Reference: Steckbauer A, Diaz-Gil C, Alos J, Catalan IA, Duarte CM (2018) Predator avoidance in the European seabass after recovery from short-term hypoxia and different CO2 conditions. Front in Mar Sci 5:350 doi:10.3389/fmars.2018.00350
Stress along the coasts
Close to 40% of the world’s population lives within 100 km of the coastal zone, with approximately three billion people relying on the resources and ecosystem services provided by its associated habitats (Agardy et al. 2005). Consequently, increased human activity has led to several negative impacts in coastal ecosystems. One such impact is called hypoxia (Fig. 1): the depletion of oxygen down to levels that become harmful, and many times deadly, to aquatic organisms. Causes of hypoxic conditions can mainly be attributed to increased runoff from activities like fertilizer application and sewage treatment, that then feed blooms of algae. These large amounts of algae eventually die, and microbes consuming the dead biomass use up all of the oxygen in the water column, leading to hypoxia. Without oxygen, economically important organisms that we like to eat, like various fish and shellfish species, struggle to survive. This is bad for your dinner options and worse for people that make their living fishing.
Another threat to coastal systems is ocean acidification, where excess carbon dioxide (CO2) from the atmosphere is absorbed into the ocean, resulting in acidified conditions and a lower pH. Too much CO2 in the air isn’t just bad for the climate, it also makes the oceans more like vinegar. These environmental conditions are harmful for several organisms, especially those that rely on calcification of hard shells or body parts for survival. Fish, in particular, use calcium carbonate to form their otoliths (also known as “earstones”) that aid with balance and hearing, which are critical for behavior like foraging for food and avoiding predators. Changes in pH can also change blood-level CO2 concentrations, also known as hypercapnia, which can have serious implications for overall performance if left unchanged.
Short but not-so-sweet?
While in many cases, long-term exposure to these harmful conditions result in severe consequences, we know much less about how short-term exposures affect organisms that inhabit impacted coastlines. Often, conditions that trigger events of intense hypoxia or acidification tend to occur in waves, such that organisms more often experience high exposure to these threats for a short time. It’s less of a problem for mobile species like fish, which can escape less-than-ideal waters by swimming away to safer conditions, consequently limiting the extent of their contact with hypoxic and/or acidified environments.
However, just how this short term exposure affects fish behavior, however, is not well understood. Are there lasting effects of hypoxia and acidification, particularly on fish’s ability to avoid predators, and can fish recover from these effects?
Let’s test it out
A group of researchers led by Alexandra Steckbauer set out to answer some of these questions by studying the behavioral response of the commercially important fish species European seabass (Dicentrachus labrax) to short-term hypoxia and varying pH conditions. They put groups of seabass in four different conditions for four days. Some fish were in a control group in normal conditions, some experienced hypoxia only, some experienced acidic water only, and some had combined conditions of both hypoxia and acidification. After this short-term exposure, they returned all individuals to normal conditions and conducted a series of behavioral tests that allowed the researchers to examine how these conditions affected their predator avoidance behavior.
In this experiment, the scientists record video of each individual in a tank before, during, and after being exposed to a dead Black scorpionfish (Scorpaena porcus), one of their known predators, while being stimulated by the presence of food. Like any qualitative behavioral response, predator avoidance can be difficult to assess, so they measured the amount of time spent away from their refuge: the more time spent wandering away from the safety provided by their refuge, the greater their risk-taking, and hence the greatest effect on their predator avoidance behavior.
Researchers found that for both the control and the hypoxia only conditions, fish spent more time near their refuge after being exposed to a predator. This is shown in the first two columns of Fig. 2, where fish before exposure to the predator (Fig. 2A) spent time feeding away from their refuge, which is indicated by the asterisk; however, after exposure to the predator (Fig. 2B), fish spent more time near their refuge. This result suggests that control and hypoxic only conditions did not affect risk avoidance behavior, because these fish still demonstrated the desire to seek refuge after exposure to a predator. In contrast, both the acidification only and combined treatments resulted in less time spent near the refuge after exposure to a predator, with the greatest effect occurring under the combination of the two treatments, suggesting that short-term exposure to hypoxic and acidic conditions resulted in less risk avoidance behavior.
What does this mean?
This altered response to predators could have serious implications for fish survival and interactions among species. The way predators and prey interact determine the structure of the rest of that ecosystem’s food web. Thus, if seabass are less likely to seek refuge when a predator is around after being exposed to acidic, or combined hypoxic and acidic conditions, they might also be less likely to survive.
It is still unclear why hypoxia alone had less of an effect on behavioral response when compared to acidic conditions alone. The authors hypothesized that it had a lot to do with the fishes’ inability to compensate for changes in such drastic changes in pH. There is more work needed to better understand the recovery from short-term exposure to stressors such as those studied here, but this is a good first step toward unraveling the complicated behavioral response to common coastal impacts.
As we continue to expand into coastlines, we need to think a bit harder about the consequences for organisms that we care about – in particular, those that provide us with food, such as the European seabass.
Agardy T, Alder J, Dayton P, Curran S, Kitchingman A, Wilson M, Catenazzi A, Restrepo J, Birkeland C, Blaber SJM, Saifullah S, Branch GM, Boersma D, Nixon S, Dugan P, Davidson N, Vorosmarty C (2003) In: Ecosystems and human well being: A framework for assessment. Island Press, Washington
Steckbauer A, Diaz-Gil C, Alos J, Catalan IA, Duarte CM (2018) Predator avoidance in the European seabass after recovery from short-term hypoxia and different CO2 conditions. Front in Mar Sci 5:350 doi:10.3389/fmars.2018.00350