The Future of Tuna Lies in the Midwater — Correlation Diagram of Deep-Sea Mining and the Food Web: Threats to Marine Ecosystems and Our Dining Tables

Lead — In the deep sea "midwater," mining-induced murky particles dilute the natural "food," quietly starving plankton and small fish. If such a chain reaction occurs, it could eventually impact commercial fish species like tuna and mahi-mahi, and even human dining tables. A new study has analyzed wastewater and sediment samples from actual test mining and particle size distribution, specifically illustrating this risk. This article organizes the latest findings, current policy status, and social media reactions to summarize the points for simultaneously applying brakes and guardrails to "hasty deep-sea mining."

1. What is happening—Overlooked developments in the "middle of the sea"

Deep-sea mining is a concept for recovering critical minerals such as copper, nickel, cobalt, and manganese from polymetallic nodules scattered on the seafloor. The slurry material sucked up from the seabed is processed on board, and the unwanted seawater and fine sediments are returned to the sea. It has become apparent that this "return destination" is more troublesome than previously imagined.

Recent discussions have often focused on the seafloor ecosystem (destruction of benthic organisms, resuspension of sediments, etc.). However, the new research focuses on the midwater, approximately 200 to 1,500 meters deep. This is a vast "midwater dining hall" where the production of phytoplankton is transported and where zooplankton and small fish (micronekton) live.

2. Key points of the new research—The "junk food effect" and the "particle size trap"

Samples were taken from the discharge water of a test mining conducted in the fall of 2022 and from the observed murky water (plume) in the actual sea area. Particle size distribution, concentration, nutritional value (amino acid concentration), and compound-specific stable isotope (amino acid CSIA) analyses were conducted. As a result, it was shown that while naturally occurring medium to large-sized particles (approximately 6 to over 53 μm) support the food web foundation, mining-derived particles "dilute" the same particle size range but have significantly lower nutritional value. Since more than half of the midwater zooplankton are particle feeders and about 60% of their predators, the micronekton, are zooplankton feeders, if the murky water spreads widely and for a long time, a **"bottom-up disturbance" that collapses from below** could occur.

Furthermore, laser particle measurement (LISST) confirmed that small particles (1 to 6 μm range) in the plume are significantly more abundant compared to background seawater, and that even in the medium to large particle size range, the amino acid concentration is significantly lower than natural particles. In other words, "it fills the stomach but doesn't provide nourishment"—the junk food effect. The small energy deficit accumulates in individual growth and survival, potentially becoming visible in changes in community composition, food shortages for higher predators, and disruptions in nocturnal vertical migration (DVM) patterns.

3. Why is the "midwater" crucial?

Midwater is a dark layer where light does not reach, but it is also a massive logistics hub connecting upper-layer production and lower-layer storage. If energy diminishes here, the flux of sinking organic matter will dwindle, and the efficiency of carbon sequestration (biological pump) may also decline. Additionally, many surface fish species (tuna, bonito, mahi-mahi, etc.) dive deep to prey on micronekton. If the midwater "dining hall" becomes impoverished, surface fishing grounds will not be immune from impact.

This zone also gathers thresholds of physical and chemical environments such as the oxygen minimum zone (OMZ) and thermocline. If "fake food" with similar particle size distribution flows in as murky water, multifaceted inhibition of behavior/sensory such as visual predation, bioluminescent communication, and olfactory receptor clogging could occur simultaneously.

4. The current state of industry and policy—Brakes and accelerators

The International Seabed Authority (ISA), which oversees the deep-sea high seas, has authorized several exploration contracts in the Clarion-Clipperton Zone (CCZ), and deliberations on commercialization rules continue. Major countries and companies are turning their attention to the deep sea as a new source of supply against the backdrop of electrification for decarbonization and geopolitical risk diversification. Meanwhile, the uncertainty of environmental aspects is significant, and many aspects of standards and monitoring frameworks for discharge depth, water quality, and total volume remain undeveloped.

In the United States, while measures to secure critical minerals are advancing, discussions are underway to review activity permits and regulations in maritime areas. It is essential to proceed with a perspective that simultaneously advances scientifically based phased rule formation and demand-side pressure relief through recycling and alternative materials, rather than a binary "stop/proceed with mining" approach.

5. Reactions on social media—Concerns, dissemination, and reevaluation

The study spread rapidly upon release, with ocean NGOs, researchers, and media accounts related to fisheries and aquaculture sharing it one after another. The message that **"serious impacts extend not only to the seabed but also to the midwater life web"** has delved into reaffirming the stance of calling for a moratorium (halt), while also generating constructive questions for the industry and regulators to visualize "discharge design conditions."

Japanese marine researchers shared the paper link with comments that "quantification of midwater risks has progressed," and in the European oceanographic community, there are many reactions that **"midwater cannot be ignored." Fisheries industry media emphasize the impact on food supply and employment with the expression "silent hunger." Rather than being solely critical, the spread of technical design and monitoring questions such as **"at what depth, particle size, and concentration is it dangerous?"** could serve as a foundation for future dialogue.

6. Practical points to lower risks

① Clarification of discharge guidance — Quantify particle size distribution, amino acid concentration, turbidity thresholds, continuous/intermittent operating conditions, and avoidance of overlap with seasonal and diurnal vertical migration periods at each stage of pre-evaluation→operation→post-evaluation.

② Advancement of monitoring (MRV) — Three-dimensional tracking of plumes and automatic alarms using a combination of LISST, fluorescent dyes, eDNA, and acoustic measurements. Consider real-time approximation of midwater biological **behavior indicators (predation, swimming, bioluminescence)**.

③ Spatial planning and avoidance — Temporal and spatial avoidance of "sensitive midwater" such as OMZ edges, high biomass zones, and migration corridors. Set rotation and rest periods for operational areas.

④ Expansion of alternative resources — Recycling of batteries and electronic devices, re-resourcing of tailings and slag, and material conversion (e.g., cobalt reduction) in the supply chain. Increase the rate of **"not having to go to the sea"** through demand-side measures.

⑤ Transparency — Open access to test mining data (particle size, turbidity, discharge volume, chemical composition, biological impact indicators) and international standardization of test design. Promote replication studies in different sea areas, seasons, and flow conditions.

7. Conclusion—"More haste, less speed" science

Midwater is invisible but a crucial layer supporting the ocean. If "junk food" increases there, the food web quietly thins. Hasty commercialization could become an expensive insurance omission in terms of both fisheries and climate. Deepen rules and technology simultaneously and design resource and environmental policies into one. What is needed now is "more haste, less speed" science.

Column: The contours of risk seen through numbers

• Midwater (approximately 200 to 1,500 meters) is a "dining hall" densely populated with zooplankton and micronekton.

• Particles from test mining are similar in size to natural particles but have low nutritional value, resulting in a strong dilution effect.

• About half of the zooplankton are particle feeders, and about 60% of the micronekton are zooplankton feeders. When these two layers of "food" thin, it affects larger fish higher up.

• Observations show that small particles (1 to 6 μm) are overwhelmingly more abundant compared to the background, and even in the medium to large particle size range, the amino acid concentration is significantly lower.
  
  
  

Reference (perspectives to broaden the discussion)

• Research gaps: Spatiotemporal spread of plumes and long-term uptake, tipping points of community changes.

• Industrial design: Optimization of discharge depth, continuity, and particle size control, advancement of recovery and filtration technologies.

• Policy: Scientific basis for ISA rule-making, hybrid design of moratorium and phased demonstration.