Interview: Ocean Acidification and Fish

Swimming in acidifying waters

Scientists suspect that the ocean acidification is primarily endangering calcareous organisms such as corals and mussels. But what about the fish having to swim in an ocean which is becoming increasingly acidic? Researchers are currently addressing this question in the national research program on ocean acidification, BIOACID II. Dr Felix Mark, physiologist at the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research also belongs to the research group. He is specifically investigating how ocean acidification is affecting the Atlantic cod – a relatively new research approach. This is because scientists assumed for a long time that fish are affected to only a small extent by the current ocean acidification. In the interview, the physiologist explains why fish are also reacting sensitively to the reduction of the pH value of the oceans and how he is examining this.

Dr Mark, why have scientists so far investigated how fish react to acidic ocean water to such a small extent?

With the exception of mammals, fish are the most highly developed inhabitants of the oceans. Their bodies have complex mechanisms enabling them to adjust to the fluctuating temperatures and alternating carbon dioxide concentrations in the water. Carbon dioxide dissolves in seawater and in bodily fluids as carbonic acid, leading to acidification, i.e. a reduction of the pH value in the liquid. This acidification must then be balanced to maintain stable bodily functions. Blood plays an important role here. It serves as a buffer so-to-speak between the outer world and the inner world because thanks to the buffer capacity the fish is able to absorb and neutralise a larger quantity of carbon dioxide without the pH value of its blood altering. Scientists have in the past examined this effect in detail and have found that fish are able to balance out a lower pH value within a few hours. They therefore assumed that in addition to the marine mammals, fish are affected the least by ocean acidification.

Dr Felix Mark
Dr Felix Mark (Photo: Alfred-Wegener-Institut)


Atlantic and Polar Cod under stress

Scientists from the Alfred Wegener Institute are heading to the Arctic. They want to find out, how increasing temperatures and ocean acidification alter life in the High North - especially how fish will react to climate change. In the waters off Svalbard the biologists therefore fish for two specific species: the Atlantic cod and the Polar cod.

Why do researchers now believe that fish do react more sensitively to the ocean acidification than previously assumed?

In the majority of research work, I would say in 90 to 95 per cent of the studies, scientists have so far examined adult fish ready to reproduce and concentrated on the general metabolism and the buffer capacity of the blood. They found little indication which could lead to the conclusion that fish react sensitively to ocean acidification. However, there are some experiments in which scientists have penetrated deeper into the metabolism of the animals, so-to-speak. They looked at what was actually happening in the cell when the carbon content of the blood rises. They also observed how fish react to a lower pH value in the different life phases. They were able to show in these experiments that particularly  juvenile fish and fish larvae are sensitive. They have not yet developed all mechanisms to cope with the rising carbon dioxide content. We therefore suspect that more fish could encounter problems in the very early stages of life when faced with greater carbon dioxide concentrations.

Do different fish species react differently to ocean acidification? 

Yes, definitely. It depends how well a species can cope with fluctuating environmental conditions such as changing water temperatures and lower pH values. Let us take the North Sea plaice as an example: plaice is used to the carbon dioxide content changing in the Wadden Sea during the course of the day and the year. It has adapted to these fluctuations by its body developing mechanisms to balance out higher and lower carbon dioxide concentrations. By contrast, Polar fish are used to very stable temperatures and carbon dioxide concentrations. We have discovered time again with these fish that they are unable to adapt to the altering environmental conditions as well. Whilst they have the same mechanism as the plaice, these characteristics have never been further developed for the simple reason that the environmental conditions have never forced this development. It costs a lot of energy to adjust these mechanisms and Polar fish try to save energy where at all possible. They need their energy to stand up to the cold temperatures.

Ocean acidification is frequently referred to the “evil twin” of climate warming. Can the effects of both phenomena have a greater effect on fish?

Most definitely. Fish are poikilothermic organisms. This means that the seawater determines their body temperature. Since metabolic processes such as breathing are also temperature-controlled, the metabolic rates also increase in warmer temperatures and as a visible consequence of this the respiratory rate increases for example. The energy and oxygen requirements also increase, of course, so that the body must now absorb more oxygen. Sooner or later, metabolism reaches its limits and can no longer provide enough energy and oxygen. At this moment, the survival limit has been set for an animal. If a factor such as ocean acidification is now added, the fish must also balance out the lower pH value. This too costs energy. The ocean acidification triggers additional stress in the fish so that the animal reaches its physical limits quicker.



From the egg to a grown-up fish

The scientists have found out that ocean acidification apparently especially affects fish in their juvenile life stages.

