Oxygen concentrations are dropping in oceans, causing dozens of fish species to migrate to shallow waters.
In a paper released in the journal Global Change Biology, researchers from the University of Carolina and UC Santa Barbara published their study findings from the last 15 years.
Meyer-Gutbrod, a professor at the University of South Carolina and the study’s lead author, says that they found declining oxygen levels at various depths. Researchers took readings and conducted surveys of the ocean at depths of 50 meters to 350 meters. Many fish species have been found to move into shallower areas where oxygen concentrations are higher.
The declining levels of oxygen in the ocean are caused by a number of factors, such as ecological changes, seasonal changes, and even storms. The paper suggests that warmer ocean water holds less dissolved oxygen, which is the most significant reason.
Fish are migrating to shallow waters because oxygen dissolves easily in shallow waters due to atmospheric mixing. Due to rising temperatures, the density differences between cold and warm surface waters have increased. The result was a clear divide between warm and cold water, preventing oxygen from dissolving into deeper waters.
Between 1995 and 2009, the researchers performed surveys of fish at several depths between Santa Cruz and Anacapa islands in Southern California, to examine the effect of decreasing oxygen levels on fish distribution.
During most of these surveys over the years, they observed 60 types of fish and discovered that four species had migrated to the deep, whereas 19 species had moved to shallower depths. Researchers also measured salinity and temperature, which stayed relatively constant over the years, as well as oxygen levels.
Despite the study being limited to a small area, the researchers claim it allowed them to eliminate confounding factors associated with surveys of large areas. As a result of this trend, many of these fish species may be forced out of their ideal habitats and may struggle to adjust to new ones. That could be reflected in stress being put on the fishing industry.