Greater Lakes

A faint whiff of an odor first experienced in childhood may conjure a host of vivid recollections. Olfactory memory is common to many animals, including fish, where it helps guide navigation, predator avoidance, food acquisition, and reproduction. Chunbo Zhang, IIT research assistant professor, studies the subtleties of olfactory perception, investigating the neural and hormonal mechanisms involved and their roles in species survival.

Zhang's current research, supported by the Great Lakes Fishery Commission and the Great Lakes Fishery Trust, focuses on olfactory memory formed during development of lake sturgeon in the Great Lakes. The future of the largest fish in the Great Lakes—prized for both their meat and eggs—is imperiled, as overfishing, pollution, and habitat loss affect their lifecycle and long-term population levels. Zhang believes an additional factor may be undermining efforts to restore self-sustaining populations of lake sturgeon to the Great Lakes—inadequate imprinting. "Olfactory imprinting is a complex and fascinating phenomenon. There are enormous questions about the underlying mechanisms involved," she says.

Species imprinting, which may involve any of the five senses, usually occurs during early development. Re-exposure to the sensory stimulus can provoke particular behavioral responses essential for survival. In fish, olfactory imprinting leaves an indelible trace in the nervous system, retained throughout the fish's lifespan.

Zhang notes that, recently, the lake sturgeon population has improved in the Great Lakes after the fish were listed as endangered, and replenished through hatchery raising and release into lakes. But questions remain as to how well lake sturgeon are naturally rebounding. "The concern is that in Lake Michigan, the stocked sturgeon either may move elsewhere or are not reproducing if they were imprinted by unexpected cues from other water bodies," Zhang says.

This is of special concern as sturgeons are slow-growing fish, requiring 8–19 years to fully mature and reproduce. Mistakes in breeding and development may manifest themselves only over lengthy time periods, leaving the full extent of the problem unrecognized for many years.

The current project seeks to establish the precise time frame of olfactory imprinting so that sturgeons receive the proper sensory cues. (One strategy would involve hatching the fish in the proper lake water.) Zhang believes this crucial period occurs in the first year of life and involves molecular, biological, and morphological changes to a brain structure known as the telencephalon.

A subtle interplay of rapidly proliferating neurons in the olfactory cortical and memory centers and a period of high thyroid hormone activity are associated with the window for proper olfactory imprinting, which closes again as specific regulatory genes are reduced in expression.

In addition to its detrimental role for fish, improper sensory imprinting may be responsible for a range of human maladies and has recently been implicated in certain symptoms of autism.