Climate change is bad, we know that much. But we've also learned that life can cope with change. Many fish can tweak their physiology to deal with warmer, less-oxygenated waters, or to combat the adverse effects of a more acidic ocean – but at what cost? A fish that must work harder to extract oxygen from the water, or to remove acid from its tissues, will have less energy available for the more important things in life, such as chasing prey, finding mates and having sex. Perhaps it's surprising then, that we often study the effects of climate change on well-fed fish in the lab, where energy limitations, such as those that occur in the wild, do not exist.

With this in mind, Louise Cominassi at the University of Hamburg, Germany, and her international team of collaborators, set out to test whether feeding changes how fish are affected by climate change. For a year, they held European seabass in different combinations of the two most dire outcomes of climate change: high temperatures (20°C) and elevated CO2 to simulate ocean acidification. Then, they put half of the fish on a strict diet and let the other half feast to their stomach's content, observing the growth of both groups over several weeks.

Not surprisingly, the fish that feasted grew faster than those on a restricted diet. So far, so good. Also, seabass like warm water, which accelerated the growth of both groups. But, here's the kicker: when these fast-growing, warm-water-loving seabass were also exposed to ocean acidification, growth of the well-fed fish slowed dramatically and those on a restricted diet barely grew at all. This fits well with the idea that living in acidified water is costly and, when on a budget, energy spent on fighting acidity is not available for growth. In fact, increasing the seabass’ energy budget with plenty of food improved their growth in acidic water over that of fish on a restricted diet, supporting the idea that a shortage of energy was limiting their growth.

So why was growth of the well-fed, warm-water seabass affected by ocean acidification? To answer this question, Cominassi had a closer look at how these animals used the food that they were provided with. When the fish in warm water were offered a feast, those in the acidified water chose to eat less, leading to slower growth. What had ruined their appetite? Cominassi found that the fish's digestion was slower in the acidic water and, generally, those with full stomachs are less peckish. The little food that they did eat, however, was also used less efficiently, meaning less growth per bite, perhaps as a result of the slowing of important digestive enzymes. In the end, when dealing with climate change, feeding changed everything.

A major effort is underway to get a grasp on how climate change will affect individual fish species and ultimately impact our ocean ecosystems. However, the waters are muddied by the interacting effects of rising temperature, CO2 and acidity levels, in addition to the falling water oxygen levels due to the increase in temperature. Surely, some animals can cope with these changes, but whether they can afford to do so is a different question. Energy is a limited resource for any organism and future fish will face the challenge of how to spend that budget to cope with climate change and still have enough left in the tank to go on with their lives. Therefore, studying the effects of climate change on well-fed fish in the lab may underestimate the severity of the problem. As with many other problems in life, dealing with climate change is easiest on a full belly.

F. C.
M. A.
Food availability modulates the combined effects of ocean acidification and warming on fish growth
Sci. Rep.
. doi:10.1038/s41598-020-58846-2