In a perspective paper out last week, Ray Hilborn argues for a new kind of analysis to better inform policy-makers and the public on the performance of fisheries management. It’s called a goldilocks plot. It is an approach that can be represented graphically (and comically below) to show how much fish is not utilized due to suboptimal abundance and/or fishing pressure.
It's all about how MSY is interpreted
Maximum Sustainable Yield (MSY) is the goal of nearly every fishery management legislation in the world. MSY is dependent on a sustainable level of fish biomass (B) and fishing mortality (F) that produce the maximum long-term amount of fish. Fishery managers commonly use the ratio of fish biomass at MSY (BMSY) to current fish biomass (B/BMSY) to determine sustainability. However, the acceptable sustainability ratio varies by management organization. For example, the UN Food and Agriculture Organization (FAO) determines that a fish stock with a ratio below 0.8 is ‘overfished’ while fishery managers in the United States and New Zealand say the ratio must be below 0.5 to be called ‘overfished,’ though it varies for individual stocks and management plans.
However, a ‘goldilocks’ plot calculates the cost of overfishing by determining how much future fish is lost by fishing too hard or having too few fish in the population. The lower a stock abundance falls below BMSY the more yield will be lost. Using fishery statistics, scientists can determine what potential future yield of a stock should be, which can then be used to determine if stock abundance is too high, too low, or just right; subsequently, fishing pressure on a stock at present could be increased, decreased or stay the same. The lost yield can be calculated either based on stock abundance, or fishing pressure. If fishing pressure is too high, fish will be lost long-term. There is also lost yield when fishing pressure is too low. The Goldilocks plot gives managers and the public and idea of how much yield in a region is being lost from stocks that are overfished and how much potential there is for increasing yield from stocks that are not filling their potential.
In his perspective piece, Hilborn shows what this could look like for the EU stocks that are assessed by ICES, the regional fisheries science organization:
The two plots show that there was significant yield being lost from the 1960s to early 2000s from stocks being too low (overfished) and subject to too much fishing pressure, but since 2010 overfishing has largely stopped and little yield is being lost, either from low stock abundance or excess fishing pressure.
This could be a good way to communicate the performance of fishery management as working towards fulfilling potential yield is the goal and information about stocks that should be fished more and less are shown.
Why is this necessary?
There is always merit in science—any kind of new information or discussion of methods is commendable, but it is worth discussing Hilborn’s rationality for writing the paper and introducing these graphs. In the paper, he derides how the sustainability ratio (B/BMSY) has been spun by advocacy organizations to delegitimize fishery science. For example, earlier this year FAO released their State of Fisheries and Aquaculture 2018 report and determined that by their measure of sustainability, 33% of stocks are overfished, 60% are ‘maximally sustainably fished’ or ‘fully-exploited’ (aka “just right” by goldilocks and scientific standards—remember this is the goal) and 7% are underfished. Yet, much of the communication about world fisheries from large, international advocacy organizations undermines science to completely misrepresent the true status of world fisheries.
Part of the reason Hilborn started this site was to improve fishery science communication online and on social media. Hopefully his new paper on the goldilocks approach to communicating fish stock sustainability will catch on.