Previous issues of Tide Bites have featured “apprentices” from our autumn Pelagic Ecosystem Function research apprenticeship. This is such an inspiring program that we asked two of last quarter’s students, Sarah and Marilyn, to write an essay for us. These two stayed on at FHL after the course was over in mid-December so they could gather more data and “fill out” their research effort. While the work they describe was messy (and often smelly!), they worked long hours in the fume hood and on the microscope, caught up in that passion for answering questions that keeps many of us in science. FHL was able to cover their expenses for that extra time because of donor funds for research; their autumn attendance had been made possible by other FHL scholarship funds. As this Tide Bite goes to press, we have over 100 student applicants waiting to hear about acceptance to FHL summer courses, and a majority of those accepted students will need financial assistance to participate. If you would like to contribute to this great cause and help foster the next generation of marine scientists, you can make a gift here.
Guts and Plastics
by Sarah Miyabi Hendricks & Marilyn Duncan
This short story is about two aspiring scientists who met at Friday Harbor Laboratories on the beautiful island of San Juan. Sarah is a student at UW, and Marilyn is at Portland State University (Figure 1). The two met in Autumn 2023 while attending the Pelagic Ecosystems Function (PEF) research apprenticeship at FHL by instructors Jan Newton, Matthew Baker, Rebecca Guenther, Alex Marquez, and Mike Sigler.
Sarah came to the program with a background in marine biology and a particular interest in Arctic ecosystems and how wildlife and the environment will cope with the increasing effects of climate change. Marilyn brought experience in environmental science and an interest in the impacts of anthropogenic pollution on marine and terrestrial life.
In the PEF program, all apprentices choose a research topic to explore during the 10-week quarter. Sarah decided to work on the Pacific salmon diet and spatial analysis, and Marilyn on the abundance of microplastics in salmon stomach contents. There was a large amount of overlap for this work, so we decided that it would be beneficial for us to work together, which is where our “Guts and Plastics” journey began.
Step 1. Guts: Sarah
All samples of salmon were collected by Andy Derksema, a charter fisherman in the San Juan Archipelago, Juan Velero, a recreational fisherman in Puget Sound, and Karen Johnson, a commercial fisherman in Alaska. The big goals for this project were comparing and contrasting the diets of salmon from 2023 to past years, and also seeing how they differ among regions. 2023 was the first year getting salmon samples from Alaska, meaning we had three different areas to compare and contrast. The salmon species we focused on were Chinook, Coho, and Pink.
To prepare for the examination of microplastics following diet content analysis, all work was performed under a fume hood to prevent potential contamination of gut contents from plastics in the air. Once the gastrointestinal tract was dissected from a salmon, the stomach was separated from other organs and weighed, with and without its contents (Figure 2). The presence of each food category, such as Clupea pallasii (herring), Ammodytes personatus (sand lance), and various crustaceans – including crab larvae – were noted by number and weight. Once all stomachs were emptied, data analysis was done. Some contents that I found (Figure 3) included crab larvae, amphipods and copepods, krill, parasitic nematodes, sand lance, herring, and squid (only in AK samples).
Overall, salmon species in different regions varied highly in their diets. For example, San Juan Archipelago (SJA) salmon had a diverse diet including sand lance, crustaceans, herring, and smelt (Figure 4). In contrast, the salmon species of Puget Sound (PS) had a very narrow diet, where they all fed on one prey type. For example, Pink salmon in PS all fed exclusively on crustaceans.
These regional differences may be due to differences in prey availability, but we do not have data on this parameter. The data for Alaska (not shown) only included 2023, so we cannot compare among years, but we found these stomachs to be almost all full of herring. The Alaska samples were all Chinook salmon, which are larger and can feed more efficiently on faster prey such as herring, which could be another deciding factor for which prey species is targeted by which species of salmon.
I also calculated stomach fullness for each individual salmon using the equation R = S/So, where R is the fullness ratio, S is the mass of the full stomach, and So is the mass of the empty stomach. The stomach fullness ratios help contrast the masses of satiated and empty stomachs, offering an insight into a fish food comparison. Chinook (=King) salmon had the fullest stomachs across all years, followed by Coho (=Silver) and Pink (Figure 5). This could be due to prey size and/or availability. Comparing regions for 2023 data (data not shown), Puget Sound and San Juan Archipelago had nearly the same stomach fullness, which means that all species of salmon caught from these two regions had about the same amount of food in them. Alaska had the highest amount of stomach fullness because every Chinook salmon that was caught had at least one to three herrings in their stomach. I also compared the stomach fullness of the different species of salmon present in the San Juan Archipelago (Chinook, Coho, Pink). Here, Pink salmon had the fullest stomachs, followed by Chinook, then Coho. This could be because Pink salmon feed mainly on crustaceans, which are one of the more prevalent critters everywhere in the ocean.
