Vibrant colors increase appetite in fish (2024)

By Elton Alisson

Agência FAPESP – Hot and vibrant colors such as red, orange and yellow tend to be used strategically in restaurants and in visual communication at fast-food restaurants to attract consumers. A study conducted by researchers at the Botucatu campus of Universidade Estadual Paulista (Unesp) and published in PLoS One magazine revealed that some of these colors could exercise the same effect on fish species.

The researchers found that red increases the appetite and the ingestion of rations in Nile tilapia (Oreochromis niloticus), a species of fish grown in several countries, in addition to Brazil, and that is very sensitive to different wavelengths of light. The increased appetite and ingestion of rations by this species, however, did not increase the weight of the animal.

“We observed that red tones stimulated feeding among fish. The increase in the ingestion of rations, however, was not converted into greater animal growth,” commented Gilson Luiz Volpato, professor at the Department of Physiology at the Unesp’s Biosciences Institute and one of the authors of the study.

The researchers kept different groups of Nile tilapia in glass aquaria covered with white, blue, green, yellow and red cellophane for four weeks. The fish were exposed to similar light levels because the intensity of light can influence the growth of fish. The animals were fed once a day.

After a period of acclimation, the scientists evaluated and measured the chronic effects of the colors on the adaptation of the fish to the new environment with colored lights, in addition to the time that the fish required to begin feeding, the quantity of rations they ingested and the conversion of what they ate into growth.

The experiment revealed that the fish kept in the aquarium with red light began to feed more rapidly and consumed larger quantities of rations that the fish raised in the aquaria with other colors of light. The weight of all these fish, however, was similar.

“There is some factor that we still have not managed to identify that makes fish exposed to red light ingest higher quantities of rations and not gain weight,” commented Volpato. “One hypothesis is that the excess food absorbed was channeled to physical processes that require energy expenditure, such as swimming. However, this question should be better investigated.”

To rule out the hypothesis that fish are more able to see food in water with red light and therefore begin feeding more rapidly and eat higher quantities of rations, the researchers conducted another experiment. They provided rations for Nile tilapia kept in the aquarium with red light with the light turned off.

Tapes of the experiment with infrared cameras that filmed in the dark showed that even without light, the fish in the red environments approached the rations more rapidly and began feeding more rapidly than the animals kept in aquaria with other colors.

To evaluate whether fish senses other than vision are involved in sensitivity to food, the researchers conducted a third experiment. They diluted a soluble ration on the surface in one corner of the aquarium in a position opposite to the position of the fish. They then charted the time that the animals took to approach the food.

Once again, the experiment showed that the fish kept in the aquarium with red light reacted more rapidly to the olfactory stimulus that the animals raised in aquaria with other colors.

“We discovered in this experiment that the other fish were undoubtedly attracted to the chemical stimulus, reinforcing the hypothesis that the red light does not solely affect the animals’ vision,” said Volpato. “The color red can probably affect the nervous system of these animals as well.”

Applications to aquaculture

In Volpato’s assessment, the study’s discoveries create the opportunity to conduct research on the mechanisms by which colors can affect animals and on the effects of colors on the central nervous system of fish.In another study published in the Brazilian Journal of Medical and Biological Research in 2001, the same Unesp group showed that colors modulate the level of cortisol hormones produced in fish stress reactions.

During the experiments, the researchers found that fish subjected to stress factors and exposed to environments with clear colors such as blue have lower levels of cortisol in their blood than those kept in white or green aquaria.

“We have data that show that, in fact, blue is a color that improves the well-being of animals, while red is a color that irritates,” compared Volpato. “Although it causes an increase in the appetite and ingestion of rations among Nile tilapia, red can be harmful to this fish species.”

The results of the studies, according to the researcher, can also help zootechnists and professionals who work in aquaculture to develop techniques to promote animal well-being in tanks, for example.“Cellophane can be placed on top of incubators to change the color of the environment and increase the animal’s well-being,” Volpato commented. “In earth tanks, this approach is more difficult but is also possible,” he added.

The researcher is currently mentoring a doctoral study that is being undertaken through a FAPESP fellowship. The study aims to evaluate whether the choice and preference of a certain color is enough to maintain the well-being of fish.

At the beginning of August, doctoral student Caroline Marques Maia, Volpato’s mentee, will begin a research internship at Pennsylvania State University, also with a FAPESP fellowship, under which she will compare the color preferences of Nile tilapia and rainbow trout (Oncorhynchus mykiss).

“This area of research and analysis of the influence of colors on feeding, growth, reproduction, stress and the survival of fish is beginning to develop in Brazil,” said Volpato.

“Our work is the first that has shown that red increases appetite but not the weight of the fish,” affirmed Volpato.

The article “Red light stimulates feeding motivation in fish but does not improve growth” (doi:10.1371/journal.pone.0059134) by Volpato and co-authors is available on PloS One’s website at www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0059134.