The journey to set fisheries targets and limits has been a long one that has evolved considerably, particularly in recent years. The journey began early in the last century and, for the first 40 years, was focussed almost entirely on the concept of maximum sustainable yield (MSY) and its fishing mortality (FMSY) and biomass (BMSY) derivatives. This concept has, and still does, come in for a disproportionate amount of criticism. However, it has become enshrined in the United Nations Convention on the Law of the Sea (UNCLOS) and many other pieces of international and national legislation, and has recently made a resurgence partly as a result of the implementation of the Johannesburg declaration of 2002, which requires fish stocks to be maintained or restored "to levels that can produce the maximum sustainable yield" by 2015.
Along the way, a number of alternative targets have been developed, some as proxies for MSY and its derivatives and some as alternative approaches. Different jurisdictions have chosen different reference points as a focus. Some have tended to favour F-based targets and some B-based targets. In ICES member countries in Europe, on the east coast of North America and elsewhere, targets have historically been based on fishing mortality. On the west coast of North America and in Australia, New Zealand and other countries, there has been a tendency to favour targets based on biomass.
Limit reference points are a more recent innovation. It could be argued that limit reference points are more powerful than target reference points, especially if they are supported by legislation or policy that requires fisheries managers to ensure that there is a high probability they are not violated. Targets, on the other hand, generally only need to be achieved on average and being somewhat above or below the target is often not treated seriously. Both targets and limits are essential for multispecies, ecosystem, economic, and social considerations.
Between about 1980 and 2000, a plethora of alternative reference points were developed, particularly by ICES. Most of these were F-based with some being targets and some limits. Introduction of the precautionary approach resulted in further developments including some former targets becoming limits. Since ICES has progressively moved towards adopting MSY-based targets over the last decade, many of the former F-based targets have fallen into disuse, or are only used for a few stocks. The development and adoption of limit reference points has however continued apace.
Over the last two decades, target and limit reference points have been incorporated into harvest control rules that may include some or all of a biomass target, a biomass limit, a fishing mortality target, and a fishing mortality limit, as well as management actions depending on the stock status relative to the reference points. I believe that, for most stocks, targets should primarily be based on fishing mortality and limits should be based on biomass. Well-managed stocks should fluctuate around some biomass level associated with BMSY or higher, and the easiest way to ensure that the fluctuations are not extreme is to maintain fishing mortality within a relatively narrow range. Biomass limits are needed to ensure that stocks remain viable and that they continue to fulfil their ecosystem roles.
I maintain that defining and implementing targets and limits for our world's wild fisheries has been instrumental in ending overfishing and rebuilding depleted fish stocks, particularly in the last two decades. This has been achieved due to considerable improvements in fisheries science and modelling, increased involvement of stakeholders and the public in fisheries management decisions, increased awareness and, perhaps most importantly, stronger legislation, policies, and binding and voluntary international agreements with time-bound deadlines for implementation. Although there has been substantial progress, there is still much that needs to be accomplished and many challenges for the future.
Pamela Mace, Principal Adviser Fisheries Science at the New Zealand Ministry for Primary