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Farmed seafood, livestock stack up differently using alternate feed efficiency measure

Study finds retention of protein and calories in feed similar for major aquaculture and livestock species.

A new study by researchers at the Johns Hopkins Bloomberg School of Public Health’s Center for a Livable Future (CLF) found that, contrary to widely held assumptions, farmed fish and shrimp convert protein and calories in feed to edible seafood at rates similar to livestock (such as cattle, pigs and chickens).

The study contributes new insights into feed conversion efficiency — the efficiency of the process by which feed is turned into meat — across species and uses a new analysis to assess this efficiency, CLF said.

The paper, “Feed Conversion Efficiency in Aquaculture: Do We Measure It Correctly?,” appeared in the journal Environmental Research Letters.

“We face the challenge of feeding a growing human population, and aquaculture is viewed as a solution due to favorable feed efficiency compared to livestock,” said Dr. Jillian Fry, director of the Public Health & Sustainable Aquaculture Project at CLF and faculty member in the Bloomberg School’s department of environmental health and engineering. “We need a complete understanding of feed efficiency, though, because aquatic species differ from livestock in important ways that are not considered in feed conversion ratios.”

The global supply of protein and calories for people is affected by how efficiently feed is turned into food. Production of food animals is increasing, and aquaculture is growing faster than other types of food animal operations, CLF pointed out.

Conversion of protein and calories in animal feed to human food during fish and shrimp production has not been fully explored; this is critical for food security, however, because growing demand for seafood and meat results in steadily growing demand for more feed inputs.

The researchers found that average protein retention and calorie retention across nine types of farmed fish and shrimp are lower than chickens and similar to pig and cattle production. They estimated that just 19% of protein and 10% of calories in feed for aquatic species are ultimately made available to the human food supply.

CLF noted that the similarity to livestock when using these efficiency measures is the opposite of the result expected from weight-based feed conversion ratios. Comparing all terrestrial and aquatic animals in the study, chickens are the most efficient at protein retention, followed by Atlantic salmon, rainbow trout, whiteleg shrimp and pigs, CLF said.

“The finding that fed aquatic species are comparable to land-based animals when measuring conversion of protein and calories in feed into animal products upends conventional wisdom,” Fry said. “This study provides strong evidence that strategies to address global food security and resource conservation should consider multiple food production efficiency measures.”

To understand how efficiently different species convert feed inputs into the products people actually consume, the CLF researchers assessed the protein and calorie contents of feed inputs and compared how protein and calories are retained in the final edible products. To produce these efficiency estimates, the research team collected data on variables that included feed composition, feed conversion ratios, edible portions and the nutritional content of major farmed terrestrial and aquatic animal species.

The study focused on intensive production methods, not extensive methods like grazing, CLF said. The species analyzed for the study included commercially farmed beef cattle, pigs, chickens, carp, catfish, Atlantic salmon, rainbow trout, tilapia and shrimp.

CLF said the study is the first of its kind to report protein and calorie retention for a wide variety of farmed aquatic species.

The most widely used measure of feed efficiency is the weight-based feed conversion ratio, calculated as the ratio of feed intake (by weight) to weight gain. By this measure, fed aquaculture and chickens are similarly efficient, and both are more efficient than pigs and cattle, meaning they require fewer feed inputs.

However, the CLF researchers said feed conversion ratio estimates are flawed measures for comparing efficiency across species because they do not account for the nutritional content of the feed, the portion of the animal that is inedible or the nutritional quality of the final product.

In light of these findings, the researchers suggested that alternative retention measures should be used in addition to feed conversion ratios, as well as environmental footprint measures like resource use (e.g., land and water), greenhouse gas emissions and negative externalities such as biodiversity loss and water pollution.

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