Researchers tested whether melatonin, a natural substance that helps with sleep and stress, could help farmed seabream fish stay healthy when packed into crowded tanks. Over 90 days, they fed 1,350 fish different amounts of melatonin and measured their growth, health, and stress levels. Fish that received melatonin grew better, had stronger immune systems, and showed less stress than fish without it. The study suggests that adding melatonin to fish food could be a simple way to improve fish welfare and reduce aggressive behavior in crowded farming conditions.

The Quick Take

  • What they studied: Whether adding melatonin (a natural sleep hormone) to fish food helps seabream handle the stress of living in crowded tanks
  • Who participated: 1,350 young seabream fish (weighing about half an ounce each) divided into six groups, kept in tanks with either normal crowding or double crowding for 90 days
  • Key finding: Fish fed melatonin grew faster, had better immune systems, and showed lower stress hormones compared to fish without melatonin. The best results came at 25 mg of melatonin per kilogram of food in normal tanks and 50 mg per kilogram in crowded tanks
  • What it means for you: If you eat farmed seabream, this research suggests that melatonin supplementation may improve the health and welfare of the fish you consume. However, this study was done in fish farms, not in humans, so direct health benefits to people are not yet proven

The Research Details

Scientists divided 1,350 young seabream fish into six groups. Three groups lived in tanks with normal crowding (50 fish per 500-liter tank), and three groups lived in crowded conditions (100 fish per 500-liter tank). Within each crowding level, fish were fed three different diets: one with no melatonin (control), one with 25 mg of melatonin per kilogram of food, and one with 50 mg per kilogram. All fish were fed the same amount of food daily for 90 days, and researchers measured their growth, health markers, and stress levels throughout the study.

The researchers used a flow-through water system, meaning fresh water continuously entered and old water left the tanks, keeping water quality consistent. They measured many different health indicators, including growth rate, feed conversion (how efficiently fish turned food into body weight), stress hormones, immune markers, liver and intestine health, and toxic nitrogen levels in the water.

This type of study is called a controlled experiment because researchers carefully controlled all conditions except for the melatonin levels, allowing them to see the specific effects of melatonin rather than other factors.

This research approach is important because fish farming is a major food source worldwide, but crowded conditions stress fish and can make them sick or aggressive. Finding a simple, natural solution like melatonin could improve both fish health and the quality of farmed fish without using medications. The study measured many different health markers, which gives a complete picture of how melatonin affects the whole fish, not just one aspect

This study has good reliability because it used a large sample size (1,350 fish), had multiple groups for comparison, measured many different health indicators, and lasted long enough (90 days) to see real changes. The study was published in a peer-reviewed scientific journal, meaning other experts reviewed it before publication. However, this research was done only in fish, so results may not directly apply to other animals or humans. The study also only tested one type of fish in one specific environment, so results might differ in other conditions

What the Results Show

Fish that received melatonin grew significantly larger and converted their food into body weight more efficiently than fish without melatonin. The best growth results occurred in the groups receiving moderate melatonin levels (25 mg/kg in normal tanks and 50 mg/kg in crowded tanks). These fish also had higher levels of growth hormone, which naturally promotes body development.

Melatonin dramatically reduced stress in the fish. Fish fed melatonin had much lower levels of cortisol, a stress hormone that increases when animals are anxious or uncomfortable. They also had lower blood glucose levels, another sign of reduced stress. Importantly, fish fed melatonin showed significantly lower levels of ammonia and nitrite in the water, toxic byproducts of fish waste that build up in crowded conditions.

The immune system improvements were striking. Fish receiving melatonin had higher levels of immunoglobulins (antibodies that fight infection), better antioxidant protection (which prevents cell damage), and improved digestive enzymes. These fish were better equipped to fight off disease and handle the physical stress of crowded living.

When researchers examined the fish’s livers and intestines under a microscope, they found that melatonin-fed fish had much healthier organs. Control fish (without melatonin) showed fatty deposits and damage in their livers, signs of stress and poor health. Melatonin-fed fish had normal, healthy-looking organs

Fish fed melatonin showed improved blood protein levels, including higher albumin and globulin, which are important for immune function and overall health. Liver enzymes (ALT, AST, and ALP) were lower in melatonin-fed fish, suggesting less liver damage. Interestingly, the benefits were dose-dependent, meaning more melatonin wasn’t always better—moderate amounts worked best. The interaction between melatonin and crowding density was important: the same dose didn’t work equally well at both crowding levels, suggesting that fish in more crowded conditions may need higher melatonin supplementation

This study builds on previous research showing that melatonin helps reduce stress in various animals. However, this appears to be one of the first studies to test melatonin specifically in farmed seabream at different crowding densities. The findings align with what scientists know about melatonin’s antioxidant and anti-stress properties, but the specific dose recommendations and the interaction with crowding density are new contributions to fish farming science

This study was conducted only in seabream fish in one specific environment (groundwater-based systems), so results may not apply to other fish species or different farming systems. The study lasted 90 days, which is medium-term; longer studies would show if benefits continue over the fish’s entire life. The research was done in controlled laboratory conditions, which may not perfectly match real-world fish farms with varying water quality and other stressors. Additionally, the study measured fish health but didn’t directly test whether melatonin reduces aggressive behavior and cannibalism, which was mentioned as a goal. Finally, while this research shows melatonin helps fish, it doesn’t prove that eating these fish provides health benefits to humans

The Bottom Line

For fish farmers raising seabream: Consider adding 25 mg of melatonin per kilogram of feed for fish in normal stocking densities, or 50 mg per kilogram for fish in high-density conditions. This recommendation is based on strong evidence from this controlled study (HIGH confidence for fish farming applications). For consumers: This research suggests farmed seabream supplemented with melatonin may be healthier and higher quality, though direct human health benefits are not yet proven (LOW to MODERATE confidence for human health claims)

Fish farmers and aquaculture producers should care most about this research, as it offers a practical way to improve fish health and reduce stress-related problems. Seafood consumers interested in the welfare of farmed fish may also find this relevant. People with sleep issues or stress should NOT assume they should take melatonin based on this fish study—human research is needed for that. This research is not applicable to people with fish allergies or those who don’t eat seafood

In fish farms, improvements in growth and stress markers appeared within the 90-day study period, with benefits visible after several weeks of supplementation. If this were applied to human consumption of farmed fish, any potential health benefits would likely take weeks to months to appear, though this hasn’t been directly tested

Want to Apply This Research?

  • If tracking seafood consumption in a nutrition app, users could note when they consume farmed seabream and track any changes in sleep quality or stress levels over 4-8 weeks, though direct causation hasn’t been proven in humans
  • Users interested in fish farming practices could use an app to research and select seafood from farms using natural stress-reduction methods like melatonin supplementation, supporting more humane aquaculture practices
  • Long-term tracking could include monitoring overall seafood intake quality and sourcing, combined with personal wellness metrics like sleep quality and stress levels, to identify any correlations with consuming higher-welfare farmed fish products

This research was conducted in fish, not humans. While the findings suggest melatonin may improve farmed fish health and welfare, this does not prove that eating melatonin-supplemented fish provides health benefits to people. Anyone considering melatonin supplementation for personal use should consult with a healthcare provider. This information is for educational purposes and should not replace professional medical or nutritional advice. Fish farming practices vary widely, and this study’s findings may not apply to all farming systems or fish species.