What are Local Fish Eating?

By: Sammie Alexander

As a member of the ecology team for the Potomac Environmental Research and Education Center’s (PEREC) summer OSCAR team project, my research goal is to investigate the predator-prey dynamics between fish and macroinvertebrates in two freshwater tidal Potomac River tributaries, Gunston Cove and Hunting Creek. This means, I examine the stomach content of 15 fish species known to inhabit both embayments in order to construct a food web for each location.
Due to variations in aquatic habitat types within the embayments (i.e., vegetated and non-vegetated), I use three different methods to collect my fish samples: fyke nets, seine nets, and otter trawls.

Fyke Net, seen from the surface

A fyke net is a passive collection technique in which fish are guided into a funnel shaped net by three leads, or guide nets. Once fish enter the funnel through narrow openings, they are unable to return to the outside of the net. The funnels on the fyke net are comprised of subsequently narrower openings that make exiting the net difficult for many fish. This method is implemented in heavily vegetated habitats where the submerged aquatic vegetation is so dense it is impossible to actively pull other types of net through.

The second method, seine nets, are an active collection technique in which two people extend a long net perpendicular to the shore and then drag it parallel to the shore for approximately 100 feet. This targets fish along the shoreline and can be used in vegetated and non-vegetated habitat. However, seine nets become very difficult or even impossible to use effectively once the vegetation reaches a certain density.

Otter Trawl being towed behind the research skiff

The third method, otter trawls, are another active collection technique in which a weighted net is dropped off the back of the boat and dragged at a constant speed for 5 minutes. Like the seine net, this method can be used in vegetated habitats to an extent. If there is too much vegetation, the trawl will become clogged or too heavy and will have to be reset, so it is best used in open water.

Once my fish samples are collected, I take them back to the lab to remove their stomachs for examination. I then sort the contents of their stomach into groups based on the lowest possible taxonomic level, which can be challenging if the organisms in the stomach have already begun to digest.

While there are published studies focused on the diet of many of the fish species I am studying this summer, none of the studies focus on populations in these specific tributaries of the Potomac River. The species found in these systems are unique, in a way, due to their close relationship with wastewater treatment plants upstream. These treatment plants feed nutrient rich effluent (i.e., discharge water) into the streams that then flow into these embayments. Adding additional nutrients to an aquatic system has the potential to influence the type of organisms that can live there.

I will use the organisms I identify in the fish stomachs to construct a food web, which will allow me to compare the diets of fish communities that reside in non-vegetated habitats, such as shorelines and open water, to the fish communities that reside in vegetated habitats, such as the submerged aquatic vegetation (SAV) beds.

Pursuing this question is interesting because in the 1980’s Gunston Cove was a hypereutrophic (i.e., very nutrient rich) system, due to the nutrient rich effluent released from the Noman M. Cole, Jr. Pollution Control Plant (NCPCP), which provided ample nutrient resources for phytoplankton algae to grow. Eventually, the algae became so abundant that it formed a thick green layer over much of Gunston Cove, which blocked the sunlight from reaching the streambed, preventing the growth of SAV. However, in an effort to reduce the occurrence of these harmful algal blooms, NCPCP reduced the amount of nutrients released in their effluent and after many years SAV beds began to reestablish (like they were prior to the 1980’s).

Stomach Content of fish caught in Gunston Cove. Photo taken through a dissecting microscope.

Now that nutrients, such as phosphorus (the first nutrient that was reduced in the wastewater effluent) and nitrogen, are better regulated, light is able to penetrate the surface and in turn SAV is able to grow. The food webs I construct will compare the potential impacts shifting from the historically non-vegetated habitat to the emerging vegetated habitat may have on the diets of fish in Gunston Cove and Hunting Creek.

As an Environmental Science student, I am very interested in the interactions that take place between organisms within an ecosystem. One day I hope to incorporate ecosystem modeling and spatial analysis into my own research, to investigate the potential impact factors such as climate change or invasive species can have on populations of aquatic species.

When I saw that Dr. de Mutsert was looking for a student to help construct a food web, I thought that this was a great place to start. Before I can create models to predict how stressors such as climate change or invasive species will impact an ecosystem, I must first understand how the species currently interact with their environment and surrounding community.

Now, I begin week seven of attempting to answer an unanswered question with high hopes and a long to-do list. I have spent many hours this summer on a boat collecting fish for my project, and macroinvertebrates for my partner’s, trolling the internet and library for resources on how to conduct a diet study and information on the history of the fish community in the Potomac River. More recently I have begun processing my 196 fish samples to try to understand what exactly is going on in the Potomac River (i.e., who is eating whom). The past six weeks have been spent collecting my fish samples and preparing the content in their stomachs to be examined.

With only two weeks left until our results are due to our advisors, it is now time I use our data to begin connecting the dots between the inhabitants of each embayment. The only thing standing between the coveted answer and myself are approximately 190 fish stomachs, but have no fear – I’ve come prepared with my microscope, tweezers, and coffee. Stay tuned to find out what happens next.