Studying toxic metals in fish in southeast Asia’s largest freshwater lake

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In the largest freshwater lake in southeast Asia, Shorna Sabikunnahar is looking into the ecological and environmental drivers of toxic metals in resident fish.

Conducting her research on Tonle Sap Lake in Cambodia, which is part of the Mekong River system, Shorna is a SAFS PhD student in the Holtgrieve Ecosystem Ecology Lab (HEEL). Designated as a Biosphere Reserve by UNESCO, Tonle Sap Lake is one of the most diverse and productive ecosystems on the globe.

Collecting and sampling over 1000 samples of fish, water, sediment and other species such as mollusks and crustaceans, Shorna visited Cambodia in both the dry and rainy reason to build her sample collection from 10 different locations. Tonle Sap Lake faces a huge transformation depending on the season. In the dry season when it drains into the Mekong River, the lake length is over 100 miles. This grows more than five times in area during the rainy season, with water flowing from the Mekong back into the lake. This brings with it different nutrients and biodiversity, but also impacts levels of toxic metals.

Shorna Sabikunnahar
Shorna collected over 1000 samples of sediment, water and fish from Tonle Sap Lake, including from floating villages and fish markets.

Shorna analyzed the presence and levels of toxic metals in 39 different species of fish found in the lake. The Lower Mekong River supports more than 60 million people, with 80% of their protein coming from fish within Tonle Sap Lake. Studying parts of the food web, including fish, is critical to shed insight into a number of different things: the fishery itself, if humans are receiving the nutrients they need from these fish, and if there are health risks posed by the level of toxic metals in fish. Some of the species of fish studied by Shorna include Parambassis wolffii, Pangasianodon hypophthalmus, Boesemania microlepis, Phalacronotus apogon, Ompok bimaculatus, Mystus, mysticetus, Hamibagrus spilopterus and Channa micropeltes.

Collecting water and sediment samples directly from the lake and fish samples from the floating villages and fish markets located on the lake, Shorna has found that it’s not always the bigger fish that have the highest level of toxic metals present, which is contrary to the usual pattern. In Shorna’s analysis, some of the largest fish had levels at the bottom of the range, whereas some of the smaller fish, hardly bigger than a finger, had the highest amount of metal concentrations. This wide and unexpected variance suggests that levels of toxic metals is driven by the feeding habits of fish such as what they are eating and where.

One of the assumptions in Shorna’s study was that climate change and numerous hydropower dams have a significant impact on the variation in toxic metal content in fish. Specifically looking at mercury, during flooding events when floodplains and forests are submerged, the decomposition of organic matter, such as leaves and other vegetation, begins. This process reduces the oxygen levels in the water and increases methylation. Methylation is the conversion of inorganic mercury into easily accessible methylmercury, which fish can readily accumulate in their bodies.

Shorna Sabikunnahar
The Lower Mekong River supports more than 60 million people, with 80% of their protein coming from fish within Tonle Sap Lake.

However, Shorna found that only the feeding ecology of fish (their feeding patterns, food sources, and trophic positions) influence mercury accumulation in fish. Environmental factors such as seasonal change and variation in water level throughout the year or flooding is not significant. This result is interesting because some previous studies in the Arctic found that environmental factors such as temperature influences mercury accumulation in fish. Another study in the Amazon River basin found that despite the absence of human-induced sources, fish contain relatively high levels of mercury due to the impact of annual flooding.

The next step in Shorna’s research is to analyze the fatty acids and other vitamins found in fish – the nutrients that humans eat fish to gain – as well as the toxic metals levels. But how do we know what fish people are eating? Household fish consumption pattern data already exists, and Shorna will combine this data on what people eat with her results on toxic metals and nutrient loads in fish. The aim? To see if these fish pose any carcinogen or non-carcinogenic risks to people as well as whether their nutritional requirements are being fulfilled or not.

The Mekong River is a huge and important area, and by studying one of the most important lakes in this system, Shorna’s work can provide deeper insight into the wider ecological system of the Mekong and the fish that call it home.

