This goal was called MSY or maximum sustainable yield. 1. the size of a fish population, and 2. Calculating MSY proved to be extremely difficult. Instead of being a fixed quantity, MSY varies in relation to environmental conditions and the interaction of species. He notes how it is difficult to model species interactions.
Reserves serve as a hedge against the limitations of management.
For example, one standard management policy which promotes conservation is gear modification, such as increasing the size of mesh on nets. This decision may be quite helpful, but it cannot solve all the problems. The nets still catch slow-reproducing and exploited species such as sharks. Reserves ensure there are some places where no sharks are caught. Reserves conserve more than fish, but also habitat which is needed especially by young fish.