Maximum Sustainable Yield –misconstrued and abused

Photo by Asc1733, Wikimedia Commons

In the new paper published in the ICES Journal of Marine Science, “MSY needs no epitaph –but it was abused” authored by FishBase creators Dr Daniel Pauly (Principal Investigator of the Sea Around Us initiative at the University of British Columbia’s Institute for the Oceans and Fisheries) and Dr Rainer Froese (Senior Scientist at the GEOMAR Helmholtz Centre for Ocean Research), they strongly propose to revive and bring back to light the original concept of maximum sustainable yield which was deeply rooted in ecology. This ‘mission’ roots on the fact that “degraded versions [now exist which] promote stock reductions to 30-40% of unfished biomass, where stocks are unable to fulfill their ecosystem roles,” according to Dr. Froese. Unfortunately, other MSY users have added ‘tricks’ to it which led to excessive fishing under the disguise of sustainability.

In fisheries management, Maximum Sustainable Yield or MSY refers to the theoretical highest catch that a fish stock can support in the long-term, given that environmental conditions do not change much.

Abstract: The MSY concept is widely considered to be outdated and misleading. In response, fisheries scientists have developed models that often diverge radically from the first operational version of the concept. We show that the original MSY concept was deeply rooted in ecology and that going back to that version would be beneficial for fisheries, not least because the various substitutes have not served us well.

Pauly: One of the reasons is because, these days, many people are interested in consuming ‘sustainable fish.’ Consumers tend to believe that when their grocery shop offers them a fish with such a label, it means that it was removed from a healthy, well-managed stock. However, if the agency or country managing that stock allowed it to decline below half its unexploited abundance, this stock is overfished, whatever the claims of ‘sustainability’ are.

Froese: Earlier – like many other scientists – we used to believe that the MSY concept was outdated and stood in the way of proper ecosystem-based fisheries management because this concept focuses on one species at a time. However, we started using a new implementation with hundreds of stocks globally and it worked perfectly well because, at stock sizes above 50% of unexploited biomass, most fish can still fulfil their natural roles as prey or predator. We now believe that the MSY concept, if applied correctly, can be more useful to ecosystem-based fisheries management than other, data-hungry assessment methods.


Pauly: Since the early seventies, some fisheries practitioners have been modifying Schaefer’s model to a point that has led fisheries management agencies in several major countries to assume that MSY occurs when a stock’s size has been reduced to 40 or even 30 per cent of its unfished biomass. Yet, they let us believe that they rely on a model which is explicitly structured around ecological considerations implying that when a stock’s biomass is reduced below 50 per cent of carrying capacity, it is overfished.

This situation is aggravated by the fact that the official stock assessments that produced these low MSY estimates are also based on truncated time-series, which omits catches from earlier period of high abundance.

Froese: Another form of abuse of the MSY concept occurs when it is applied to an ensemble of discrete populations. One case that we refer to in the paper uses a combined MSY for numerous seamount-specific populations of orange roughy. This doesn’t make sense because the declining biomass of one population does not affect the density and population growth of the other, and this misuse of MSY led to the collapse of numerous stocks of orange roughy.


Pauly: In principle, most fisheries scientists and legislations agree that MSY should be a limit, and not a target for fisheries management because if it were a target, this target would be exceeded about half of the time just because of uncertainties in estimation and application, resulting in overfishing and stock decline. This implies that target catches should be set below and target biomass above the MSY level.

Also, at biomass levels of, for example, 60 per cent or more of carrying capacity, populations are much more capable of fulfilling their ecological roles than at the currently common 30–40 per cent levels, while at the same time supporting good catches.

Froese: Smart fisheries management, such as applied in Australia, aims for biomass 20 per cent higher than that needed to generate MSY. This allows them to maximize the economic benefits from fisheries and to radically reduce the risk of unintentional overfishing.


About Joann Glorioso

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