May 15, 2019 — 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.”
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.