The River Landing

The Gizzard Shad in Nature's Economy - Part 1 of 2

One cold winter day while walking along the Illinois River near Havana, I noticed thousands of fish swimming in the clear water near shore. Most of them were gizzard shad, small- to medium-sized members of the herring family. Gizzard shad tend to be rather numerous throughout the river, forming an important food source for populations of many bird species, especially overwintering bald eagles.

Adult gizzard shad (Dorosoma cepedianum), from The Fishes of Illinois, by P. W. Smith, used by permission.

The shad I saw in the shallow water that day, in fact, were easy prey, especially those that were showing signs of stress from the recent period of prolonged frigid temperatures. Common goldeneyes, mergansers, and gulls had a feast on the dead shad floating in the open water. And soon I saw a bald eagle leave its perch on an upper branch of a tall cottonwood tree to fly over the expanse of frozen river. The eagle decreased its altitude in a series of graceful turns, then extended its talons forward just as it was barely above a patch of open water; it grabbed a fish from the water, and then flew to a silver maple tree to feed. Of no surprise to me, the fish in the eagle’s talons was a gizzard shad.

Illinois River near Havana, 10 degrees F below zero.

From those simple observations, it was easy to see the relationships between gizzard shad and the fish-eating birds. But it made me wonder about the shad themselves. What do they feed on? And what aspects of life history and behavior explain their importance to other river inhabitants?

Obtaining Food

Water in the Illinois River teams with a variety of microscopic life called "plankton," which is derived from the Greek word planktos, meaning "drifting" or "wandering." The animals of the plankton are called "zooplankton," the plants "phytoplankton." There are a bewildering variety of types and sizes of these plankters, from one-celled protozoans, invisible to the unaided eye, to minute crustaceans that one can just barely see darting about below the water’s surface. These are small animals, indeed, with perhaps many thousand per cubic meter of water, depending on river conditions and time of year. The phytoplankton are generally even smaller than the zooplankton, and even more numerous. But these small organisms, drifting about in the river’s sluggish currents, are a major component of a gizzard shad’s diet, although the proportion of zooplankton or phytoplankton in the diet changes with age of the fish.

Below a length of about one inch, a young shad looks and behaves like a carnivore. Its mouth is lined with small teeth and is farther forward on the body than in the adult fish; these features facilitate the capture of live prey. The digestive tract of shad at this stage is short, a reflection of the easier digestibility of animal versus vegetable matter. At this immature life stage, the shad compete with the young of other fish species for the same prey, which is mainly zooplankton. Most fish species switch to other prey as they grow older. Young shad travel about in schools to increase foraging efficiency and to protect themselves from larger predators, such as largemouth bass. They form schools to confuse predators with a complicated motion – much as birds in a flock.

When a young shad grows to about one inch in length, its body goes through a major transformation, and its diet gradually begins to include more plant material. The digestive tract becomes long and twisted, a necessity due to the longer time required to digest plants. In addition, the teeth are lost and the mouth gradually moves slightly toward the bottom side of the fish, reflecting its new non-predatory lifestyle. Adult gizzard shad feed heavily on phytoplankton in addition to a variety of dead, decaying animal and plant matter collectively called "detritus." They also ingest mud and attached algae, which are simple plants that grow on solid objects in water clear enough to allow sunlight to filter through. Adult shad are, therefore, plant eaters, or herbivores, as well as detritivores.

Adult shad forage by filter-feeding, as opposed to actively pursuing zooplankters as they did when young. They do not, however, passively filter the water while moving slowly forward with their mouths agape as one might imagine; instead, they actively pump-filter the water by drawing in a definite volume into the mouth cavity. Food particles and minute organisms are then strained from the water by special structures called "gill rakers" at the back of the mouth cavity.

Because each food particle filtered from the water contains only a small amount of energy, large volumes of water must by pumped into the mouth and filtered for food items. The ability to acquire energy from such diffuse resources as phytoplankton and detritus hints at the important role played by adult gizzard shad in the riverine environment.

[Note: This story originally appeared in slightly different form in the Summer 1995 issue of Illinois Audubon magazine, Number 253.]

