The Use of MSY in Fisheries Management

The concept of MSY has been imbedded in many national and international fisheries management regulations and agreements. For example, the Magnuson-Stevens Fishery Conservation and Management Act of the United States established the objective to maintain fisheries at optimum yield, which is defined (in the Sustainable Fisheries Act amendment) as the MSY from the fishery, as reduced by any relevant economic, social, or ecological factors. The MSY concept has frequently been included in fisheries management through the use of biological reference points based on the biomass (BMsY) and fishing mortality (FMsY) that produces MSY. These have been used to define overfishing (fishing mortality is greater than FMsY) and overfished (biomass is less than BMsY). Historically, these were used as target reference points in an attempt to manage fisheries at levels that would produce MSY. However, they have recently been considered too risky as target reference points, and are now considered limit reference points. A limit reference point indicates a limit beyond which the state of a fishery or population is not considered desirable and management action should be taken. It should be noted that, theoretically, catches above MSY are not sustainable, but fishing mortality rates above FMsY and biomass levels below BMsY can be sustainable. Many populations have been sustainably overfished, but some populations have also seen drastic collapses without recovery. Depensation or other factors that occur at low population sizes may cause unsustainability, and therefore fisheries managers are generally urged to take a precautionary approach to avoid low population sizes.

MSY and the associated quantities BMsY and FMsY require substantial information to produce estimates with the certainty required to make them useful for management advice. For many populations the estimates are simply too uncertain or not available at all. Therefore, several proxies have been developed to represent BMsY and FMsY. The rate of natural mortality (M) has been used as a proxy for FMsY, but this ignores most of the life history characteristics of the population and the characteristics of the fishery. Natural mortality is also notoriously difficult to estimate. The fishing mortality rate that produces the maximum YPR (/max) has also been used for a proxy for FMsY, but this assumes that recruitment is not reduced by fishing, and this proxy is likely to be higher than FMsY. The rate of fishing mortality that reduces marginal yield per recruit to 10% of its unfished value (F01) is a more conservative reference point that is commonly used. However, F01 ignores the information about the age at which the fish mature compared to the ages that are caught in the fishery. The fishing mortality that reduces spawning biomass per recruit (or equivalently the expected lifetime contribution to the spawning stock biomass for a single recruit) to a predetermined percentage of the unfished level (Fx%) takes the maturity information into consideration, but does not require an understanding of the SR relationship. As mentioned above, the SR relationship is often the most uncertain component of fisheries population dynamics. Simulation analysis is used to determine the percentage used to define Fx% that is robust to different SR relationships. F40% is commonly used in fisheries management (Figure 4).

MSY is a biological concept, and ignores the economic and social factors considered in managing fisheries. Management regulations and agreements often refer to optimal yield, which is MSY as modified by any relevant economic, social, or ecological factors. Optimal yield is a somewhat vague concept. Generally, yields only slightly less than MSY can be obtained with substantially less effort. The maximum economic yield (MEY), which takes into consideration the cost of fishing, generally occurs at a lower fishing mortality rate than that required to produce MSY because it trades off yield with effort to maximize profit. Other factors, such as discounting future yield, make the calculation of the economics of fishing

FMEY M FMSY F01 FMAX

FMEY M FMSY F01 FMAX

Figure 4 YPR and yield as a proportion of their maximum plotted against equilibrium fishing mortality as a proportion of the fishing mortality that would produce MSY (FMSY). The levels of several management reference points and FMSY proxies are indicated on the top of the figure.

Fishing mortality as a fraction of FMSY — YPR — Yield

Figure 4 YPR and yield as a proportion of their maximum plotted against equilibrium fishing mortality as a proportion of the fishing mortality that would produce MSY (FMSY). The levels of several management reference points and FMSY proxies are indicated on the top of the figure.

more complicated than MEY. Social factors are even more difficult to incorporate into these analyses.

Project Earth Conservation

Project Earth Conservation

Get All The Support And Guidance You Need To Be A Success At Helping Save The Earth. This Book Is One Of The Most Valuable Resources In The World When It Comes To How To Recycle to Create a Better Future for Our Children.

Get My Free Ebook


Post a comment