Georgetown College. Georgetown University.

There are mysteries hidden in the Chesapeake Bay, including questions about fish population changes that Georgetown’s Dr. Matthew Hamilton seeks to unravel.

In the 1970’s the numbers of striped bass in the Bay declined dramatically, causing concern about pollution and over-fishing in the community and drastic management actions to save striped bass populations. Even though the numbers of striped bass have now nearly returned to previous levels, a new problem has emerged. Many fish are infected with mycobacteria. The Maryland Department of Natural Resources estimates that at least 60 percent of striped bass in the Chesapeake Bay have this disease and the numbers of infected fish are rising.

Hamilton, an associate professor of biology at Georgetown College, is researching how gene variants in a fish population change over time and how these genes affect susceptibility to diseases. Hamilton and his students are asking: “In striped bass alive today, are the genes that control the immune response different from those in the early 1970’s?” Did the period of population decline—a “population bottleneck”—result in fish that today are more susceptible to this bacterial infection than in previous decades? Are there changes in immune response genes?

Hamilton and a team of students regularly visit the bay and it’s tributaries to “go fishing.” For them, this is much more than recreational. Each fish is carefully identified and its age determined. Select specimens are collected and then stored in the research freezer in Hamilton’s lab for subsequent DNA analysis.

To study this problem, Hamilton must make comparisons between today’s populations and fish from the 1970s, none of which are alive today. How can researchers obtain fish long since dead? Fortunately for Hamilton and researchers like him, biologists from the MD Department of Natural Resources (DNR) carefully catalogue and keep samples of fish scales each year. Studying these specimens provides the clues Hamilton needs for his study.

Developments in the field of molecular biology now permit DNA to be extracted from very small samples, even when the samples are 35 year-old dried fish scales. The scale samples were originally collected during surveys as far back as the 1960’s and used to determine the ages of fish studied. Without any expectation that their samples might be used for DNA analysis, the DNR biologists were essentially storing “time capsules” of the genetic make-up of this fish population, with accompanying data on where and when the fish were caught. From these samples, Hamilton can explore and compare the genetic variation of striped bass over a 30-year period.

Using the markers cloned in his lab, Hamilton and his students will analyze the DNA from fish across time, map changes and pay special attention to the genes involved in immune response. If all goes well, the clues that Hamilton finds will help explain the increase in the prevalence of mycobacteriosis.

Hamilton develops conceptual models of population genetic processes and then applies these to specific species. In earlier striped bass research, he and his colleagues cloned molecular markers that are now used for genetic studies by other scientists. Demonstrating that striped bass genetic variation is evenly distributed among different river populations. These results emphasized that management for striped bass needs to be coordinated across the entire Bay. Hamilton also includes other species in his studies, such as the canopy tree Corythophora alta, a member of the Brazil nut family (Lecythidaceae) found in Amazonian tropical rainforests.

Hamilton develops the mathematical aspects of his biology work through a collaboration with math professor Dr. Judy Miller. Together they have been awarded an NSF grant to study the theoretical basis of genetic quantitative traits, the features of an organism that we recognize right away, such as height. By balancing both abstract and hands-on research methods, Hamilton ventures into new areas of population genetics, always fishing for new clues and insights.

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