You Are What You Eat: The Difference Between Salmon and Seals

D’Agnese, E., McLaughlin, R. J., Lea, M.-A., Soto, E., Smith, W. A., & Bowman, J. P. (2023). Comparative microbial community analysis of fur seals and aquaculture salmon gut microbiomes in Tasmania. Oceans, 4(2), 200–219.

Upon first glance, it seems that fur seals, mammals that can walk on all fours, and salmon, fish that are often found at the supermarket, are vastly different creatures. However, the two may have more in common than one might think. 

Australian fur seal pup. Image source: Michael Sale via Flickr Creative Commons 

Salmon and Seals

In Tasmania, a small island south of Australia, both the Atlantic salmon and the Australian fur seal are increasing in numbers. Atlantic salmon are non-native fish that are farmed as important sources of food for humans. Male fur seals often feed near salmon farms and often catch salmon through fishing nets, which reduces salmon populations, harms the seals themselves, and threatens the success of salmon fishers. However, another result of fur seals eating farmed salmon is the transfer of salmon gut microbes to fur seals, an act that may introduce new pathogens, or microorganisms that cause diseases, to the species.

What is a microbiome?

Every organism has a unique microbiome: a community within them made up of microbes like bacteria and fungi that interact in their gut. While the composition of these microbiomes vary between individuals depending on their lifestyle, geographic location, and species, when one animal eats another, these microbes can be transferred through the food chain.

Farming Atlantic salmon in Tasmania. Image source: Arthur Chapman via Flickr Creative Commons

The Salmon-Seal Connection

In this study, the authors collected scat (animal droppings) and feces samples from both male fur seals found feeding at salmon farms and fur seals on islands further away. The samples were tested for DNA sequences, or unique biological codes, which were matched to a database of known bacterial DNA sequences, to determine which bacteria made up the gut microbiome of each seal. The results were compared to previously collected data on Atlantic salmon microbiomes. 

The results of the study showed that the gut microbiomes of fur seals that fed on farmed salmon and those that did not were composed of different bacteria species. The samples taken from the fur seals that lived near the salmon contained a novel genus of bacteria, which was interestingly not found in the salmon themselves. Salmon and fur seals found near salmon farms did have 13 microbe DNA sequences in common, including one often found in soil that causes infection in both humans and animals. 

Why does it matter?

The apparent transmission of microbes to salmon-eating fur seals has the potential to introduce both humans and seals to new dangerous pathogens. While it is not clear whether all the microbes found in the fur seals were a direct result of eating the salmon, their proximity to the salmon could also introduce them to threats that come with being closer to humans, nutrients in soil that enter the ocean, and boats. Conversely, humans could also be at risk of being introduced to harmful pathogens found in seals, which can be released into the water and soil through seal feces, broken down by bacteria, and enter important food sources like fish and shellfish. 

This study uncovered an interesting connection between two seemingly unrelated animals that could have greater implications for the complex marine food web. Further research on other salmon and fur seal populations, species that live close to human developments, and the harmfulness of new bacterial communities would provide a better picture of the complexity of the problem and could help guide scientists to a potential solution.

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