The context for the study is India’s rapidly expanding cage aquaculture sector. With marine capture fisheries near stagnation, the Indian government has turned its attention to the country’s 3.5 million hectares of reservoirs as a frontier for fish production. Cage farming of Pangasianodon hypophthalmus, an introduced catfish species better known as striped catfish or pangas, has been a major driver of reservoir fish production growth. But the boom has created enormous demand for feed, pitting fish farmers against other livestock sectors for prime ingredients such as soybean oilcake, fish meal, and cereal products. The resulting cost escalation has made catfish cage culture economically precarious, pushing researchers to hunt for alternative, locally available ingredients that can keep the venture profitable.

Brewery waste, in the form of dried distillery residues and distillers dried grains with solubles (DDGS), emerged as a strong candidate. Rice-based DDGS is available in bulk in India, and the material has already been tested successfully in feeds for Indian major carps and minor carps. Worldwide, the co-product has found uses ranging from silage and single-cell protein production to biosorption of heavy metals and biogas generation. What remained unknown was whether pangas, a catfish with a relatively short gut, could digest and utilize this cereal co-product as efficiently as carps with longer digestive tracts. The mean relative gut length of a fish influences how well it handles plant-rich diets, so the researchers set out to test the ingredient scientifically before recommending it for feed formulation.

The experiment took place at the Maithon reservoir in the eastern state of Jharkhand, where nine square cages measuring 5 by 5 by 3 meters were stocked with a total of 29,997 pangas fingerlings, each weighing on average 3.8 grams. After a 15-day acclimation period, the fish were assigned one of three diets formulated to be equal in protein and energy content. The reference diet, designated T1, contained 44.5 percent soybean meal. The two test diets replaced the soybean meal entirely with 40 percent brewery waste, one processed as an extruded floating pellet and the other as a hand-pressed sinking pellet. All three diets contained a constant 5 percent fish meal, with maize making up the balance at 44.5 percent in the reference diet and 49.5 percent in the brewery waste diets.

Feeding was carried out twice daily, at 10:00 and 16:00, at a rate of 5 percent of body weight, with rations adjusted every 15 days as the fish grew. The team tracked weight gain percentage, specific growth rate, feed conversion ratio, and protein efficiency ratio using standard formulas, and analyzed whole-fish carcass composition through proximate methods: moisture by drying at 105 degrees Celsius, crude protein via micro-Kjeldahl nitrogen multiplied by 6.25, lipid by Soxhlet extraction, and ash by incineration at 550 degrees Celsius. Limiting amino acids were measured by high-performance liquid chromatography after acid hydrolysis, and gross energy was estimated using established conversion factors for carbohydrate, protein, and lipid.

Water quality remained within acceptable ranges throughout the trial, with average temperature of 28.5 degrees Celsius, pH of 8.1, conductivity of 201.4 microsiemens per centimeter, and dissolved oxygen of 6.95 milligrams per liter. Cage management included regular cleaning to prevent biofouling, removal of dead fish and uneaten feed, routine health checks, and preventative treatment with potassium permanganate and copper sulfate. These conditions ensured that differences observed between dietary treatments could be attributed to the feeds themselves rather than to environmental stress.

The results were striking. Fish fed the brewery waste-based floating feed, treatment T2, achieved the highest weight gain, specific growth rate, and protein efficiency ratio, along with the lowest feed conversion ratio of all groups. Their growth performance was statistically significantly better than that of fish fed the brewery waste-based sinking feed, but not significantly different from fish on the soybean-based floating control. Survival, however, told a more nuanced story: fish on the floating brewery waste diet showed significantly lower survival than the other two groups, a finding the researchers noted even as they emphasized that growth and feed utilization were otherwise unaffected. Fish on the sinking brewery waste diet performed worst overall, suggesting that feed form matters as much as feed composition for this species.

The carcass analysis added another layer of insight. Body lipid content rose significantly in all treatment groups over the 60 days compared with initial values, likely reflecting the high nitrogen-free extract content of the diets and the restricted swimming space within cages. Fish on the soybean control had the lowest carcass lipid at 10.15 percent and the highest moisture at 72.5 percent, while the two brewery waste groups showed similar lipid and moisture levels to each other. The team observed a clear inverse relationship between carcass fat and water content: as the fish accumulated fat, they lost water. Crucially, these compositional shifts indicate that brewery waste does not compromise flesh quality in ways that would concern consumers or processors.

The economics may prove to be the study’s most persuasive contribution. Producing one kilogram of the soybean-based floating feed cost 28.29 rupees, while the brewery waste-based floating and sinking feeds cost 19.11 and 17.98 rupees per kilogram respectively. Translated into production costs, the brewery waste floating feed reduced the feed expense of raising one kilogram of fish by 31.45 percent compared with the soybean control, and the sinking version achieved a 29.5 percent reduction. For smallholder cage farmers operating on thin margins, savings of this magnitude could double earnings, according to the authors, who noted that the result is consistent with their earlier work on the minor carp Labeo bata, where a brewery-based floating feed cut costs by 33.8 percent relative to a soybean oilcake formulation.

The broader implications extend beyond a single species or country. Previous studies have shown that distillers grains can partially or wholly replace fish meal in channel catfish diets, and that Nile tilapia fed high levels of corn distillers grains with lysine and tryptophan supplementation achieve improved feed conversion and protein efficiency. The new findings extend this evidence to pangas cage culture in tropical reservoirs and demonstrate that feed presentation, floating versus sinking, is a decisive variable for a surface-feeding species. Because brewery waste requires no additional land or water to produce, it qualifies as a genuinely renewable feed resource, and it can be further valorized into bioethanol and biogas, creating additional revenue streams. The authors caution that further research is needed to identify optimal inclusion levels for different species and life stages, but the central message is clear: what breweries once paid to discard may soon help feed the world’s growing appetite for farmed fish, turning an environmental liability into nutritional and economic security.

Subject of Research: Use of brewery distillery waste as a low-cost feed ingredient for cage-farmed striped catfish

Article Title: Clean valorization of distillery industry co-product for fish cage aquaculture: a waste-to-wealth approach

Article References: Hassan, M. A., Aftabuddin, M., Meena, D. K., Puthiyottil‬, M., Das, B. K., & Sharma, A. P. (2024). Clean valorization of distillery industry co-product for fish cage aquaculture: a waste-to-wealth approach. Blue Biotechnology, 1(1), Article 18. https://doi.org/10.1186/s44315-024-00015-5

Image Credits: AI Generated

DOI: 10.1186/s44315-024-00015-5

Keywords: aquaculture, brewery waste, distillers dried grains, Pangasianodon hypophthalmus, cage farming, fish feed, soybean meal replacement, feed conversion ratio, reservoir fisheries, waste valorization, feed economics, India