with efficiency in low light conditions. Couple that with its marbled camouflage, and the rock bass, lurking in the shadows, easily takes unsuspecting prey. Its marbled appearance also protects it from becoming the prey of herons and smallmouth bass.

With a large mouth and brush-like teeth, the rock bass can easily capture shiners, darters, and aquatic insects, as well as the hapless terrestrial bugs that slip into the water. Its eye muscles allow it to look down to pick food off the stream bottom, making crayfish a particular favorite on the menu. While the rock bass takes numerous creatures from the stream, it also gives back, serving as host to the hitch-hiking larvae of stream-dwelling mussels.

Rock bass take up housekeeping in quiet, slow-flowing pools and don't seem to wander a great deal. They may, however, move short distances when it comes time to spawn in the spring. As the water warms in May, the males dig a saucer-shaped depression in gravel along the stream edge. They usually position their nests beside a boulder or a log; scientists believe that doing so not only reduces stream current near the nest but also minimizes the area the males have to guard from nest invaders. Both factors save energy they can devote to being good caregivers.

Ripe females congregate in deeper pools while excited males encourage them to lay eggs in the new nests. With some coaxing and orchestrated swimming, the pair converges on the nest, and the female deposits a few of her 10,000 eggs, which the male fertilizes. The eggs stick to the clean gravel bottom where the male stands guard for days. He fans them clear of choking silt, brushing oxygen-rich water over them. Soon, the young emerge, eventually moving to shallow, slow-flowing water — a nursery of sorts — where they stay for a few weeks. If abrupt spring floods don't wash the young ones downstream to an almost certain death, they may grow to be about an inch and a half long by the end of their first growing season.

Young rock bass feed on zooplankton, then dragonfly, midge, and mayfly larvae. But when they are only an inch long, they start to show an affinity for fish fare. By their fifth year, rock bass may reach a respectable size for the angler; eight-inch fish are not uncommon.

The rock bass is distributed widely over the Midwest, and its conservation status is secure. However, on a local level, land use changes like development and agriculture can cause stream quality to deteriorate with heavy siltation and irregular or "flashy" water flows. The rock bass lives mostly in the outlying reaches of Chicago Wilderness, preferring clear tributaries such as those of the Kankakee River in southern Will County to the muddy, channeled portions of many urban waterways. It also inhabits rocky stretches of Lake Michigan shoreline.

A 19th-century visitor to urban rivers such as the Chicago and Des Plaines could once see through many feet of water to a rocky riverbed, perhaps to a nesting rock bass. Many groups are working to return these rivers to their former glory and welcome the bass back to its old haunts. Some efforts focus on the reestablishment of riverside vegetation, including trees with their beneficial roots.

So if you want to catch rock bass or witness their courtship antics, put your own goggle-eye to the sky to find one of those tall streamside cottonwoods.

— Craig Springer