Catch data are first and foremost measures of fishery output (D. Pauly, R. Hilborn and T. A. Branch Nature 494, 303–306; 2013). A long-term decline in catch can be a warning that conditions have changed, either in the abundance and productivity of fish populations or in fishing practices. Although increases in fishing effort can sometimes temporarily restore yields, it is important to investigate the causes of declines.

When time series of commercial catch data are the only information available, there are ways to supplement them with life-history information (such as maximum body length) to expose important changes in population abundance (see C. Costello et al. Science 338, 517–520; 2012).

In areas of the world that cannot afford investigations of abundance, a reduction in fishing intensity is prudent precautionary management (see Code of Conduct for Responsible Fisheries UN Food and Agriculture Organization; 1995).