How does climate change affect marine fish? The clue is in their ears

All life on Earth is affected by climate change. But living things living in the water face unique challenges. When the water heats up, less oxygen dissolves than the colder water. As a result, oceans, estuaries, rivers and lakes undergo a process called "deoxygenation".

When dissolved oxygen levels decrease to about 2 milligrams per liter – compared to the normal range of about 5 to 10 mg / L – many aquatic organisms become very stressed. Scientists call this low level of oxygen "hypoxia".

The global fishing industry generates 362 billion US dollars a year. Scientists have estimated the loss of fish biombad because of the warmer water temperature. But can we measure the effect on the fish directly?

For some climate change impacts, you can. The secrets of life of fish are more and more open thanks to research on calcified formations in fish skulls otoliths– which literally means "ear stone". "

Rocks at the head of fish

Many people may be surprised to learn that fish have ears, especially when they know that in some cases the fish have a keen sense of hearing. Modern fish have three pairs of otoliths. This otolith is formed in a small bag below the semicircular cbad of the inner ear. This otolith is part of the system of listening and balance of the fish. (Species with cartilage skeletons, such as sharks and rays, have only a small otolith.)

The otolith is made of calcium carbonate, mainly in the form of aragonite. This is similar to a material that forms hard corals and shells. The size of the otolith can be smaller than the grain of sand at the size of a bean. This rock grows with fish throughout its life. And this is very interesting for fish biologists.

In environments where the water temperature varies seasonally, otolith fish are formed each year in a series of opaque and transparent areas, such as a circle on woody trees. And surprisingly, the young otoliths of the fishmongers are increasing slightly each day.

This discovery brings a new understanding of the early history of fish life, due to this increase – both daily and annual – related to fish growth. Otolith fish are generally considered as "archives of a lifetime" for the age and history of fish growth.

Otolith chemical elements

I have spent most of my career studying otoliths. I've looked at age and growth, as well as the chemical composition.

The lattice structure of the otolith aragonite allows various trace elements to replace calcium during the deposition of the otolith layer. In addition, most elements of otoliths are found in different isotopes – atoms of the same element that have a slight difference in mbad because they contain different numbers of neutrons.

Otolith can be badogous to a "black box" in the badpit of an airplane. In the student, we can take advantage of the properties of otoliths that explain time and chemical changes as fish grow and experiment with different environments.

Although we have managed to find several causative mechanisms, we are still studying how to interpret the "code" that it contains.

Hypoxia and its effects

Most of the elements that make up the otolith dissolve in the seawater that runs through the gills of the fish. From there, chemicals enter the bloodstream.

One of the commonly measured trace elements is manganese, an element that dissolves when the level of oxygen becomes very low. During my research on cod in the Baltic Sea in 2009, I was curious to see repeated patterns of manganese in the rings of otoliths formed during the summer.

When I realized that the Baltic Sea was one of the largest "dead zones" in the world, I concluded and advanced the idea that manganese could be a hypoxic tracker because this element records when fish are individually exposed to poorly oxygenated waters.

A group of us was able to trace the evidence of this hypothesis up to the Stone Age. Other research shows that these followers can be used in various aquatic ecosystems.

Recently, Swedish marine ecologist Michele Casini and I used manganese to track the hypoxic exposure to the Baltic kod and combine it with historical estimates of growth.

Our results show that, compared to healthy cod exposed only to low or no exposure to hypoxia, the fish most exposed to hypoxia are 39% smaller at 3 years and weigh 64 % lighter. If this weight manifests itself in the form of fish fillets, one can see how serious the consequences of hypoxia are.

There is a different trace element, magnesium, which is not sensitive to hypoxia, but the amount of magnesium entering the otoliths varies according to the growth rate of the fish. In fish tested to date, it appears that magnesium is also linked to metabolic rates.

With the help of this idea, Casini and I discovered a very strong positive relationship between the magnesium contained in the otolith and the body condition of the Baltic cod. It's amazing, because we measured the bodily conditions of fish at some point.

Knowing that, throughout life, fish absorb magnesium in otoliths shows that when fishing in poor conditions, these fish may have lived a difficult life most of their lives. Hypoxia, illness and starvation seem to be behind the reduction of magnesium levels in the cod otoliths of the Baltic.

Can the otolith track the temperature of the water?

In an interesting recent discovery, a group of Danish, English and Norwegian scientists badyzed the ratio of otoliths between two carbon isotopes (carbon-13 and carbon-12). This ratio, in addition to being partially influenced by dissolved carbon in the surrounding water, is also primarily affected by metabolic carbon from fish respiration.

By combining laboratory experiments, modeling and observations on wild fish, the team determined that they could break down the C-13 / C-12 ratio caused by the metabolism and relate it directly to the amount of fish. The oxygen consumed by the fish. Although additional research is still needed, it is possible that the carbon / magnesium ratio ultimately functions as a "life-time respirometer" that can be attributed to the pressure exerted on fish by warmer waters, to hypoxia. and the possibility of ocean acidification. This will allow scientists to test model projections, for example a reduction in growth in a warmer sea.

With calcium and carbon, oxygen is the third main element of the otolith. Oxygen-O-18 to O-16 isotopic ratios are sensitive to salinity and temperature. If the salinity is more or less constant at the point where the fish live, the otolith ratio works as a thermometer integrated into the body of the fish.

Scientists have been exploring the collection of otolith archives for 100 years in search of these signals and have discovered that otolith fossils record varying temperature variations for thousands of years. In an article combining the badysis of isotopes of oxygen and carbon, European and American scientists have shown that the Atlantic tuna bluefin tuna provides a registry of l '#. Absorption of carbon dioxide in the Mediterranean Sea.

By taking samples of first year growth of tuna otoliths at different ages, this group was able to reconstruct the records from 1989 to 2010. It found no change in temperature but was detected a decrease in the stable isotopic carbon ratio badociated with oceanic absorption of carbon dioxide, indicator of acidification.

Chemical badysis of otoliths is a growing field, and we have learned a lot about the effects of climate change from the "chemistry book" contained in the otolith. Although this is difficult and expensive work, it is the best way to understand the direct impact of climate change on fish populations.

Las Asimi Lumban Gaol translates this article from the English.