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Unformatted text preview: BIOECONOMIC MODELS (continued) These bioeconomic models that we have developed are all for longrun equilibrium situations. To examine the shortrun dynamics we need a model for changes in fishing effort. Clark (1985) discusses the following simple model for a fishery system. Change in Biomass: dB dt r B ⋅ 1 B K ⋅ q f ⋅ B ⋅ = Change in Effort: df dt a p q ⋅ B ⋅ c ( ) ⋅ f ⋅ = The differential equation for biomass is just the normal GrahamSchaefer model using the assumption that the instantaneous rate of fishing mortality is proportional to fishing effort (which here is the number of active fishing operations). In the differential equation for fishing effort the rate of entry and exit of effort is proportional to the current flow of profits, and parameter a is the constant of proportionality. It is sometimes described as a stiffness parameter because its role in the equations is similar to the stiffness of a spring. These two differential equations are said to be coupled differential equations , meaning that both equations involve both dependent variables., meaning that both equations involve both dependent variables....
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 Fall '09
 dt, isoclines, fishing effort

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