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Green To Measure Growth: Assessing Eutrophication's Effects on Mummichogs

As part of his research on preserving healthy and productive salt marshes, James Nelson, postdoctoral researcher at the Plum Island Sound Long Term Ecological Research site, turned 28,500 mummichogs green last summer.

Dr. Nelson explains: "Salt marshes, such as the Great Marsh along the northern Massachusetts coast, are highly productive landscapes that provide a great number of ecosystem services. Some of the more well known ecosystem services provided by salt marshes are protection from storm surge and runoff filtration. There is another major service salt marshes provide that ecologist have long theorized about but rarely demonstrated and that is the export of food to adjacent habitats in the form of fish.

Each year in the spring the productivity engine of the marsh gets switched on and begins to produce enormous amounts of biomass in the form of small fish. These fish, primarily mummichog (Fundulus heterclitus), follow the pulse of the tides to feed on the abundant food in the relative safety of tidal creeks throughout the spring and summer. As the temperatures dip into the fall season these little fish must move out of the shallow tidal creeks and into deeper waters, where they become prey for larger fish such as striped bass (Marone saxatilis). Essentially, salt marshes act as food factories that package large amounts of production in a short period and then export that production to organisms in other systems. In fact, we have observed a strong correlation between the amount of mummichog produced and the number of striped bass. Therefore, maintaining healthy and productive salt marshes will help to maintain productive offshore fisheries.

"Human-caused nutrient pollution, however, is a major threat to the stability of this important ecosystem service. My work focuses on the effects of 'eutrophication' or human-caused nutrient pollution on the secondary (fish) productivity of salt marshes. To examine this I employ a number of techniques with the primary goal of answering two main questions: first, how does nutrient pollution change the number of fish the salt marsh can support? And second, what are the effects on fish growth from nutrient pollution? By knowing the number of fish and their rate of growth we can calculate the total production of fish in the salt marsh. Of course, actually determining fish growth in the wild can be quite difficult.

"This summer I utilized a new technique that marks the bones, and other hard parts, of fish with a fluorescent dye (Calcein) that can be used to determine a fish’s growth rate. The fish are marked by immersing them in a non-toxic solution of Calcein and salt water for two minutes. A major advantage of this method is the number of fish we can mark at one time. This summer I was able to mark 28,500 fish in a two-day period. The dye does not harm the fish and cannot be seen in normal light, so the fish’s natural behavior is not affected. Once marked with the Calcein the bony structures will glow green under ultraviolet light. The marked fish were released back into the creeks from which they were captured to grow during the summer. We conducted monthly sampling of the fish in these creeks to recapture the marked fish. In the lab an ultraviolet light will be used to illuminate the Calcein mark. We will then measure the additional growth beyond the calcein mark on several bony structures to determine the fish’s growth during its time at liberty. If we are successful we will be able to answer the question of how eutrophication affects the growth rate of these fish and draw conclusions about the overall effect eutrophication will have on the export of secondary production from marshes to fisheries species offshore."