A new role for MSY in single‐species and ecosystem approaches to fisheries stock assessment and management

Abstract

In 1977, Peter Larkin published his now‐famous paper, ‘An epitaph for the concept of maximum sustained yield’. Larkin criticized the concept of single‐species maximum sustained yield (MSY) for many reasons, including the possibility that it may not guard against recruitment failure, and the impossibility of maximising sustainable yields for all species simultaneously. However, in recent years, there has been a fundamental change in the perception of the fishing mortality associated with MSY (FMSY) as a limit to be avoided rather than a target that can routinely be exceeded. The concept of FMSY as a limit is embodied in several United Nations Food and Agriculture Organization (FAO) agreements and guidelines, and has now been incorporated into the US Magnuson–Stevens Fishery Conservation and Management Act. As a result, the United States now requires the development of overfishing definitions based on biological reference points that treat the FMSY as a limit reference point and must also define a lower limit on biomass below which rebuilding plans with strict time horizons must be developed. This represents a major paradigm shift from the previously mandated (but often unachieved) objective to simply maintain fishing mortalities at levels below those associated with recruitment overfishing. In many cases, it requires substantial reductions in current fishing mortality levels. Therefore, the necessity of the new paradigm is continually questioned. This paper draws on examples from several fisheries, but specifically focuses on the recent US experience illustrating the practical difficulties of reducing fishing mortality to levels below those corresponding to MSY. However, several studies suggest that even more substantial reductions in fishing mortality may be necessary if ecosystem considerations, such as multispecies interactions, maintenance of biodiversity and genetic diversity, and reduction of bycatch and waste, are taken into account. The pros and cons of moving beyond single‐species assessment and management are discussed. A US plan for improving stock assessments indicates that even a ‘basic’ objective such as ‘adequate baseline monitoring of all managed species’ may be extremely costly. Thus, the suggestion of Larkin (1983, 1997) that the costs of research and management should not exceed 10–20% of the landed value of the catch may preclude comprehensive ecosystem management. More importantly, neither single‐species nor ecosystem‐based fisheries management is likely to improve appreciably unless levels of fishing capacity are aligned with resource productivity, as is currently being promoted by FAO and several individual nations.

Effect of complexity on marine ecosystem models

“…In general, such models improve our understanding of systems by reflecting the 2-way nature of system dynamics. Human impact on one part of a system can spread to 3 other parts of the system, but system feedbacks and interrelations can also mean that an impact can have unexpected consequences even for those groups directly affected by anthropogenic activities (Hollowed et al 2000, Fulton 2001, Mace 2001. There are also a number of potential problems common to multispecies and ecosystem models: (1) they inevitably require more information than single-species models (which incur associated costs); (2) they are more likely to suffer from issues about optimal complexity (i.e.…”

“…There are also a number of potential problems common to multispecies and ecosystem models: (1) they inevitably require more information than single-species models (which incur associated costs); (2) they are more likely to suffer from issues about optimal complexity (i.e. what groups and processes should be included); (3) operational objectives and monitoring indices can be hard to define for the real systems to be modelled; (4) it can be difficult to define appropriate indices to summarise model output; (5) there are often alternative hypotheses about system structure and function (Silvert 1981, Jørgensen 1994, Mace 2001. These features are particularly important when dealing with ecosystem models.…”

“…These features are particularly important when dealing with ecosystem models. However, as multispecies and ecosystem models are the only models with the potential to answer the environmental questions that single-species and pure hydrodynamic models cannot (Hollowed et al 2000, Mace 2001), the advantages of intelligent and attentive application of such models can outweigh their potential pitfalls. One of the main criticisms aimed at ecosystem models is that their potentially immense complexity can make predictions highly uncertain (Duplisea 2000).…”

“…Unfortunately, although the need to integrate comprehensive biological, physical and chemical models is recognised, reconciling and reducing the dimensions of complexity required in each of the areas is a large but poorly understood task (Nihoul & Delhez 1998, Mace 2001. Much of the handling of model complexity has been dealt with by drawing on experience from prior modelling efforts (Murray & Parslow 1997, 1999a.…”

Incorporating uncertainty into the estimation of biological reference points for a spiny lobster(Panulirus penicillatus)fishery

“…The importance of incorporating uncertainty into fisheries stock assessments has been well documented in the literature (Hilborn & Walters 1992;Restrepo 1999;Mace 2001;Heiser et al 2001). Per recruit analysis is commonly used to estimate yield, spawning biomass or egg production (Fogarty & Idoine 1988) an average recruit may contribute over its life span (Beverton & Holt 1957).…”