The Gizzard Shad in Nature's Economy - Part 2 of 2

Energy Flow Through the Ecosystem

For ecosystems (a term that refers to the community of all living organisms more or less within a definite area, plus their habitats), sunlight is the driving energy force; and, needless to say, without sunlight, an ecosystem would eventually run down. Plants are the primary producers; they are able to combine carbon dioxide with sunlight and water through photosynthesis to form sugar, a form of stored energy. The plants, whether eaten directly by herbivores or as dead plant material in detritus, form the base of the ecosystem’s food chains. Mid-level carnivores (for example, ten-day-old shad) may feed on zooplankters that in turn had fed on plants. Top carnivores, in the strictest sense, do not feed on plants; but if we look at the food of their prey and so on, eventually we will come to plants. So a largemouth bass could have fed on a warmouth sunfish, which had fed on an aquatic insect that in turn had grazed on plant materials; but the warmouth may also have fed on a crayfish that ate mostly plant detritus, or may even have scavenged on a dead bluegill. Where does the gizzard shad fit into this picture? We have seen that it holds many roles in the riverine ecosystem, beginning life midway up the food chain as a carnivore, then settling down in adulthood closer to the bottom. Food chains, in fact, are so interwoven that all of the food chains in an ecosystem can be referred to as a food web.

As energy moves up a food web, from the sun to plants to top carnivores, most of it is lost due to the inefficiency of transfer from one level to another. Because of the inevitable energy losses with each level up a food web, the closer an animal can feed to the base of the web (the plants), the more energy will be available. This is where the gizzard shad plays a significant role. To be sure, there are many other fish species in the river, but no other species that feeds at the base of the food web is as abundant, widespread, and easily available as a food resource for carnivores. The rapid growth of shad (to about four inches in length after the first growing season) ensures a minimum of delay and energy loss in transferring energy from the sun through the shad’s plant foods into animal life that can be utilized by such large carnivores as the bald eagle.

Producing the Next Generation

The gizzard shad belongs to a general category of organisms of diverse kinds whose life history strategies include high reproductive potential with little or no parental care of the young. At the other extreme are organisms that invest much time, care, and resources into raising only a few young. Birds belong to this second category, an extreme example of which is the California condor that lays only one egg, and may not breed every year. Between these two extremes are many variations. Sunfish, for instance, lay many eggs, but actually construct nests and protect the eggs and newly hatched young from predators.

Gizzard shad in the Illinois River probably spawn in backwater areas, sloughs, or quiet coves beginning in April or May when rising water temperatures are between 50 and 70^o F, and they may continue through June. A spring flood in the river is beneficial to the shad because the greater volume of water increases the available spawning and rearing habitat. When spawning, females release thousands of eggs into the water column. The eggs, each about three quarters of a millimeter in diameter, are fertilized by the males, then sink and become attached to any solid object. They hatch about four days later. About five days after hatching, the young shad begin actively feeding.

The high reproductive output of the gizzard shad is one reason why the population can endure winter die-offs and heavy predation. Even so, their populations tend to vary considerably from one year to another.

A Web of Interactions

The changes in shad populations from year to year must surely have effects on other riverine life. For instance, more shad might mean more food for bass, allowing for greater survival of bass to larger sizes and possibly better reproduction from healthier individuals; then, more and larger bass might result in greater predation on other fish species including shad. More shad would also result in greater predation on plankton. One might imagine many other similar scenarios.

In 1869, John Muir wrote that "when we try to pick out anything by itself, we find it hitched to everything in the universe." It is a concept worth thinking about when watching a bald eagle forage for gizzard shad or, at a much greater scale, when viewing the night sky full of stars; the concept might even make it easier to appreciate life in the depths of the murky Illinois River or a community of aquatic plants and animals invisible to the naked eye. It might even help us understand where we humans fit into the big picture, within the world’s great food web. Such an ecological perspective is necessary today more than ever, as the human population continues to grow and prosper, demanding increasingly greater amounts of the earth’s natural resources.

Chautauqua National Wildlife Refuge, totally frozen in mid-winter. At times like these, fish-eating birds, such as the bald eagle, must find open water elsewhere.

[Note: This story originally appeared in slightly different form in the Summer 1995 issue of Illinois Audubon magazine, Number 253.]