Rita Steyn

When is a fish, a thing that lives in the ocean and swims, like a canary, a thing that lives in the air and flies? Well, when both the canary and the fish are telling us something important about the environment.

You may be familiar with the saying ‘a canary in the coal mine’ – meaning the macabre tradition in which miners would carry small caged birds into mines with them, and if there was a danger of exposure to gases such as carbon monoxide, the poor little bird would die before the levels of the gases became deathly for humans. In this way, the bird acted as an indicator of potential danger in the miners environment (the mine), like an early-warning mechanism.

The state and health of our oceans is influenced by both environmental factors (water, air) and biological species (fishes, algae). Some of these species act just like the canary, as indicator species, and we use the observed health of these animals as a way of measuring general ecological condition, because healthy habitats house healthy organisms (say that fast three times). So does that mean we are sitting around waiting for the fish to die before we think there is a danger? No! Why would you think that? How awful. We go out and catch them instead.

In recently published (2018) research, Dr Isabel Romero, a chemical oceanographer from the University of South Florida St. Petersburg, studied several fish species over a ten-year period. This study was conducted in the Gulf of Mexico, and scientists found that by monitoring the levels of certain chemical contaminants found in seven species of fish, they could infer the general health and state of pollution in the water column, thus using these little fish as indicators for the overall health of the Gulf. Researchers collected samples (fish, water) in the years 2007-2016 - before, immediately after, and then much later after the Deep Water Horizon oil spill of 2010. That spill is considered to be the largest marine oil spill in history, with devastating effects to marine life found in the Gulf.

The specific chemicals monitored are known as Polycyclic Aromatic Hydrocarbons, or PAHs, and the kicker is that these chemicals can occur both naturally (in oil or gas), or they can be manufactured. You may be familiar with the PAHs naphthalene (mothballs) or anthracene (coal tar), neither of which is good for human OR fish health. Trust me (but I’m not a doctor!). In humans, high levels of PAHs can lead to blood and liver abnormalities, AND cause cancer.

In fact, the results of this research show that fishes (and other marine life!) were being exposed to background levels long before the oil spill, then incredibly high levels during and immediately after, and then back to lower levels in the years after the spill. This exposure can lead to early death and abnormal development in young fish, and interrupt muscle movement (swimming is important) and cause wounds (lesions on gills and skin - yuck) in adult fish. PAHs can even cause DNA damage (think of the next generation y’all).

And as if all of that isn’t bad enough, the research also indicated that even though PAH levels in the water column decreased, the long-term generational effects in the fish persisted. Exposure to these biologically available - meaning the fish can easily ingest/acquire these - toxins happens at ALL life stages.

The danger (and lasting effect) of PAHs doesn’t stop – these fish accumulate toxins over their lifetime, and then pass those on to whatever eats them. Mid-water column, open-ocean (mesopelagic) fish are both the eaten, AND the eaters. These fish are important prey items and an integral part of the marine food web. They regulate the populations of smaller organisms, acting as a link between the tiny things they eat, like zooplankton, and the much bigger things that eat them. The much bigger things are open water fish and marine mammals, like tuna and dolphins, and these larger predatory fishes are usually the species targeted for consumption by humans.

So for ten years, researchers followed seven species of little fish, and the levels of PAHs in fish organs, tissue, and eggs, tell us more about the persistent levels in the environment, and the overall state and health of the ocean. This ecosystem is home to over 1500 species of fish, rare marine mammals (manatees), and many sea birds. The Gulf of Mexico holds immense biological, cultural, economic, and ecological significance, and should be fiercely protected. The canaries in our watery coal mine are suffering, and we should worry about what that means for our future, and the future of the oceans we love so much.

Many thanks to Dr Isabel C. Romero, from the College of Marine Science at USF and the DeepEnd Project, for providing her research paper and answering questions.

IF YOU’VE ENJOYED READING THIS ARTICLE, LOOK OUT FOR MORE ARTICLES FROM RITA AND FOLLOW HER HERE @RITASTEYN // @ritasteyn

Want to write for us? Check out our ‘Get Involved’ section