Mesopelagic Harvests Threaten Bigeye Tuna Diets and High-Value Fisheries

Large-scale commercial fishing of mesopelagic, or midwater "twilight zone," fishes could strip a crucial food source for bigeye tuna and imperil lucrative tuna fisheries, according to a new study led by Woods Hole Oceanographic Institution scientists. The peer-reviewed analysis couples ecological food-web modeling with economic assessment and finds that removing mesopelagic forage to produce fishmeal and fish oil risks long-term declines in bigeye abundance and the revenue those fisheries generate.

The study models predator-prey links and market dynamics to quantify trade-offs. Bigeye tuna derive a large share of their diet - cited at roughly 50 to 60 percent in reporting - from mesopelagic fishes. When those forage stocks are reduced, the models show declines in bigeye feeding success and population-level impacts that translate into lower longline and purse-seine catches over time. The economic portion of the analysis finds potential short-term gains from selling mesopelagic catch into the fishmeal and aquaculture feed sectors, but those gains may be outweighed by sustained losses to high-value sashimi markets and the coastal livelihoods they support.

Ciara Willis and Di Jin are listed among the study's co-authors; the research emphasizes the need for ecosystem-based, precautionary management before any expansion of commercial mesopelagic fishing. The authors note that international waters - where many mesopelagic stocks would be targeted - currently face gaps in oversight, raising the stakes for coordinated management through regional fisheries bodies and other agreements.

For tuna fishers and local communities, the findings translate to concrete risks. Longline fleets that target bigeye for sashimi markets, processors that depend on stable premium-grade supply, and shore-side support businesses could all feel knock-on effects if midwater forage is industrially harvested without accounting for food-web links. Managers weighing permits or quotas for mesopelagic fleets will need to consider not just catch volumes but ecological connectivity and the value of maintaining forage for predators that underpin high-value fisheries.

The bottom line for people who fish, process, or invest in tuna is clear: proposals to turn the twilight zone into a fishmeal bonanza carry potential for a bait-and-switch outcome - short-term revenue at the cost of long-term supply. Monitor upcoming management discussions, look for measures that require ecosystem-based assessments, and track research on mesopelagic biomass and predator diets. That oversight will determine whether mesopelagic harvests become a new, sustainable chapter for fisheries or a costly experiment for bigeye and the communities that depend on them.