The ongoing absorption of atmospheric carbon dioxide (\(\text{CO}_2\)) by the world’s oceans causes a chemical change known as ocean acidification (OA). This process results in a measurable decrease in the ocean’s pH, shifting conditions toward more acidic levels. The marine environment that fish have evolved in is slowly changing, introducing physiological stresses that affect their survival. Understanding the consequences of this chemical imbalance on marine fish species is necessary to predict the future health of ocean ecosystems.
The Mechanics of Ocean Acidification
Ocean acidification begins when the ocean absorbs excess \(\text{CO}_2\) from the atmosphere due to human activities. When \(\text{CO}_2\) dissolves into seawater, it reacts with water (\(\text{H}_2\text{O}\)) to form carbonic acid (\(\text{H}_2\text{CO}_3\)). This weak acid quickly dissociates, releasing hydrogen ions (\(\text{H}^+\)) into the water, which causes the seawater’s pH to drop.
A decrease of \(0.1\) pH unit, already observed globally since the industrial revolution, represents a \(30\%\) increase in hydrogen ion concentration. This chemical shift reduces the availability of carbonate ions needed by calcifying organisms like corals and shellfish. It also directly alters the chemical environment, creating an imbalance that fish must actively counteract. This necessity to compensate for the external environment introduces metabolic demands on all fish species.
Disrupting Internal Body Chemistry
Fish must expend energy to maintain the acid-base balance within their internal fluids, a process known as \(\text{pH}\) homeostasis. Elevated \(\text{CO}_2\) concentration in the water increases the \(\text{CO}_2\) partial pressure within the fish’s blood and tissues, a condition called hypercapnia. To prevent their blood \(\text{pH}\) from dropping, fish actively regulate ion exchange, primarily by increasing the concentration of bicarbonate (\(\text{HCO}_3^-\)) in their extracellular fluids.
This physiological compensation is achieved by regulating the excretion of hydrogen ions and the uptake of bicarbonate ions across the gill membranes. While this mechanism defends the blood \(\text{pH}\) in many adult fish, it requires a significant metabolic cost. The energy diverted to this ion regulation process can no longer be dedicated to other functions, such as growth or reproduction.
The sustained need to maintain this internal balance can result in reduced growth rates, with some studies documenting an \(18\%\) decrease in growth for certain larvae. Furthermore, altered blood chemistry can impair the efficiency of gas exchange at the gills, which handle both respiration and ion regulation. This trade-off means that even fish that appear to tolerate \(\text{OA}\) may have compromised overall fitness.
Altering Fish Behavior and Senses
Ocean acidification significantly interferes with a fish’s nervous system, leading to changes in behavior and sensory perception. Elevated \(\text{CO}_2\) levels disrupt neurotransmitter function in the central nervous system, particularly by altering the activity of the \(\text{GABA-A}\) receptor. This receptor, which normally inhibits nerve signals, is sensitive to changes in chloride (\(\text{Cl}^-\)) and bicarbonate ion concentrations.
This neurological interference impairs basic survival behaviors. Fish often lose their ability to detect and respond to olfactory (smell) cues, which are necessary for finding food, locating habitats, and recognizing predators. For example, fish exposed to high \(\text{CO}_2\) sometimes fail to avoid the smell of a known predator, demonstrating a breakdown in risk assessment.
Other sensory functions, including hearing and vision, are also affected. Certain reef fish exposed to future \(\text{CO}_2\) levels showed slowed retinal function, impairing their ability to react quickly to fast-moving events. The result is an increase in risky behaviors, such as greater boldness and activity, which lowers the likelihood of survival in a complex marine environment.
Vulnerability During Reproduction and Development
The earliest life stages of fish—eggs and larvae—are particularly susceptible to ocean acidification. These developmental phases often lack the fully developed regulatory mechanisms that allow adults to compensate for external \(\text{pH}\) changes. Consequently, even small shifts in water chemistry can severely impact population viability.
Exposure to increased \(\text{CO}_2\) can lead to reduced fertilization success and slower embryonic development. Larval fish may experience increased daily mortality rates; for instance, studies on Atlantic cod showed a doubling of mortality under projected end-of-century \(\text{OA}\) conditions. The cumulative effect of these stressors results in a steep decline in the number of individuals successfully recruiting into the adult population.
For some species, overall survival rates of eggs and larvae decline by as much as \(74\%\) under acidified conditions. Larvae that survive often show reduced protein biosynthesis, leading to smaller size and poorer overall condition. Since smaller, slower-growing individuals are more vulnerable to predation and less successful at feeding, the long-term health of fish stocks is jeopardized.