The sea is a big place. Most fish are small. So it stands to reason that it is difficult to work out with any degree of accuracy just how many fish live in the sea. One way is to measure how many fish we pull out of it. But is that the best way? Or even an accurate way? In two Comment pieces this week, starting on page 303, fisheries scientists debate the issue. It is a crucial one. Worldwide, more than US$200 billion of fish were caught or farmed in 2010. How long can that continue?

In one piece, Daniel Pauly argues that ‘catch data’ of the number of fish caught are a vital tool for assessing the health of fish stocks. In their counterpoint piece, Ray Hilborn and Trevor Branch warn that over-reliance on this measure misses important subtleties and can misleadingly distil the health of entire ecosystems down to a landed tonnage. This is far from an academic debate. If scientists cannot estimate fish numbers, and so the health of stocks, there is little hope that this resource can be exploited in a sustainable fashion.

Disagreements such as this can be problematic for policy-makers. They want a simple answer to the question of how much fish should be caught. But it is crucial that they happen, and happen openly. Fisheries science, and marine science generally, may never have been more important.

It is unquestionable that some fisheries have been horribly mismanaged, and some species driven to dangerously low levels. But equally, there are positive signs of change. There are examples of well-managed fisheries, and, more importantly, there now seems to be a political will to listen to scientists. In the past, quotas for fishing were frequently set much higher than recommended. Europe’s rightly derided Common Fisheries Policy (CFP) is a leading example of this. Tuna populations also show the dangers of repeatedly ignoring scientific advice.

Last year, the International Commission for the Conservation of Atlantic Tunas surprised some by sticking to scientific advice on how many of the valuable fish should be caught, despite suggestions that numbers are increasing. And European politicians are pushing for a reform of the CFP that may finally put science in the driving seat in setting catch limits. Schemes to tell consumers which fish they can eat with a clear conscience have never been more popular, and are also attracting increasing, healthy scrutiny (including in these pages; see J. Jacquet et al. Nature 467, 28–29; 2010, and related Correspondence).

Marine conservation more broadly is also gathering pace. Huge marine reserves are being created around the world, although these are not without teething problems and whether they will ultimately boost fisheries is hotly debated. Billionaires vie to explore the depths, bringing with them slick technology, show-business élan and even more public attention. Last week saw the launch of the Global Ocean Commission, with senior political figures aiming to produce recommendations on how to preserve the ecosystems of the high seas outside national jurisdictions, to feed into United Nations discussions set for 2014.

One message from the Comment pieces this week is just how little reliable information we have about fisheries. Pauly admits that catch data are massively under-reported in many countries, and Hilborn and Branch cite the value of more-detailed scientific assessments of stock while acknowledging that these exist for only 40% of the total catch in the global database of the Food and Agriculture Organization of the UN.

Fisheries scientists unwilling to face this reality can take heart. We don’t have the basic information to judge the health of many human stocks either (see page 281). Those who have the more difficult job of sifting the oceans must be brave enough to outline the uncertainties — such as those over catch data — even as they fight to reduce them.