Climate change affects all life on Earth, but it poses unique challenges for aquatic species. For example, as water warms it holds less dissolved oxygen than cooler water. As a result, the world’s oceans, coastal seas, estuaries, rivers and lakes are undergoing a process known as “deoxygenation.”
When dissolved oxygen levels fall to about 2 milligrams per liter – compared to a normal range of roughly 5 to 10 mg/L – many aquatic organisms become severely stressed. Scientists call this low oxygen threshold “hypoxia.”
Globally fisheries generate US$362 billion annually. Scientists are already forecasting loss of fish biomass due to warming water. But can we measure effects on fish directly?
For some climate change impacts, the answer is yes. Increasingly, a window on the secret lives of fishes is opening up through study of tiny, calcified formations inside fish skulls called otoliths – literally, “ear-stones.”
Karin Limburg, CC BY-ND
Rocks in fish heads
Many people may be surprised to learn that fish have ears, and in many cases an acute sense of hearing. Modern fishes have three pairs of otoliths that form inside small sacs underneath the semi-circular canals of their inner ears and function as part of the fish’s hearing and balance system. (Species with skeletons made of cartilage, such as sharks and rays, lack otoliths.)
Otoliths are made of calcium carbonate, mostly in a form called aragonite, which is similar to the material that makes up hard corals and clam shells. Otoliths can be smaller than sand grains or as large as a fava bean. They grow as the fish grows throughout life, which makes them interesting for fish biologists. In environments where water temperature changes seasonally, sequences of opaque and translucent zones form in fishes’ otoliths over the course of a year, like tree rings. And amazingly, young fish deposit tiny increments on a daily basis.
This discovery revolutionized understanding of the early life histories of fish, because these increments – both daily and annual – are related to fish growth. Fish otoliths are widely regarded as “lifetime archives” of age and growth histories.
I have spent much of my career studying otoliths, researching not only age and growth but also their chemical composition. Otoliths’ aragonite crystal lattice structure permits various trace elements to substitute for calcium as the otolith layers are deposited. In addition, most elements in the otolith exist as different isotopes – atoms of the same element that have slight mass differences because they contain varying numbers of neutrons.
Otoliths in a sense are analogous to cockpit black-box recorders. By studying them, we can take advantage of time-keeping properties and changes in chemistry as fish grow and experience different environments. Although we have been able to work out some of the causal mechanisms, we are still learning how to interpret their “codes.”
Hypoxia exposure and effects
Most of the elements incorporated into otoliths are dissolved in seawater, which flows over fishes’ gills. From there the chemicals enter the bloodstream.
One of the commonly measured trace elements is manganese, which dissolves when oxygen levels become very low. When I studied Baltic Sea cod otoliths in 2009, I wondered why I saw recurrent patterns of elevated manganese in rings deposited during summertime. Realizing one day that the Baltic Sea is one of the world’s largest “dead zones,” I put two and two together and proposed that manganese could be a hypoxia tracer, recording an individual fish’s exposure to low-oxygen waters.