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What’s up, doc? How scientists are developing health charts for fish populations

Journal article: La Rosa et al. (2020) Carbon:nitrogen ratio as a proxy for tissue nonpolar lipid content and condition in black sea bass Centropristis striata along the Middle Atlantic Bight Marine Biology (2020) 167:77 https://doi.org/10.1007/s00227-020-03688-9

Health, an emerging concern, for us and wildlife

Increasingly, health and wellness are taking center stage in our daily lives. We are often faced with a wide spectrum of ways to assess our health: from measuring our weight, body mass index, cholesterol levels, to blood sugar, just to name a few, doctors often given advice for how to lead healthy lives. And for us, health is critical for our livelihood, our work, and caring for our families.

Health chart that many of us are familiar with. Photo credit: https://www.publicdomainpictures.net/en/view-image.php?image=164004&picture=doctor

Just as many of us keep track of our health with Fitbits or the like, marine scientists constantly seek ways to track the health of fish populations. Fish populations can experience changes in their overall health status due to several factors, including habitat alteration, pollution, and invasive species. Often, researchers attempt to determine the suitability of a habitat for a species of fish, and they might do this by counting the number of different prey organisms or by monitoring water quality. However, scientists often want to measure the health of the fish themselves from different habitats.

How do scientists track the health of fish populations?

Common measures that scientists take from fish include length and weight, which can provide information on food availability, age, and reproductive status of the fish. However, other, more informative measures of fish health exist. Ideally, healthy fish have high levels of lipids, or fats, because these are present in highly nutritious food. These fats are stored in different fish tissues, including liver and muscle tissue, where the fats can be transported to different parts of the fish to provide energy needed for the long-term, including migration, overwintering, and reproduction. These features are important for sustaining future healthy generations of the population. In turn, healthy fish populations help keep marine food webs in balance, and provide food and income for local coastal communities that depend on fishing for their livelihood.

In an ideal world, scientists would measure the amounts of all the different lipids present inside of a fish, as this can provide the equivalent of a health chart for the fish. Unfortunately, measuring lipids is expensive and requires long hours in the lab to process samples. This makes it challenging for scientists when they want to measure fish health from a variety of locations, habitats, or different seasons.

The study

To find out if a less time-consuming metric could be a good substitute for lipids, La Rosa and colleagues at the Chesapeake Biological Laboratory, University of Maryland Center for Environmental Science measured carbon to nitrogen ratios (C:N ratios) in the tissues of black sea bass (Centropristis striata).

(Featured image) Black sea bass swimming by a reef. Photo credit: https://commons.wikimedia.org/wiki/File:Black_sea_bass_at_Gray%27s_Reef_National_Marine_Sanctuary_in_Georgia.jpg

C:N ratios measure the amount of carbon relative to nitrogen in a tissue sample. Because lipids tend to contain lots of carbon-rich hydrocarbons but few nitrogen-rich amino acids (which are the building blocks of proteins such as DNA),  a higher C:N ratio in a certain tissue may be a good proxy for lipid content.

In their study, the researchers analyzed C:N ratios in muscle and liver tissues from black sea bass collected from the coast of New Jersey, Maryland, and North Carolina. They chose to study black sea bass because it is a common species that lives in the Atlantic Ocean, where it plays a central role in the food web. This species is also a target of both commercial and recreational fishing; in fact, U.S. wild-caught black sea bass is a good choice for seafood because the populations are sustainably managed (NOAA).


Map of the study area, highlighted in blue, which spans the coast of New Jersey, Maryland, and North Carolina. Photo modified via: NOAA http://www.nefsc.noaa.gov/nefsc/publications/crd/crd0211/images/f21.gif

Back at the lab, the researchers took samples of muscle from the top part of each fish, near the dorsal fin, as well as from the liver. After collecting these, the tissues were dried in an oven, ground to a fine powder, and analyzed for C:N ratios using an Elemental Analyzer, an instrument that can detect and measure how much carbon and nitrogen is in the sample being analyzed. The lipids were extracted from the fish using a series of glass columns containing special chemicals that separate out the different types of lipids.

Black sea bass samples being prepared for dissection of muscle and liver tissues by lead author La Rosa. Photo credit: Ginni A. La Rosa-Ponsini and Dr. Ryan Woodland.

Lead author La Rosa placing dissected black sea bass muscle and liver tissue into a drying oven. Photo credit: Dani Quill.

After running their samples, the authors used statistical models to form an equation that described the relationship between C:N and lipid content in fish muscle and liver tissue. The researchers expected that C:N content would reflect that of the lipid content.

What did they find?

The authors found that C:N ratios in black sea bass muscle and liver tissue increased as lipid content increased. This positive relationship meant that C:N looked promising as a tool for scientists to estimate the health condition of this species.

Interestingly, the researchers noted that this relationship differed between muscle and liver tissue. In muscle tissue, C:N ratios and liver lipid content increased together, while in liver tissue, C:N ratios increased with liver lipid content up to a certain point, and then formed a flat line. Muscle and liver C:N ratios differed among the different regions sampled, which indicates this metric can potentially detect regional variation in the condition of black sea bass. These differences indicate that C:N ratios need to be interpreted with caution based on the tissue being analyzed, the total amount of lipids actually present, and regional effects.

What does this study mean for black sea bass and other fish species? I actually had the chance to speak with La Rosa to discuss this study. She emphasized that C:N ratios hold promise as a cost-effective method that can serve as an additional measure, but certainly not the only metric, of fish health for monitoring and managing populations. La Rosa suggested that C:N ratios should be used alongside other measures such as fish diet, body size, and age structure. Looking to the future, La Rosa said more long-term studies are needed so that scientists can assess fish population baselines from many different places and over long time periods. Then, scientists may then be able to detect major changes in fish health, and hopefully implement management strategies to protect the long-term survival of the species.



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