The high seas – the area outside any country’s national waters – cover nearly two-thirds of the oceans and are largely ungoverned. Some fishers do venture into the open ocean and over the past century have contributed to some fish stocks, such as tuna, becoming over-exploited.

Fishing on the high seas is also unfair – large, multinational fishers can effectively fish just outside poorer nations’ national waters, leaving them with less to catch.

In research published recently in Nature Scientific Reports, I and others show that closing the high seas to fishing would help protect fish stocks and make fishing fairer, but barely impact catches.

The high seas

For the most part now, countries claim an exclusive economic zone (EEZ) of about 200 nautical miles (370 km) offshore, although some of these claims are disputed by neighbouring countries.

When we are talking about the “high seas”, or international waters known as the Commons, we generally mean areas that are not part of these claims. This is about 60% of the 364 million square kilometres of global oceans.

Waters more than 370 km offshore are generally well off the continental shelf and quite deep. As a result, undertaking any activity on the ocean floor here, be it mining or fishing, is difficult and expensive. There is, however, some fishing in these areas for the migratory stocks of tunas and other larger fishes.

There are even some seamounts in the high-sea areas where depths are shallow enough to support some other fisheries such as alfonsino and orange roughy. The sustainability of these fisheries is often a worry.

Overfished and unfair

The high seas are a headache for the people who manage fisheries. These fisheries are common resources, accessible by anyone and not subject to the same controls that apply to fisheries in national waters.

Historically, this has led to over-exploitation. A “snapshot” prepared by the Food and Agriculture Organisation of the United Nations reported that of the 600 marine fish stocks monitored by FAO:

3% are underexploited

20% are moderately exploited

52% are fully exploited

17% are overexploited

7% are depleted

1% are recovering from depletion.

In the Pacific and Indian Oceans (where Australia sits) the worst-affected species included southern bluefin tuna – one of those species frequently caught on the high seas.

Most stocks of fish that are commercially taken spend at least some of their time in the national waters of one or more countries. Some countries profit from selling access rights to fishing within their national waters.

However, in some cases fleets do not pay for access but simply take stocks in high seas waters just outside the national waters of other countries. Typically these would be larger, sometimes multinational fleets fishing stocks such as tuna, which are vital to support some of the poorest maritime countries.

A global ‘fish bank’

Though closing the high seas presents challenges, because of the difficulties in enforcing the ban, it is worthwhile examining what might happen. By careful study of where global catches are taken and what they are worth, we determined likely outcomes for each country.

Our analysis showed that overall there would be very little reduction in current fish catch if there were no fishing in high seas waters. For the most part, the same fish stocks would likely be caught within the national waters of one or more countries rather than further offshore.

Reg Watson

This would improve the economies of these countries and reduce income inequality. Money derived from fishing would be distributed more fairly within societies. Small fishers would get an income – not just large fishing companies with foreign interests. It would allow greater self-sufficiency and income for small island nations.

If high seas areas were closed to fishing, some countries would benefit directly and others indirectly through the protection of some fish stocks. Importantly, the management of exploited stocks would be in the national interest by ensuring greater incentives for management and better equity. Indeed, some could view the high seas as a global “fish bank”.

Difficult though it may be to achieve, we believe that a world where fishing is closed on the high seas would provide better managed fisheries, reduce energy use and the cost of fishing, and ensure that the benefits of marine resources go to those most deserving - all without reducing global catch.

Tuna stocks might come back to higher levels and return to abandoned coastal fisheries. All of this would benefit most countries, including Australia and our near-neighbours.