Scientists study sea trout in food web

Many of us enjoy fishing as a pastime or a livelihood, but have you considered the origins of the fish that you're catching? Kevin Dillon, Ph.D., and Richard Fulford, Ph.D., at The University of Southern Mississippi's Gulf Coast Research Laboratory have. In fact, they've made a detailed study of it.

As part of a larger project involving aquaculture, Dillon and Fulford studied one particular species that is both recreationally and commercially important in Mississippi and Alabama waters: Cynoscion nebulosis, better known as the spotted sea trout. The pair wanted to know more about the diet of the sea trout at different life stages and during different seasons. The information can be invaluable to aquaculture and resource management efforts, and it demonstrates effects of influences on the ecosystem.

By knowing what an organism eats at different stages in its life and times of the year, scientists can better simulate wild conditions within the controlled environment of a farm. While a small decrease in the percentage of a certain plant in the wild may not seem a drastic change, analysis may indicate that  it has disastrous effects in the big picture.

The data collected and analyzed in this study, funded by Mississippi-Alabama Sea Grant, can be used in computer simulation models to forecast the results of changes in a quantity of food source on the population of the species in the region.

So, how does one find out what a sea trout has been eating? By examining muscle and liver tissues of the C. nebulosis using a process called isotopic analysis, scientists were able to narrow it down. Because all living things contain an element called carbon, scientists examine carbon signatures to determine what a particular animal ate.

Scientists collected fish from several sites and across a range of seasons to provide the greatest variability. Tissue samples from both the muscles and liver were removed from the fish and dried at a specific temperature to a constant weight. Samples were sent to the Marine Environmental Research Laboratory at Cedar Point in Ocean Springs, Miss. The lab has a specialized piece of equipment called a mass spectrometer. The spectrometer is able to determine the elemental composition of a sample. More specifically, it can determine carbon and nitrogen signatures.

Have you ever heard the old saying, "You are what you eat?" By examining a carbon signature of a creature, scientists can get a general idea of the types of food it consumed. Because muscle tissue undergoes relatively little change over time, the carbon signature found in it can provide a good indicator of the animal's diet over a long period of time (months to years). Conversely, the liver changes frequently because of its role in the digestion process, and it is a good indicator of what the creature has consumed recently (weeks to months). This information is important because it can tell scientists what plants and animals are important food sources to the fish throughout its life cycle.

Sea trout do not consume plant material all of their lives. As they grow in size, they feed on small fish and animals. This is where nitrogen levels come into play. A creature's nitrogen signature aids in determining the trophic level, which is the animal's place in the food web. The higher the trophic level, the further the animal is from a primary producer. For example, phytoplankton (plant plankton) are primary producers, because they produce their food from the sun and inorganic materials. Thus, plankton are the first trophic level. Menhaden eat phytoplankton and are called primary consumers and are the second trophic level. Mackerel eat menhaden, so they are secondary consumers and occupy the third trophic level. Sharks eat mackerel and are apex predators: predators that have few or no predators of their own.

By applying both nitrogen and carbon signatures to a graph, scientists were able to get a clearer picture of the sea trout's diet and position in the food web. Add in data from multiple species and locations, and they can see the structure of the food web as a whole.