Maximum Sustainable Yield, MSY, is an old and remarkably bad idea as a goal for fisheries management. Yet it has recently been resurrected in European and global fisheries policy, probably because it sounds good and few people understand the implications.

The intent of MSY is much as it sounds – maximum sustainable catches. So far, so good. The problem is that it is a literally formulaic approach whose equations are based on heroically simplifying biological assumptions – it might predict the biggest crop that could be taken from bacteria grow in a flask kept topped up with nutrients in carefully controlled conditions, but that’s about it. MSY doesn’t take into account natural fluctuations, or that the maximum yield from any fishery will change if we also fish the species that they feed upon; fish those which prey upon it; or indeed fish all three levels. These all undermine the assumptions behind MSY and have a major impact on the outcome. Few fisheries scientists or economists are enthusiastic about it as a target of fisheries management, although if it was used as a ‘limit reference point’ – meaning that stocks should never be allowed to drop lower that that which could support the (theoretical!) MSY – that would be an improvement on the current situation.

Economically, as fish get rarer, if it costs more to catch them, then it is usually assumed that the greatest profits, the Maximum Economic Yield, MEY, will come from smaller catches than MSY. This leaves more fish in the sea, so in theory MEY is better both economically and environmentally than MSY. Practically, as with MSY, the necessary real data to calculate it is often not known, requiring dubious assumptions to be made. Moreover MEY doesn’t get over the problem of the needs and interactions between other species, and indeed general fluctuations and uncertainty. For this reason one has to leave even bigger stocks than indicated by the MEY, as explained in the FAO’s Technical Guidelines.

However, all of this assumes that the goal of fisheries is indeed for each stock to be fished sustainable. It may even be believed that the system is self-regulating, and that economics kick in to prevent over-fishing. Yet this has rarely happened, at least over the last century. There are various reasons why. Stocks may be serially depleted, perhaps in the belief that stocks will bounce back while attention is diverted elsewhere. Indeed, fisheries until now has been very financially rewarding for the bigger operators, due to a combination of subsidies, moving to new locations, and ‘fishing down the food chain’. Indeed, species lower down the food chain may actually be worth more (for example shellfish may be worth more that the fish that eat them) – in which case there may be little interest in sustaining maximum yields of their predators. Alternately – if greater economic returns come from some other business – the goal may simply be to mine the natural ‘capital’  and turn it into into cash as rapidly as possible so that this can be invested elsewhere. Or it may be completely irrational behaviour, induced (as one senior fisheries manager said) “by the most powerful drug known to man – money”.