In an experiment, yellow trout grew larger than the wild strain

An experiment conducted in the state of São Paulo, Brazil, concluded that yellow trout can grow larger than the wild strain, which is the most commonly farmed phenotype of rainbow trout (Oncorhynchus mykiss).

The yellow, cobalt blue, and white phenotypes result from crossing wild strain females with white albino males. Previous studies have shown that albinism negatively affects trout growth and weight gain.

Supported by FAPESP, the study was published in the journal Aquaculture Reports.

“The performance of the yellow phenotype was surprising because the wild type usually grows the most. Those with albinism generally have lower efficiency in converting food into muscle and in other parameters. The wild type actually grew more at the beginning of the experiment, but growth slowed after 120 days,” says Vinicius Vasconcelos Silva, the first author of the study. Silva conducted the study during his doctoral research with a FAPESP scholarship at the Fisheries Institute of the São Paulo Agency for Agribusiness Technology (IP-APTA), which is affiliated with the São Paulo State Department of Agriculture and Supply. He also conducted research at the Faculty of Animal Science and Food Engineering at the University of São Paulo (FZEA-USP) in Pirassununga.

The study is part of the project “Evaluation of Muscle Growth in Rainbow Trout Strains”, which is supported by FAPESP and coordinated by Vander Bruno dos Santos, who is a researcher at the Fisheries Institute.

“The difference in coloration has potential for aquaculture, as it can generate different products from the same species, but mainly for sport fishing. There’s a demand for different fish at catch-and-release facilities,” says Santos.

The experiments were conducted at the Dr. Ascânio de Faria Regional Salmonid Research Center in Campos do Jordão, the Fisheries Institute’s facility that is best suited for cultivating species from temperate climates. Salmonids, such as salmon and trout, are native to the Northern Hemisphere and are adapted to milder climates, like the one found in Campos do Jordão at an altitude of 1,525 meters above sea level.

Performance

The fish were raised in tanks under identical temperature, pH, and feeding conditions. Measurements were taken at 60, 120, and 180 days and again at the end of the experiment at 270 days. 

According to Brazilian standards, trout are typically at harvest size at this age, weighing about 250 grams. Known as “portion-sized trout,” they are ideal for one person’s consumption. In North America, harvest size can start at 500 grams.

All of the evaluated fingerlings were the offspring of a cross between five albino males and oocytes from 80 wild-type females with normal coloration, typically brown with black spots.

External fertilization was performed by manually extracting the females’ oocytes and the males’ sperm, mixing them in a container, hydrating them, and transferring them to incubators. The larvae remained in the laboratory for 30 days before being transferred to tanks for the experiments.

A total of 2,031 albino, 1,969 yellow, 1,971 cobalt-blue, and 2,306 wild-type fingerlings were produced. For regular evaluations, 90 animals of each phenotype were selected.

In vitro fertilization involves mixing male sperm with female oocytes in a container and then adding water (photo: Vander Bruno dos Santos/IP-APTA)

By 120 days of age, the wild-type had gained the most weight, reaching an average of 54 grams, while the albino type weighed 33 grams. The latter gained the least weight in all phases. 

However, at 180 days, the wild-type fingerlings averaged 119 grams, while the yellow ones averaged 123 grams. By the end of the experiment (270 days), the yellow fingerlings averaged 300 grams, while the wild-type ones remained in the 237-gram range. The white ones remained at 145 grams.

At the top, from left to right, wild and white albino phenotypes. Below them, yellow and cobalt blue. In an experiment, the yellow phenotype outperformed the other three (photo: Vinicius Vasconcelos Silva/IP-APTA)

Mechanisms

The reasons for the slower growth of albino white trout are not fully understood. However, studies conducted by other groups have suggested some hypotheses. 

The group led by Ricardo Shohei Hattori at the Fisheries Institute concluded in a 2020 study, supported by FAPESP, that in addition to reduced weight, the cobalt-blue and white phenotypes had a smaller pituitary gland compared to wild and yellow trout. The pituitary gland regulates metabolism, growth, and other glands. 

“Other studies suggest that the set of genes responsible for pigmentation in trout may negatively affect growth and specific defense mechanisms,” notes Santos.

According to these studies, the absence of melanin production in the skin, the defining characteristic of albinism, is associated with disruption of the melanocortin regulatory pathway. Melanocortin is a hormone involved not only in pigmentation but also in inflammation, hunger, and energy. Consequently, it negatively impacts feeding behavior, growth, stress response, and immune function. 

With an annual production of about 1,600 tons, considered low, rainbow trout serve a niche market. However, researchers see potential in producing so-called salmon trout, which possess similar nutritional properties and replicate the color of salmon flesh through the introduction of a pigment in their feed. 

Studies aimed at understanding and increasing its productivity could improve production and reach new markets. The group is now working to increase the nutritional value of trout in potential partnership with the Fish for Health Research Center (NPPS), a Science Center for Development (SCD) supported by FAPESP.

About FAPESP

The São Paulo Research Foundation (FAPESP) is a public institution with the mission of supporting scientific research in all fields of knowledge by awarding scholarships, fellowships and grants to investigators linked with higher education and research institutions in the state of São Paulo, Brazil. FAPESP is aware that the very best research can only be done by working with the best researchers internationally. Therefore, it has established partnerships with funding agencies, higher education, private companies, and research organizations in other countries known for the quality of their research and has been encouraging scientists funded by its grants to further develop their international collaboration.

 

---

---