We all know stress is bad for health. Luckily, humans can normally let us know if they need our help with managing their stress. However, the feelings of fish can be a bigger mystery, which presents problems for any farm trying to raise healthy stocks for seafood markets.
“Farmed fish frequently face stress from handling, crowding and infections, making them more disease-prone and slow-growing,” explained Caroline Wee, a Principal Scientist at the A*STAR Institute of Molecular and Cell Biology (A*STAR IMCB). “As such, it’s important to understand and monitor fish stress, as early detection could help farmers adjust their aquaculture practices for optimal productivity and quality.”
In fish as with humans, stress triggers a release of the hormone cortisol into the bloodstream. While cortisol is a well-known biomarker of stress, measuring cortisol in a fish typically involves sampling their blood or tissue. Unsurprisingly, this can agitate the fish further, causing a cortisol spike that muddles the picture of its overall stress levels.
Previous studies have shown that fish also release cortisol into their fins, mucous and surrounding waters when stressed, which offers potentially less invasive ways to monitor them. However, studies in this area have mostly been done in European commercial fish species.
To shed more light on cortisol dynamics in fish relevant to Asian aquaculture settings, Wee and Laura Sutarlie, a Principal Scientist at the A*STAR Institute of Materials Research and Engineering (A*STAR IMRE), launched an investigation with colleagues from their respective institutes as well as Republic Polytechnic, James Cook University Singapore, and the National University of Singapore. These included A*STAR IMRE Senior Principal Scientist Xiaodi Su and Republic Polytechnic’s Shubha Vij and Marie Tan. The project was funded by an A*STAR Agritech and Aquaculture Horizontal Technology Programme Office seed grant.
Schematic of the experimental recirculating aquaculture system (RAS) with water flows and sample collection points indicated.
The team focused on a popularly-farmed fish species in Singapore: the Asian sea bass Lates calcarifer, also known as barramundi. In a farm-scale experimental setup mimicking a typical recirculating aquaculture system (RAS), the team examined how intense, short-term stress events that often occur on farms—such as being chased by nets or briefly exposed to air—affected cortisol in various L. calcarifer tissue and tank water samples over 48 hours. They then compared those dynamics with those of blood cortisol to see if cortisol levels changed similarly over time.
The team observed that plasma cortisol in the distressed fish predictably peaked within an hour, while mucous cortisol rose more slowly, remaining elevated for 24 to 48 hours. Notably, both plasma and tank water cortisol levels rapidly peaked within 40 minutes of stress events.
“Our results indicate that measuring water cortisol may be a viable approach for the non-invasive monitoring of fish stress in local farmed species,” said Wee. “We also gathered preliminary data on changes caused by poor water quality and high fish stocking density.”
Wee added that while the team focused on stress monitoring, follow-up studies could aim to identify effective stress reduction approaches.
“We’ve also concurrently developed and validated a nanomaterial-based cortisol sensor for fish tank water, which we aim to enhance in future studies,” concluded Sutarlie.
The A*STAR-affiliated researchers contributing to this research are from the A*STAR Institute of Materials Research and Engineering (A*STAR IMRE) and the A*STAR Institute of Molecular and Cell Biology (A*STAR IMCB).
