Sturgeon are giant, ancient fishes that can grow up to 600 kg and live upwards of 100 years. Although these iconic fish have existed for more than 100 million years, they now face significant threats from humans – including habitat loss and climate change – making them some of the most endangered species on Earth. For example, one population of white sturgeon (Acipenser transmontanus) in British Colombia has not produced offspring that have survived to adulthood since the 1960s. To assist the dwindling population, humans are currently raising white sturgeon in a lab for over a year before releasing them into the wild to increase their chance of survival during this vulnerable life stage. One possible explanation for why the developing fish are struggling in the wild is the increasing occurrence of climate change-induced heatwaves in their river habitat, which simultaneously warms the water and lowers the amount of oxygen in it. Researcher Madison Earhart worked with colleagues at the University of British Columbia, Canada, the University of Glasgow, UK, and Vancouver Island University, Canada to investigate how sensitive juvenile sturgeon are to the stressful changes in river conditions caused by heatwaves.
To start, the scientists measured the hottest water temperature and the lowest amount of oxygen that the fish could tolerate before warming the water in their tanks from 13 to 20°C to simulate a heatwave for 20 days. Because every species of fish can react differently to heatwaves, the team was not sure how the sturgeon would respond. However, they figured two things could happen: either the heatwave conditions would lower the fish's ability to deal with extreme heat and low oxygen, or the heatwave could increase their ability to handle the harsher conditions. So, Earhart and colleagues repeated their tests after the heatwave, and to the team's delight, the young fish dealt with the heatwave remarkably well. In fact, the fish were more resilient to both the extreme heat and the low oxygen in the water after they had experienced the heatwave, suggesting they were better prepared to respond to future stress.
Delving deeper, the team wanted to uncover the secret behind the fish's high resilience after the heatwave. To do so, the researchers measured the activity of genes associated with resilience to high temperature and oxygen deprivation in the young fish. What Earhart and colleagues found was fascinating. In response to the heatwave, the sturgeon lowered the amount of methylation on their DNA, which enabled rapid increases in gene expression, meaning that the fish could quickly turn on their heat and oxygen stress tolerance genes when needed. By examining the sturgeon's response to the heatwave at the molecular level, the researchers revealed a mechanism facilitating the survival of these fish in harsh environments.
It appears that juvenile white sturgeon have a heatwave superpower, which could explain how these ancient and long-lived fish have endured the test of time. Their surprising toughness when faced with hot waters and low oxygen will be especially important as they continue to navigate the challenges of a changing climate. Despite other threats to their survival in the wild, this discovery offers a glimmer of hope for the future of white sturgeon and their conservation.