The egg of a polar cod
Sensitive life stage: a cod egg 24 hours after fertilization (Photo: Flemming Dahlke)
Polar cod larvae
Also fish larvae react sensitively to a decreasing pH value. (Photo: Flemming Dahlke)
Atlantic cod in a lab aquarium
Once grown-up fish can better handle ocean acidification. (Photo: Flemming Dahlke)
A fish aquarium in a lab in Tromsø, Norway
A fish aquarium in a lab in Tromsø, Norway (Photo: Flemming Dahlke)
cod in a pool in Kristineberg, Sweden
Cod swimming in a lab aquarium (Photo: Felix Christopher Mark)
AWI PhD student Flemming Dahlke working with cod
AWI PhD student Flemming Dahlke working with cod (Photo: Flemming Dahlke)

Can ocean acidification lead to one species prevailing over another?

We are currently examining precisely this in two species of cod: the Atlantic cod and the Polar cod. One of them, the Atlantic cod, is a generalist. It lives both in the North Sea, around Iceland and in the Barents Sea and along the East and North Coast of North America. It tolerates water temperatures between zero degrees and 20 degrees Celsius and presumably does also not react so sensitively to lower pH values in the sea. By contrast, the Polar cod only occurs in the Arctic, at water temperatures from around zero to four degrees Celsius and will therefore presumably encounter difficulties in coping with higher temperatures and carbon dioxide concentrations in the water. As water temperatures rise, the Atlantic cod swims further and further to the North and therefore into the habitat of the Polar cod. We presume that both species hunt the same prey and are therefore in food competition. If the Polar cod is weakened by the rising water temperatures, for example, then it may certainly be the case that the Atlantic cod wins the upper hand and spreads to the detriment of its arctic cousin.

How do you determine whether and how ocean acidification impacts the cod?

We firstly look at the entire fish: does its metabolic rate alter at a lower pH value? Does it swim faster or slower in acidic or warmer water? We conduct a type of fitness test with the fish in our swim tunnel in Bremerhaven. The fish is in a chamber and must swim against water current. By slowly increasing the flow speed, we can measure its metabolic rate and optimum swimming speed. The principle is the same as for humans: if we run, we also need more energy and that will be reflected in an increased metabolic rate. In the same way as people on a treadmill, we can also measure when the maximum speed of the fish is reached in the swim tunnel. For example, we can examine whether the performance limit is associated with different temperatures and carbon dioxide concentrations in the water. These measurements are an example of how we examine a fish at the so-called whole animal level. But we can also analyse what for example is happening at the genetic level , i.e. in the DNA, of the fish. Ultimately, we wish to create an overall picture, from the processes in the cell through to the whole animal level. Only in this way can we understand how a fish reacts when the water becomes warmer or more acidic.

How are these laboratory results then transferred to the real ecosystem?

We cannot of course simulate all the environmental conditions in the laboratory as they occur in the ecosystem but we can test the impact of individual conditions consecutively. For example, we can first investigate how a fish reacts to warmer water and how it reacts to more acidic water. Finally, we can then test how both factors, i.e. warming and acidification, together influence the fish. We can even go further by not only observing one species but as many species as possible. However, there are limits to this because we do not hold the complete ecosystem in the aquarium and cannot work with every individual animal. Therefore, we select certain species which, for example, together form a food chain or a part of a food chain. This is because it could of course be that one fish species can adapt to the ocean acidification but that its main prey is particularly endangered. The more factors we take into consideration, the better indications we can give as to what is happening in the ecosystem.

experiments with cod larvae
Flemming Dahlke preparing an incubation facility (Photo: Felix Christopher Mark)
Incubation facility for cod eggs
Incubation facility for cod eggs (Photo: Felix Christopher Mark)
Dana Graulich studying cod larvae
Dana Graulich studying cod larvae (Photo: Felix Christopher Mark)
Fisheries could be affected by ocean acidification (Photo: Sina Löschke)

To what extent do scientists and representatives from politics and industry cooperate to combat ocean acidification?

There are different possibilities for scientists to exert national and international political influence. At a national level this is the so-called Inquiry Commission of the Bundestag which also has scientists in its ranks. It is here that politicians set catch quotas and environmental protection zones, for example. If scientists participate in political work, the perception of scientific work will rise. This may then mean that ministries facilitate further research projects. The Federal Ministry for Education and Research supports our national research program on ocean acidification, BIOACID II, in the second funding period already. This is certainly an indication that we firstly generate good data and secondly that these data are also used.

The interview was conducted by Kristina Bär.

How could ocean acidification impact fishing?

Ocean warming has led to cod stocks shifting and to them disappearing from the North Sea. In Bremerhaven, for example, cod fishing used to be the main source of income for the city. Nowadays, hardly anyone captures fish and most definitely not cod. By contrast, the stocks around Greenland and Newfoundland are slowly beginning to recover thanks to the strict catching quotas and a slight warming of the waters here. This may in future have a positive effect on cod fishing. Whether the effects for the ecosystem are equally good is currently difficult to say. This is why we are currently researching how the interaction between ocean warming and acidification impacts the stocks of Polar and Atlantic cod. The fishing industry is also interested in these data because it ultimately wishes to estimate where income will develop in the next decades.