Fishing is a very mobile and adaptive industry, and fishing locations move with the distribution of fish stocks. Deep-sea mineral extraction sites, on the other hand, are relatively stable.
Even in a scenario where deep-sea mining is very well established, it will occupy only a tiny fraction of the vast ocean meaning that fishing and mining vessels will have plenty of room to maneuver around each other.
Importantly, fisheries areas do not typically overlap polymetallic nodule exploration areas.
Nevertheless, environmental impact assessments will need to consider the potential impact of deep-sea mining on tuna in the vicinity of specific mining sites.
Tuna species are seldom found deeper than 400 meters beneath the ocean’s surface, although some species are able to dive to 1,000 meters for occasional foraging.
Polymetallic nodule collection would take place at much lower depths – on the seafloor between 4,000 and 6,000 meters below the ocean’s surface.
Sediment stirred up by the action of collecting nodules and removing them from the sediment will not reach the fish stocks. Field trials have shown most of the suspended sediment is found within two meters of the seafloor and particle concentrations are equivalent to the quality of drinking water in major cities (e.g. New York, London).
Seafloor nodule collector systems are being designed so that nodules are washed before they are transported from the seafloor to the surface vessel. This is to minimize the amount of sediment that is brought up with the nodules. Still, the nodules will need to be dewatered (i.e., separated from the seawater they were brought up with) and the resultant seawater may contain some sediment fines as well as some nodule fragments. This will need to be returned to the ocean.
A study published in 2021 that examined the potential interface of DSM and the fishing industry stated that any sediments should be discharged at depths deeper than 1500 meters to avoid both the commercial species and their vertically migrating prey.
The DSM industry has taken this recommendation on board and currently the shallowest known planned discharge depth for return water is 2000 meters, which is 1600 meters below where tuna reside and avoids their vertically migrating prey.
Some operators have said they will still strive for discharges deeper than this, including to the ultra-deep ocean above the seafloor, to avoid discharges to the midwater column altogether.
What if this return water was discharged at shallower depths where tuna are found?
If return water was to be discharged in the midwater column, the noise created by the outfall could attract or even deter fish (more likely the former) but this is not anticipated to cause any harm.
While there may be some concentration of sediment near the outfall, this is expected to dilute rapidly and any exposure is likely to be short-lived.
What about discharges from the surface vessel?
Any discharges from the vessel as part of normal ship operations (e.g. macerated food waste, discharge from the vessel’s sewage treatment plant) will be governed by International Convention for the Prevention of Pollution from Ships (MARPOL).
As with any ship, these discharges occur at the surface and could provide a localised food source for marine life, including tuna.
In addition, noise and vibration from the vessel, as well as lights at night, could result in the vessel acting like a fish aggregating device.
However, there are number of measures that can be taken to mitigate the effect such as reducing light levels at night. While some fish might congregate around the vessel, deep-sea mining is not expected to impact overall fish stocks.
Will metals in the sediment plumes present a toxicity risk?
The collection of polymetallic nodules is unlikely to pose a toxicity threat to tuna fisheries. The deepwater plume does not overlap with tuna habitats or their food sources and studies to date suggest deep-sea mining is unlikely to release toxic concentrations. While the return water plume may contain metal-bound particles, these are expected to dilute rapidly. Additionally, tuna’s migratory nature and diverse diet minimize potential exposure. Ongoing environmental impact assessments are investigating these aspects further.