Nature-Like Fish Farm Environments Enhance Welfare

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Animal welfare practices in farming generally seek to provide a better environment for animals, that in turn reduces stress and improves the quality and sustainability of the final product at market. Welfare has been considered in land animal agriculture for years, but fish welfare measures — in both senses of the term — are still in their infancy.

This study was one of the first to assess fish welfare in farms, particularly breeding spaces for “nesting, feeding and sheltering.” The authors used environmental stimuli and dietary supplements with farmed Nile tilapia and evaluated their welfare. Nile tilapia (Oreochromis niloticus) are one of the most widely eaten fish by humans with 4.5 million tons produced globally. Tilapia farming has increased recently, particularly in Brazil where production rose nearly 8% from 2018 to 2019. 

The research was carried out at the Faculty of Veterinary Medicine and Animal Science at São Paulo State University (UNESP), Botucatu, Brazil. The authors used 16 glass aquariums with a recirculation system providing filtered water, aeration, and a heater to keep the temperature from changing. Weekly water samples measured pH, ammonia, nitrite, and dissolved oxygen to ensure they were within the range for captive breeding. The researchers studied 640 juvenile male Nile tilapia for 100 days. They designed four sets of tanks: 1. “Environmental enrichment using shelter (PVC pipes)”; 2. “Environmental enrichment using artificial water hyacinth (frayed nylon rope simulating the natural water hyacinth root)”; 3. “food supplementation with tryptophan”; and 4. “a control tank without enrichment or supplemented food.” Fish were fed four times per day and evaluated once per month for biological changes, specifically “average final weight, average weight gain, and food conversion ratio.” 

In terms of fish welfare, the authors studied “general health status, aquarium foraging distribution, number of confrontations, number of repetitive behavior[s] (scratching) and ventilatory frequency [respiration rate].” Researchers watched for “lesion presence, scale absence, fungi presence and signs of possible common diseases in tilapia.” Fish behaviors were particularly studied 15 minutes before and after feedings.

The study found that placing artificial water hyacinth in tanks with tilapia and giving them the supplement tryptophan relieved their stress. The fishes in the study also had better weight gain and fewer incidents of aggressive behavior. Previous research has demonstrated that tryptophan, an amino acid that adjusts serotonin, can heighten social interactions among fish and lower the cortisol levels that regulate stress.

In more detail, results showed that the fish were in good health throughout the study. Fish growth gains demonstrated better health. The fishes in water hyacinth and shelter environments had much higher final mean weight and mean weight gain than those in the food-supplemented tanks.  Tilapia had the fewest number of repetitive behaviors (scratching) in the water hyacinth environment, and fishes in the hyacinth tanks had lower respiration rates than the other tanks. Fishes were less aggressive in the food-supplemented aquariums versus those in the control, hyacinth, or shelter tanks. 

Tryptophan clearly reduced stress and made the fishes feel full after eating. The authors noted that “the low rate of confrontations that individuals showed [sic] under this treatment can be explained by the fact that tryptophan is a serotonin precursor, a neurotransmitter involved in the control of aggressive behavior and the susceptibility to stress.” Territorial fishes actually had more disputes in the water hyacinth and shelter tanks because “Nile tilapia males showed higher aggressiveness in territories enriched by more resources to be defended,” a previous study found. Skirmishes were generally minor and limited to the area around environmental features, though, compared to the control tanks where aggressions were more widespread. This is important to welfare, as aggressiveness can lead to injuries and death.

Fishes in the tryptophan supplement tanks, however, tended to scratch tank glass more often, which signals a feeling of confinement and decreased welfare. The authors pointed out that rubbing “fish can remove the protective mucus layer, become injured and more prone to fungal and bacterial contamination, resulting in high mortality and production losses.”

This study is unique in its analysis of environmental stimuli and fish welfare. The authors argue that using these methods leads to “better quality of life” for fishes without impacting profitability. The study showed that tilapia prefer environments that feel natural and provide “refuge and shelter.” A previous study demonstrated that Nile tilapia had improved “learning and memory” in a nature-like environment. The researchers conclude that “we can highlight artificial water hyacinths as the treatment that presented the best and most consistent results. Such a means of environmental enrichment can be easily implemented in fish farms since it is low cost, easy to handle, and highly durable.” Animal advocates should petition fish farms to install environmental stimuli, and can use this as a starting point for improving fish welfare.