Fish Population Dynamic and Stock Assessment

1.4.7. Maximum Sustainable Yield (MSY)

MSY is the maximum long-term average yield that can be achieved through conscientious stewardship by controlling F through regulating fishing effort or total catch levels. MSY is a reference point for judging the potential of the resource.  However, it is not necessarily the goal of fishery managers to always set the maximum yield.  Other factors influence the choice of a management objective, such as socioeconomic considerations or conservation and ecosystem concerns for other marine life indirectly affected by fishery harvests.  

One of the major objectives of fish stock assessment is regularisation or optimisation of effort.     

Standardization of fishing effort – increasing the effort in the long term gives highest yield. The basic principle is smaller a population, when effort is not put at required level, the greater will be production. At optimal level of effort, the production will be maximum for a given Fmsy.  

For all practical purposes, the effort should be monitored at various levels of fishing mortality.

The basic principle in stock assessment models is to provide estimates of optimum yield. The environmental factors, economic factors are to be taken into account for arriving at appropriate management decisions.

When intensity of fishing is not monitored, the population of a stock show sign of depetion and there will be a decrease in mean length of fish and length at minimum maturity.   

In principle as age increases the number of survivors will be less.

In a virgin (unfished) biomass, the population is at equilibrium. The population neither grows nor declines which means that each year’s recruitment is balanced by each year losses due to mortality.   

When a stock is brought below b_¥ level, the population grows. The maximum surplus production is achieved when the stock biomass is reduced to ½ of the level of B_¥.

In principle, the highest yield achieved on a long term basis for a particular effort is the Fmsy and the corresponding yield is MSY or Maximum Sustainable Yield.