It may seem obvious that suffocation is not good, but determining a fish’s tolerance for low oxygen is increasingly important as hypoxia increases worldwide.

Hypoxia (low dissolved oxygen concentrations) occurs naturally—it develops in the hot summers and cold winters beneath ice in small lakes and ponds. Hypoxia can also occur as a result of human activities. When excess nutrients flow into the system, hypoxia may develop in more lakes or become more severe in lakes where it naturally occurred. In extreme cases, part of a body of water can become hypoxic or anoxic (lacking oxygen completely) permanently.

Lethal levels of hypoxia vary among species and among individuals. Much research has focused on defining these lethal concentrations and understanding how species behave differently and experience physiological changes (i.e., changes in internal functions) due to severe oxygen depletion.¹ But since fish and other mobile aquatic organisms can escape lethal hypoxia by moving to less harsh areas, they are more likely to experience hypoxia at moderate levels. Therefore, we examined the responses of yellow perch (a species that regularly encounters hypoxia) to moderate hypoxia.

Typical behavioral responses to hypoxia. Fish avoid hypoxia if possible, and studies show that more fish leave as hypoxia becomes more severe. However, fish often have to balance their need for oxygen and their need for food. Some fish are known to move in and out of a hypoxic area to search for preferred food. Fish that cannot avoid hypoxia or that choose to remain in affected areas may have to cope with negative physiological consequences.

Typical physiological responses to hypoxia. To conserve energy and reduce oxygen use, fish may reduce their activity, feeding, or reproductive development. Fish may also increase their blood hemoglobin concentration—a protein that carries oxygen throughout the body.  Increasing hemoglobin may allow for greater uptake of oxygen from the environment. Humans also do this when they move from an area of low elevation (high oxygen) to an area of higher elevation (low oxygen). However, some fish can make this adjustment in a few days or even hours, whereas humans usually take at least a week to acclimate to low oxygen conditions.

Many of these responses are controlled by the increase or decrease in production of certain proteins. Therefore, measuring the activity level of the genes that control the production of these proteins can provide a good indication of stress, like exposure to hypoxia.

What we expected. We used three different experiments to test how yellow perch responded to hypoxia. In each experiment, we tested at least two hypoxia scenarios. We always had one control treatment, in which the dissolved oxygen concentration was normal (i.e. not hypoxic). The other treatments were either two hypoxic treatments of Low and Moderate oxygen concentrations (Experiments 1 & 3) or one hypoxic treatment of moderately-low oxygen concentration (Experiment 2). We predicted that moderate hypoxia would affect yellow perch only slightly, given their frequent encounter with moderately-low oxygen concentrations in nature. However, we expected that we would find the greatest changes in perch when they were exposed to the lowest dissolved oxygen concentrations.

Experiment 1: Willingness to forage in hypoxic water

 In the first experiment, we examined yellow perch movement behavior when choosing between a hypoxic environment² in which food was available and an environment without food but with normal oxygen concentrations. This simulated a choice that yellow perch often have to make in lakes that become seasonally hypoxic. When certain prey is most abundant (Chironomid midge larvae), the bottom of the lake where the larvae live becomes hypoxic. Because of this, yellow perch have been observed dashing in and out of hypoxia to have their larval midges and breathe too, as if the best buffet³ had their favorite “All you can eat” menu, but they had to hold their breath the whole time.

Experiment 2: Physiological response to acute hypoxia

Our second experiment tested physiological responses to moderately-low hypoxia (~2.0 mg/L) for 30 minutes. The goal was to mimic the oxygen concentrations a yellow perch might experience while dashing in and out of hypoxic water to look for food. We wanted to see what physiological changes occur when yellow perch hold their breath for that super cheesy mac-and-cheese chironomid buffet.⁴ We expected hemoglobin concentration to be higher and the production of certain proteins to change in the treatment when we dropped the oxygen to hypoxic levels and held them there for 30 minutes.

Experiment 3: Physiological response to chronic hypoxia

Finally, we examined yellow perch food consumption, growth, blood hemoglobin concentration, and gene expression after a ten-day exposure to three hypoxic treatments (Low, Moderate, and Control)⁵. Although fish avoid severe hypoxia, they might not avoid moderate hypoxia for longer durations if they can tolerate the low oxygen levels and there are other ideal habitat characteristics.⁶ We expected that the Low treatment would decrease food consumption and growth and lead to changes in hemoglobin concentration and gene expression.

Findings

Yellow perch, the honey badger of moderate oxygen – they just don’t care

In all three experiments, we found little evidence that yellow perch are affected by moderate hypoxia. Yellow perch movement frequency and time spent in the Treatment tank did not differ between treatments in Experiment 1. Hemoglobin concentration was not affected in either of the physiological experiments (Experiment 2 & 3), and growth and consumption were not affected by hypoxia in Experiment 3. In terms of gene expression, there was little indication that this was strongly affected by hypoxia in either Experiment 2 or 3. It is likely that yellow perch use another compensation method that we did not measure (such as increasing their breathing rate) to adjust to moderate hypoxia.

Together, these results indicate that yellow perch are well equipped to deal with moderate hypoxia. Some studies found that yellow perch consumption decreased under moderate hypoxia, but this may be due to differences in temperature between studies. At higher temperatures, yellow perch have a higher metabolic rate and consume more food. Therefore, a change in appetite and food consumption due to lack of oxygen may be more apparent in warmer temperatures. Our study was conducted at a cooler temperature (~15 °C), which is closer to the temperature yellow perch would experience when hypoxia develops at the bottom of a large lake, like Lake Erie, in late summer. Therefore, our results may better represent the typical hypoxic environment a yellow perch would encounter.

Hypoxia can alter food webs by affecting species distributions and changing predator-prey interactions. If yellow perch are not affected by moderate hypoxia, as our study suggests, yellow perch may be able to use hypoxic habitat as refuges from predators and continue to forage on their preferred prey. Therefore, they may even prefer moderately hypoxic habitats in some cases. Yellow perch may also be more susceptible to commercial or recreational fishing if they congregate at higher densities in the “Goldilocks Zone” of moderate hypoxia that lies between fully oxygenated habitat and severely hypoxic habitat. Understanding the responses of yellow perch to moderate hypoxia can help natural resource managers predict the behavior and growth of perch and to monitor yellow perch populations better for adaptive management.

–by Zoe Almeida, guest blogger. Zoe conducted this research as a MS student at Purdue. She is now working on her PhD at Ohio State where she’s studying how food quality for young walleye in Lake Erie may affect individuals and the population in relation to changing climate conditions.
Footnotes:

¹Severe oxygen depletion is often defined at less than 2.0 mg/L of dissolved oxygen

² The oxygen concentrations in Experiment 1 treatments were about 2.5 mg/L for “Low”, 4.5 mg/L for “Moderate”, and 9.0 mg/L for “Control.”

³ A classy one, not a gross one with dirty sneeze-guards

⁴ Author insight: Yes, mac-and-cheese is a food for which I would hold my breath to eat a ton. It is my chironomid buffet.

⁵ The oxygen concentrations in Experiment 3 treatments were about 2.5 mg/L for “Low”, 4.0 mg/L for “Moderate”, and 9.0 mg/L for “Control.”

⁶ Yellow perch’s ideal habitat characteristics include optimum temperatures, good food (that cheesy buffet), and probably a Bulbasaur or a few of the other rare Pokémon.