Overall, this project on dietary and spatial analysis of Pacific salmon showed that salmon in the San Juan Archipelago region feed more on sand lance compared to salmon in Puget Sound or Alaska. They also have a diverse diet compared to Puget Sound salmon which have a narrow prey selection, such as the Pink salmon feeding exclusively on crustaceans. If I were to do this project again, it would include more samples of Alaska salmon and also examine more salmon species from there, such as Coho and Pink. I would also like to examine why the diet breadth differs among regions.
Step 2. Plastics: Marilyn
PEF is a challenging and one-of-a-kind course that pushes you to become a confident researcher. For me this meant seeking out a project that I am passionate about: plastic pollution.
I was able to work with PEF mentor Matthew Baker, who fully supported my interest in researching microplastics in Pacific salmon. Microplastics are persistent anthropogenic pollutants that are prevalent in ecosystems all over the world. The main concern with microplastics is their potential to bioaccumulate up the trophic web where they can cause false satiation (stomach fullness), resulting in decreased consumption of food. Microplastics also contain harmful chemicals and additives that can leach into ecosystems and cause harm to wildlife.
I wanted to study how proximity to large human population densities impacts the consumption of microplastics in salmon and forage fish. Since Puget Sound has a high human population density and relatively low level of oceanic flushing, I hypothesized that salmon stomachs and their contents from this area would have significantly more microplastics than those from the San Juan Archipelago or Sitka, Alaska. I also hypothesized that I would see a difference in concentrations of microplastics between the three species of salmon due to differences in diet or life history that could expose them to various levels of contaminants.
Working alongside Sarah while she conducted stomach content analysis created an efficient lab setup. My methods involved digesting the stomach tissue in 10% potassium hydroxide, then spending hours looking under a microscope (Figure 6) and counting plastic particles.
The results confirmed my hypotheses. The salmon collected from Puget Sound had the most microplastics, with total counts decreasing with greater distance from urban environments (Figure 7). San Juan Archipelago salmon stomachs had significantly fewer microplastics than those from Puget Sound, and only a few individuals from Alaska had any microplastics. Although Alaska has a lower human population density, microplastics can travel easily by wind and currents, making it important to quantify them across a spatial gradient.
I took the data that Sarah collected on stomach fullness and compared it to the microplastic abundance and concentration in each sample. Puget Sound showed a negative relationship between stomach fullness ratio and total microplastics. This could point to possible retention of microplastics within the stomach tissue. Alaskan samples were mostly full stomachs with no microplastics; however, the four salmon individuals that contained microplastics had empty stomachs, further supporting the possibility of retention in salmon tissue.
I then compared the microplastic concentrations across the three salmon species (Figure 8). This takes into account that Chinook, Coho and Pink salmon vary in size and have different diets and life histories. Coho and Pink salmon stomachs had far more microplastics than I found in Chinook stomachs. This suggests that Chinook salmon may consume or retain fewer microplastics, and is an area for future research.
With the support of our PEF mentors, FHL staff, and some additional financial aid, Sarah and I were able to stay for a month after the PEF program ended to continue our research together. I processed more salmon stomachs from Puget Sound and added samples from the Arctic to strengthen the spatial comparison. I found no microplastics in the Arctic samples, however this is not to say that microplastic pollution is not impacting these far-reaching areas, as they have been found in the Arctic (Lusher et al. 2015). I hope that future PEF students will conduct research regarding microplastics in marine ecosystems because it is essential for informed decision-making, policy implementation, and the development of effective strategies to mitigate the impacts of microplastics on ocean ecosystems.
As someone that had never been on a boat before the PEF apprenticeship, I can confidently say that this experience changed my life and introduced me to many wonderful researchers. Sarah and I got to spend a whole quarter with amazing classmates and supportive mentors aboard the research vessels Thomas G. Thompson, Rachel Carson and most of all the Kittiwake. Surrounded by exceptional peers and mentors, we delved into research integral to a longstanding 20-year apprenticeship. We hope that our methods for combining stomach diet analysis with microplastic analysis will help future PEF students interested in working as part of the “fish team” with Matthew Baker. Being a research apprentice not only enriched our academic pursuits but also solidified our commitment to contributing meaningfully to the scientific community.
Reference:
Lusher A.L. et al. 2015. Microplastics in Arctic Polar Waters: The First Reported Values of Particles in Surface and Sub-Surface Samples. Scientific Reports: 5(1), 14947–14947. https://doi.org/10.1038/srep14947.
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