Shorna Sabikunnahar
By studying Tonle Sap Lake, Shorna is delivering deeper insight into the wider ecological system of the Mekong and the fish that call it home

Diving deep into how fish impact carbon cycling in the ocean

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Recently featured in Hakai Magazine’s article “All The Fish We Cannot See”, SAFS PhD student Helena McMonagle is conducting a deep dive into a hidden cache of fish that might play an unexpected role in how the ocean sequesters carbon. We caught up with Helena to find out more about it.

Since starting this research in 2019, I’ve been using data collected at sea and in the lab, along with bioenergetic models, to estimate how much carbon these mesopelagic fish, such as lanternfish, transport from surface waters into the twilight zone. Also known as the mesopelagic zone, the twilight zone spans from roughly 200-1000 meters (roughly a tenth to a half mile deep) and comprises the part of the ocean where there is still some sunlight, but not enough to support photosynthesis.

Paul Caiger, formerly at WHOI and now at University of Auckland
Several species of lanternfish, also called myctophids. There are nearly 250 species of lanternfish, some of which have been considered for commercial harvest to make fishmeal, nutritional supplements, and other products.

Despite this lack of primary production (the biomass generated by photosynthetic organisms like the microscopic, plant-like organisms known as phytoplankton), there is still a large biomass of fish and other marine life in this part of the ocean. Many of these twilight zone animals perform diel vertical migration, in which billions of fish and zooplankton all over the globe swim from the mesopelagic zone to the sea surface to feed where there is more food available. After filling their stomachs at night, they swim back down to the mesopelagic zone to hide from visual predators during the day.

At these deep depths, where carbon can be stored for longer time periods than if it stays near the sea surface, they release much of the carbon they consumed the night before by breathing out carbon dioxide, pooping, and in some cases getting eaten by predators that stay in the deep ocean, thus moving that carbon deeper.

WHOI Creative
Ocean zones, including the twilight zone (also known as the mesopelagic zone), which spans from roughly 200-1000 meters below the sea surface.

One of the most abundant types of mesopelagic fishes that performs this daily migration is the lanternfish (not to be confused with the rarer anglerfish), a family of fish that includes almost 250 species. With my colleagues Tim Essington (SAFS), Joel Llopiz (Woods Hole Oceanographic Institution) and Ray Hilborn (SAFS), we found that estimations of how much carbon a single fish, like a lanternfish, transports each day into the twilight zone is associated with about a 6 fold uncertainty (McMonagle et al. 2023), which can be constrained a little further with great data coverage from the specific location where you’re estimating the carbon transport of that fish. Even then, scaling up to how much carbon entire populations of these fishes transport requires fish biomass estimation at a given study site, and biomass estimation is associated with even more uncertainty than all the bioenergetic uncertainties for an individual fish combined, based on preliminary results from a study that our team is currently working on.

Marley Parker, ML Parker Media
Helena McMonagle pictured on a 2021 research cruise getting ready to deploy the net system used for collected mesopelagic fish.

As a result, even in a well-studied area, there can be well over an order of magnitude of uncertainty in the contribution of fish to biologically-driven carbon transport. One of these rare, well-studied areas is the Porcupine Abyssal Plain Sustained Observatory, which is located in the Northeast Atlantic, which has both long-term data collection as well as focused cruise efforts like one that I took part in during 2021.

Marley Parker, ML Parker Media
L-R: Julia Cox (WHOI), Kayla Gardner (WHOI), Helena McMonagle ,and the lead scientist for the mesopelagic fish work, Joel Llopiz (WHOI).

This huge uncertainty from both biomass and bioenergetic uncertainty is based on preliminary results from my current project using data from the 2021 cruise. This data was gathered on board a Spanish research vessel operated by CSIC (Spanish National Research Council) in Spain, which was led by NASA EXPORTS (EXport Processes in the Ocean from RemoTe Sensing) and the WHOI Ocean Twilight Zone (OTZ) project. My fellowship funding was provided by the National Science Foundation’s (NSF) Graduate Research Fellowship Program.

Marley Parker, ML Parker Media
The three research vessels that sampled fish, zooplankton, and sinking particles (e.g., dead phytoplankton) in the North Atlantic in May, 2021. This expansive sampling effort was done to better understand how carbon moves from the sea surface into the twilight zone.

