Costello et al. Authors’ Note

In Costello et al. (2016) we showed that appropriate management reforms at the global level could lead to a bright future for fisheries, with the the potential to increase biomass of live fish in the sea, fishery catches, and fishery profits, and that the typical fishery could recover within a decade. Our analysis used estimates of current status for almost 5,000 individual fisheries around the world. The current biological status of these fisheries ranges widely, from many small, coastal fisheries that are severely overexploited, to many large industrial fisheries that are well-managed, and even some stocks that are underexploited. Across these fisheries, we estimated the median B/Bmsy of global fisheries to be 0.78. This means that current biomass for this median fishery is about 78% of the biomass expected to produce maximum sustainable yield. Importantly, the B/Bmsy target for maximizing economic yield would be even larger than the target for maximizing yield (perhaps B/Bmsy=1.2 rather than 1), so according to these estimates, the median fishery in the world is below most management targets. We referred to this median global fishery as “overfished,” invoking the FAO definition in the 2014 State of World Fisheries (where a fishery is “overfished” if B<Bmsy). But there is also a legal definition of “overfished” in the United States which differs from the FAO definition provided above. In the US, a stock is typically defined as “overfished” only in the much worse case in which B/Bmsy < 0.5. So while a fishery with B/Bmsy=0.78 is clearly below management targets in the US, it would not be legally defined as “overfished” by US standards. This distinction is made clear with the following comparison: Overall, 76% of all fisheries and 47% of large fisheries (MSY>10,000 MT) are overfished given FAO’s definition (and therefore these stocks are below management targets for the FAO and the US); while just 22% of all fisheries and 17% of large fisheries would be classified as legally overfished under the US definition.

The discussion above considers each fishery equally – so a small artisanal fishery counts as much as a large industrial fishery towards our estimate of median fishery status. Another summary statistic of interest is the weighted average of fishery status. We plotted catch-weighted arithmetic average status as squares in Figure 1 of Costello et al. (2016). That approach produced a global weighted average status of B/Bmsy=1.18, suggesting that on average the fisheries contributing the most to global catch today are in better condition than the median fishery. But other approaches to weighting provide different results. Using geometric mean weighting (because B/Bmsy values are ratios) and weighting by MSY rather than catch (because overfished stocks may have low current catch and would thus be under-represented) provides an alternative measure of average status. Using that alternative approach produces an MSY-weighted geometric mean status of B/Bmsy=0.79, which is quite similar to the median status of B/Bmsy=0.78 discussed above, though lower than the catch-weighted arithmetic mean B/Bmsy (1.18). Importantly, the MSY weighted geometric mean F/Fmsy is 0.66, substantially lower than the catch weighted arithmetic mean F/Fmsy of 1.5. These alternative ways of representing average status have no bearing on the status of individual fisheries or on the results of the analysis in Costello et al. (2016) regarding the bright potential future of fisheries. They do however illustrate diverse ways in which we can consider the current overall status of global fisheries. In the figure below, we plot MSY-weighted geometric means of B/Bmsy and F/Fmsy as the square.

Note: B/Bmsy is the ratio of current biomass to the biomass that would produce maximum sustainable yield. F/Fmsy is the ratio of current fishing mortality to the fishing mortality that would produce maximum sustainable yield (MSY).

Signed,

Christopher Costello, Daniel Ovando, Tyler Clavelle, C. Kent Strauss, Ray Hilborn, Michael C. Melnychuk, Trevor A. Branch, Steve D. Gaines, Cody S. Szuwalski, Reniel B. Cabral, Douglas N. Rader, and Amanda Leland