Although few people are aware of it, wild-caught fishes are a common ingredient in feeds given to farmed fishes, mainly as fishmeal and fish oil. And, according to the present study, the impact of this practice is likely far greater than previously thought.

Drawing from four independent datasets, including reports from the Food and Agriculture Organization of the United Nations, Monterey Bay Aquarium Seafood Watch, and industry surveys, researchers developed a more complete picture of aquaculture’s resource demands. Their analysis incorporated previously overlooked factors and adjusted traditional calculation methods to better reflect real-world conditions.

The researchers found that the ratio of wild fish inputs to farmed fish outputs (FI:FO) ranges from 0.36 to 1.15 — up to 307% higher than earlier estimates of 0.28. When accounting for fish mortality during capture and excluding “unfed” farming systems (systems that don’t use compound feeds), this ratio increases further to between 0.57 and 1.78.

These updated calculations considered multiple factors that previous studies had overlooked or underestimated. For instance, the researchers identified that roughly 66% of fish trimmings used in feed come from wild-caught fishes. Trimmings are traditionally categorized as byproducts and therefore not included as wild fish inputs. Even whole fishes can be classified as byproducts when they’re not the target species, meaning they’re not typically counted either. Additionally, the high mortality rates of fishes during capture operations, particularly through “slipping” (when fishes die after escaping from nets), hadn’t been considered before.

The environmental impact of fish farming goes beyond its heavy reliance on wild fish populations. A shift away from marine-sourced to terrestrial-sourced feed ingredients has also created new environmental challenges. Between 1997 and 2017, while the industry reduced its reliance on wild fishes, it increased its use of feed crops by 468% — more than twice the rate of aquaculture production growth during the same period. This indicates that aquaculture’s environmental impact hasn’t decreased but rather shifted from marine to terrestrial ecosystems.

The environmental footprint varies significantly among different types of farmed fishes. Species groups using herbivorous fish feeds consume more than twice as much land and 43% more freshwater than those using carnivorous feeds. However, carnivorous species like salmons and trouts require substantially more wild fish inputs. For example, salmon farming alone can require between 1.86 and 6.24 pounds of wild fish for every pound of salmon produced.

The findings are particularly concerning given aquaculture’s rapid growth and its promotion as a sustainable solution to global food security. The study suggests that fish and crustacean farming results in a net loss of both calories and protein, challenging the notion that aquaculture effectively produces more food than it consumes.

For advocates and policymakers, this research provides several important takeaways:

1. Current sustainability certifications and metrics may significantly underestimate aquaculture’s environmental impact and should be reevaluated.
2. The industry’s shift from marine to land-based feed sources isn’t necessarily more sustainable. It simply trades one set of environmental impacts for another.
3. Different aquaculture species have vastly different environmental footprints. Policies should consider these variations when promoting aquaculture development.
4. Greater transparency is needed in feed formulation and industry practices, as many parameters still rely on voluntary industry disclosures.
5. Future food security strategies should carefully weigh the true costs and benefits of aquaculture expansion, considering both marine and terrestrial impacts.

The researchers recommend implementing more comprehensive sustainability assessments that consider both marine and terrestrial impacts. They emphasize that common sustainability accounting methods have been too narrow and overly optimistic, suggesting that directives to expand aquaculture on sustainability grounds should be reconsidered. Together with research showing that aquaculture’s animal welfare risks are unknown and underestimated, these findings underscore the need for continued advocacy for both farmed and wild fishes.