by Sammie Alexander

In spring 2016, I completed an OSCAR Undergraduate Research Scholars Program project under the guidance of Dr. CJ Schlick (then, PhD candidate) investigating the influence of temperature on the growth of young-of-year (YOY) Alewife and Blueback Herring in Potomac River tributaries. At the time, I was a second-year undergraduate and had not yet had much research experience beyond the Honors College course HNRS 110: Principles of Research and Inquiry, wherein I conducted a literature review to answer a research question I had developed. Through the project with Dr. Schlick, I gained hands-on laboratory experience dissecting YOY fish to extract their otoliths (i.e., ear-bones) for aging using a method similar to counting rings on a tree – each ring on the YOY otolith represented a day of life, so the number of rings equated the number of days since hatching. Once all fish were aged, we then began comparing the determined ages against the known lengths of the fish to then incorporate into a commonly used growth model for fisheries research known as the von Bertalanffy growth function.


We wanted to know if there was evidence to suggest that temperature was driving growth in these two Species of Concern, so we needed to find a way to compare the growth function to known temperatures throughout the study period. After months of trial and error, I received some very helpful feedback at the Tidewater Chapter of the American Fisheries Society conference where I was presenting a poster. (I won the Best Poster Award at this meeting!) Another presenter suggested I look into using the growing degree-day (GDD) metric often applied in agricultural science to our fish species, and after doing some research of my own this eventually became one of our primary modes of inquiry. The GDD metric essentially applies a value to each day of life a fish experiences above their known minimum temperature threshold for growth, and then all of the calculated values are added together for the period of time the fish was alive (i.e., we knew the date we collected the fish on and the number of days they were alive, so we could trace back to which day they hatched on and use temperature data from a nearby buoy to calculate the daily GDD values). This method allowed us to analyze the relationship between temperature and growth in our populations of fish sampled between 2013 and 2015.


Ultimately we were able to determine that species length and cumulative GDD were highly correlated within each year for both species, but were significantly different between species and years for each species. Overall, our results illustrated the close relationship between temperature and growth for the youngest age-classes of two regionally important fish species. While it was already known that the inland migration from the Atlantic Ocean into the Potomac River by adults of these two species was triggered by increased water temperatures, the relationship to YOY was not as clear. Our results indicate that changes in timing and magnitude of seasonal warming with future climate change may not only alter spawning timing for adults, but may also have strong influence on the growth of YOY directly and indirectly by promoting a timing mismatch between YOY and their seasonally available food source.


This was the first of many research projects I would conduct in Dr. Kim de Mutsert’s Fisheries Ecology Lab, and it contributed to my decision to pursue an accelerated master’s at Mason. Following this project, I completed an OSCAR team project in summer 2017 (https://www.youtube.com/watch?v=cTpO-M2q4o8&t=51s) looking at diet composition of fishes in Gunston Cove and Hunting Creek (i.e., two of the areas studied in first OSCAR project), and then began my master’s in Environmental Science and Policy in 2018 using environmental DNA (eDNA) and habitat assessments to evaluate the implications of roadways on stream connectivity and the distributions of native and invasive fish species in northern Virginia. Since becoming a master’s student at Mason, I have served two years as Social Chair for the Environmental Science and Policy Graduate Student Association, become a 2019-20 Catherine and Richard Becker Graduate Research Fellow, and received two program development grants from Virginia Sea Grant. I hope that this manuscript is the first of many reflective of the work I have conducted during my time at George Mason University.


More information surrounding my work can be found on my personal website and information surrounding all of the work in the de Mutsert Fish Lab can be found on the lab website (https://demutsertlab.gmu.edu/).

Read the new publication here: Alexander, S. B., Schlick, C., & de Mutsert, K. (2020). Growth models and growing degree-days: Assessment of young-of-year Alewife and Blueback Herring in Potomac River tributaries. Environmental Biology of Fishes. https://doi.org/10.1007/s10641-020-01012-4

Photo Credit for last two photos: Aileen Devlin, Virginia Sea Grant

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