Low oxygen leaves fish breathless
BY Brittney Borowiec
ON March 20, 2018
Unlike most land-dwelling animals, aquatic animals like fish often have to deal with low levels of oxygen, called hypoxia. Hypoxia is a natural part of underwater life, and happens for a lot of reasons. For example, oxygen levels can be low in ice-covered lakes because the ice layer keeps air (and oxygen) from entering the water, like plastic wrap. Hypoxia can also happen in deep water because it takes a long time for oxygen to reach the bottom of the lake or river.

Hypoxia can also develop from human causes. In the Great Lakes region, nitrogen and phosphorus pollution from fertilizers used on farms is a major cause of hypoxia. The fertilizer runs into the water, and causes a population boom of aquatic plants called algae. Algae don’t live very long, and once they begin to die-off and decay, huge amounts of oxygen are used. This leaves very little oxygen for everything else living in the water. These low oxygen “dead zones” form all over the world, including Lake Erie and Lake Ontario during the summer.

Whether its natural or caused by humans, water with too little oxygen is dangerous for fish. Like humans, fish need oxygen to survive. This makes it hard for fish to get enough energy to survive, swim, and grow.

I’m a PhD Candidate in Dr. Graham Scott’s lab at McMaster University. We study a variety of animals, including ducks, mice, and fish. We want to understand how they do amazing things that no human could ever do. How does a mouse live on top of a mountain, where the air is very thin, cold, and dry? How does a tiny fish survive in a tidal marsh, where everything from temperature to oxygen level to salinity is always changing?

We can measure how well a fish deals with hypoxia by exposing it to a very low level of oxygen, and timing how long it takes for the fish to faint. It’s an easy and straight forward test, doesn’t hurt the fish, and gives us some useful information. Knowing the hypoxia tolerance limits of different fish can help us understand which
We often angle bluegill and pumpkinseed sunfish. Other times, such as when we want smaller fish, we will instead use a beach siene.
 species are the most at-risk to episodes of low oxygen, or help explain why some species avoid certain parts of their habitat.

Studying local species, like fish in Cootes paradise, can tell us a lot about the ecosystem in our own backyard. Thus we have studied local animals, like sunfish. Both bluegill and pumpkinseed sunfish are common in Cootes Paradise, and you’ve probably caught them if you’ve ever been fishing in Ontario. Work by other fish researchers, and our own studies, showed that even though pumpkinseed and bluegill sunfish look almost the same, and live side-by-side, one copes with hypoxia much better than the other. When exposed to about 10% of normal oxygen levels, pumpkinseed sunfish can last over twice as long as bluegill sunfish.
What was it about pumpkinseed that made them so much better in low oxygen than their cousins, the bluegill? We first hypothesized that pumpkinseed could supply more oxygen to their organs. We were wrong though - both sunfishes had the same sized gills (“fish lungs”) and the same amount of oxygen-carrying red blood cells. Pumpkinseed weren’t any better
It can be hard to tell a bluegill (top) from a pumpkinseed (bottom)
than bluegill at supplying their body with oxygen.
Hmm, what could it be then, if not oxygen supply? Our new hypothesis was that pumpkinseed are better at producing energy when oxygen levels were low. We looked at the activity of some special proteins, called enzymes, in the tissues of our sunfish. Enzymes are responsible for almost every chemical reaction that happens in a cell, including reactions that make energy for animals to use.
(Image from: Du et al., 2018. Environ. Sci. Technol. 52, 801−811.)
Fish gills stained with hematoxylin-eosin and imaged under a microscope.

Pumpkinseed had much higher activity of the enzyme lactate dehydrogenase than bluegill.
This enzyme is related to hypoxia tolerance because it helps animals to make energy from anaerobic (oxygen free) metabolism. If you’ve ever run quickly up the stairs or played any sports, you’ve probably used this enzyme too. The burning you feel in your muscles when you exercise is caused by lactic acid, which is produced by lactate dehydrogenase. The higher activity of lactate dehydrogenase in pumpkinseed sunfish means they are better able to produce energy when oxygen levels were low. 

Due to climate change, hypoxia is getting more common and more severe. It’s important to understand how fish cope with hypoxia so we can predict how populations will respond to drastic changes in their environment in the near future. Since hypoxia is a fact of life for so many fish, understanding how they respond to hypoxia can also tell us a bit about natural history and evolution.

References & Resources:

Borowiec et al., 2016. Interspecific and environment-induced variation in hypoxia tolerance in sunfish. Comp. Biochem. Physiol. A. 198: 59−71.

Du et al., 2018. Metabolic costs of exposure to wastewater effluent lead to compensatory adjustments in respiratory physiology in bluegill sunfish. Environ. Sci. Technol. 52, 801−811.

Johnston, L. 2017. What the heck is Lake Erie’s dead zone? Rock the Lake.

West, K., and Evers, J. (eds.) 2011. Dead zone. National Geographic Society.

US EPA. 2017. Hypoxia 101.

Author Bio - Brittney Borowiec
Brittney is a PhD Candidate in Biology (Environmental Physiology) at McMaster University, where she studies how fish cope with low oxygen conditions. She has written for a number of popular science and science media outlets, including Massive, Oceanbites, and Let’s Talk Science CurioCity. Tweet her @this_is_brit
Brittney is a PhD Candidate in Biology (Environmental Physiology) at McMaster University, where she studies how fish cope with low oxygen conditions. She has written for a number of popular science and science media outlets, including Massive, Oceanbites, and Let’s Talk Science CurioCity. Tweet her @this_is_brit

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