Apart from biological carbon transport, physical and chemical processes also play a major role in moving dissolved carbon dioxide from the sea surface into the deep ocean, and together some portion of that carbon that is transported is also sequestered. Translating carbon transport into carbon sequestration is another can of worms in itself, because carbon transported into the twilight zone is not necessarily sequestered from the atmosphere on climate-relevant time scales. Therefore, the role that fish play in not only moving carbon deep, but locking it up from the atmosphere on climate-relevant time scales, is a topic that is largely still unstudied and unknown.

Acknowledging the difference between carbon transport within the ocean and carbon drawdown and sequestration from the atmosphere is part of my current research on fish carbon transport. My goal is to rigorously study the role of fish in carbon transport while avoiding miscommunication or misinterpretation of the carbon transport research into the realm of climate policy and carbon offsets, as this potential application will require much more research that we have currently available to verify with any certainty. 

There is a large suite of climate mitigation strategies that we already know with high certainty will reduce atmospheric carbon dioxide concentrations and mitigate climate change. Although no one solution is a silver bullet that will curb human-driven climate change on its own, it’s important to urgently prioritize well-established climate mitigation strategies in climate policy, while exploring other mechanisms of carbon transport and carbon sequestration in the meantime in the research sector.

Helena McMonagle
Back in the lab, the bioenergetic model is written up in code using a freely available coding language and software called “R” and “R Studio”. A snippet of this code for the bioenergetic model, which is used to estimate fish carbon transport, is shown above. This code comes with online instructions, is publicly available on McMonagle’s Github account, and is free to use. This is done so that the science is open and reproducible: other researchers can see how the analysis was done, regenerate the results, or edit and build off it for their own research.

Coexistence and origin of trophic ecotypes of pygmy whitefish, Prosopium coulterii, in a south-western Alaskan lake

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Coexistence and origin of trophic ecotypes of pygmy whitefish, Prosopium coulterii, in a south-western Alaskan lake

Journal of Evolutionary Biology, doi: 10.1111/jeb.12011

Authors: C. P. Gowell*†, T. P. Quinn† & E. B. Taylor‡
*Department of Biology, University of Puget Sound, Tacoma, WA, USA
†School of Aquatic and Fishery Sciences, University of Washington, Seattle, WA, USA
‡Department of Zoology, Biodiversity Research Centre and Beaty Biodiversity Museum, University of British Columbia, Vancouver, BC, Canada

Abstract
Ecologically, morphologically and genetically distinct populations within single taxa often coexist in postglacial lakes and have provided important model systems with which to investigate ecological and evolutionary processes such as niche partitioning and ecological speciation. Within the Salmonidae, these species complexes have been well studied, particularly within the Coregonus clupeaformis–C. laveratus (lake and European whitefish, respectively) group, but the phenomenon has been less well documented in the other whitefish genera, Prosopium and Stenodus. Here, we examined the morphology, feeding biology and genetic structure of three putative forms of the pygmy whitefish, Prosopium coulterii (Eigenmann & Eigenmann, 1892), first reported from Chignik Lake, south-western Alaska, over 40 years ago. Field collections and morphological analyses resolved a shallow water (< 5 m depth) low gill raker count form (< 15 first arch gill rakers), a deepwater (> 30 m), low gill raker form and a deepwater, high gill raker count (> 15 gill rakers) form. The two low gill raker count forms fed almost exclusively on benthic invertebrates (mostly chironomids), while the deepwater, high gill raker count form fed almost exclusively on zooplankton; differences in diet were also reflected in differences both in d13C and d15N stable isotopes. All three forms were characterized by the same major mitochondrial DNA clade that has been associated with persistence in, and postglacial dispersal from, a Beringian glacial refugium. Analysis of variation at nine microsatellite DNA loci indicated low, but significant differentiation among forms, especially between the two low gill raker count forms and the high gill raker count form. The extent of differentiation along phenotypic (considerable) and genetic (subtle) axes among the Chignik Lake forms is similar to that found among distinct taxa of Prosopium found in pre-glacial Bear Lake (Utah–Idaho, USA) which is probably at least ten times older than Chignik Lake. Our analyses illustrate the potential for the postglacial differentiation in traits subject to divergent natural selection across variable environments.

pp 2432 – 2448