Like high school sweethearts, rising atmospheric carbon dioxide levels and the acidity of Earth's surface waters go hand-in-hand. And just as the chemistry in puppy love is beautifully simple, so too is aquatic acidification – one carbon dioxide molecule combines with one water molecule and a proton is freed. Sadly, this is where our love story ends because the more that protons are freed, the more acidic the water becomes, and the more it affects creatures that live and breathe the life aquatic. Indeed, aquatic acidification is linked to a variety of sub-lethal effects in marine fishes that raise concern for the long-term impact of climate change on oceanic ecosystems. But little is known about effects on freshwater fish and of the consequences of growing up in acidified water. So, Colin Brauner at the University of British Columbia, Canada, and a team of Canadian collaborators decided to test how current and projected carbon dioxide levels affect developing pink salmon. Among all the species of Pacific salmon, pinks are the youngest when they migrate to the open ocean from their freshwater nurseries, so any developmental effects caused by acidification of their natal environment could have a profound impact on their survival during the critical life history stage of seawater transition.
Brauner's team, headed by Michelle Ou, began their acid test by dividing salmon embryos into three freshwater groups that varied in carbon dioxide content from present day levels to predicted end-of-century levels. They then reared the fish in these conditions for 10 weeks – to an age when natural populations begin swimming out to sea – and compared several growth parameters to see how the fish measured up to each other. In all categories, fry from the highest carbon dioxide exposure group fell short, meaning that growth was impaired in the salmon that developed in acidified freshwater. This stunted growth could have dire consequences for the young fish because runts are easy targets for predators and less competitive for food.
The team then compared the olfactory responses of the salmon raised in different levels of carbon dioxide. First, they tested the aversion response of the fish to an alarm cue (made from their ground-up salmon siblings…ew!). They added the alarm cue to one side of a split aquarium, leaving the other side clean, and then measured how much time each fish spent swimming on either side. Compared with fish raised in present-day levels of carbon dioxide, fish raised at elevated carbon dioxide levels spent nearly double the time in the side of the aquarium with the alarm cue, meaning their aversion response was blunted. Next, the team placed microelectrodes in the noses of the fish so that they could quantify how well the fish smelled different chemicals. The team presented these wired-up fish with isolated amino acids – chemicals that are used by adult fish for homing back to their natal streams to spawn – and again found a blunted response in the salmon reared in elevated carbon dioxide water compared with those reared in present-day conditions. Together, these experiments show that developing in freshwater with elevated carbon dioxide levels can impair the ability of salmon fry to respond to important olfactory cues in their environment.
Stunted and blunted, aquatic acidification is of clear concern for freshwater fish.