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Abstract
Uncertainty has long been recognised as an important aspect of renewable resource
assessment and management. Stochastic optimal control provides a framework in which to
incorporate uncertainty, whether arising from fluctuations in the biological or economic
environment or from lack of a precise understanding of inter-relationships within a system.
However, overlaying complex and interdependent biological, physical and economic
relationships with uncertainty often results in an optimal control problem which is
analytically complex.
In this paper, a parametric approximation to the control equation is combined with genetic
search algorithms to solve the stochastic control problem. The parametric approximation to
the solution of optimal control problems is compared with a collocation approach. The use
of these two numerical solution techniques is explored in the context of a harvest model for a
multi-species fishery.
While the two techniques yielded similar solutions, they offered different advantages and
disadvantages. The use of collocation methods facilitates the understanding of the problem
and the nature of the solution. However, for multi-dimensional state space problems,
collocation techniques require exponentially increasing computational time. Parametric
approximation techniques require prior specification of an explicit relationship between the
state and control variables. As a result, the approximation may impose or miss features of
the solution. However, when combined with a genetic search algorithm, the technique is very
robust and computation time is significantly less than for the collocation technique. The use
of collocation techniques to characterise the solution to the problem followed by the
application of an appropriate approximation technique may to prove to be an expedient
method for dealing with larger scale problems.