Risks and benefits of catching "pretty good yield".

What is it about?

ICES have published ranges of "pretty good yield" for a number of North Sea fish stocks, as well as their best estimate of "maximum sustainable yield". The rationale for this is that many fish stocks provide similar yields for different levels of fishing, provided they are within a factor of 2 or so of the optimum. By offering ranges of good yield, it becomes easier to manage trade-offs in a mixed fishery, i.e. it may be possible to manage within the ranges for most stocks, so potential fisheries choke points are minimised or avoided. There ranges are calculated using single species models, assuming fishing on other stocks are fixed. But whilst each may be plausible individually, what happens if all the stocks are fished at the top of their ranges.... In this paper we determine a multispecies reference point by calculating a Nash equilibrium in which the yield on all stocks is simultaneously maximised. From this central estimate, we then determine pretty good yield ranges for each stock, and fish the community at different points on the ranges to see whether there is a significant difference in outcomes. We find that risks go up and yields fall off in the upper part of the MSY ranges, so it is a bad idea to fish all stocks at the top of their ranges for a long period of time.

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Photo by Michael Yero on Unsplash

Photo by Michael Yero on Unsplash

Why is it important?

MSY ranges are topical and are routinely offered as advice, but it is not always appreciated that they are determined in isolation, and taken together, not all parts of the ranges are equally good. This work has the potential to improve the way in which the ranges are implemented, and result in better long-term management outcomes.

Perspectives