Fish slow to adapt behaviors to climate change

A large portion of the carbon dioxide emitted into the atmosphere ends up dissolved into the world's oceans—a phenomenon commonly known to cause ocean acidification. As the level of dissolved CO2 in the ocean rises, reef fish begin to experience behavioral changes, leading to a reduced capability to detect and avoid predators. Past research has shown fish are capable of making certain adaptations to CO2 relatively quickly — in a matter of just a few generations. However, when it comes to behavioral adaptations to CO2, the timeline may be much longer.
 
In a recent study, researchers from James Cook University in Queensland tested whether exposing reef fish to elevated CO2 levels would confer an adaptive benefit to their offspring. Groups of spiny damselfish were reared in low, moderate, and high CO2 environments. Moderate and high CO2 conditions were based on estimations from moderate and high emission scenarios looking forward to the year 2100. The offspring from each of the parent groups were then exposed to the three CO2 conditions. If adaptation had occurred, the offspring from parents raised in high CO2 conditions would be expected to perform differently from those fish with parents raised in low CO2 conditions.
 
In testing fish behavior, the researchers observed how individual offspring chose between currents of water within a larger water column. One of the currents contained a chemical known as a "chemical alarm cue" (CAC). CACs function as an early warning system for fish of nearby predators, and are especially important for juvenile fish in the most vulnerable stages of their lives. Normally, a fish avoids water that contains a CAC, but elevated CO2 dramatically changes a fish's response: fish in ambient CO2 level will spend only 10 percent of their time in the CAC-containing current, whereas the fish spent around 80 percent of their time in the CAC-containing current when exposed to high CO2 conditions.
 
According to their findings, published in the journal Nature Climate Change, the researchers found no adaptation in offspring as a result of parental exposure. Regardless of the CO2 concentrations in which the parent fish had been raised, the fish had become attracted to the alarm cues that they were hardwired to avoid. This implies that adaptations to elevated CO2 over shorter time scales—just a handful of generations — are unlikely.  If CO2 levels in the ocean rise more quickly than fish populations are able to adapt to them, the survivability of these populations could be threatened due to weakened predator avoidance behaviors.
 
Although the fish in the study did not show adaptation over two generations, the variation in the responses of individual fish hints at potential for future adaptation. If certain behavioral traits are inheritable and improve the survivability of certain individuals, this could imply an evolution towards adaptive behaviors. Indeed, early field studies may suggest this trend, but additional work is needed to confirm whether or not these traits are being inherited.
 
As atmospheric CO2 levels continue to rise, issues such as ocean acidification will intensify. One result of this CO2 buildup is interference in the signaling cues that some fish rely on for avoiding predators. A weakening in predator avoidance behavior threatens the survivability of these populations. As of now, there is no evidence that fish can adapt to overcome CAC interference in just a few generations. Longer-term genetic adaptations through natural selection may be possible, but researchers do not yet understand the extent to which behavioral traits in fish can be inherited.