Many fish stocks are in decline worldwide. Overfishing has led to the completecollapse of numerous fisheries including those for important and well-publicizedspecies. In addition, over-exploitation of fisheries has led to reductions in biodiversity and modified ecosystem functioning. Despite management attempts to reduceoverfishing, little progress has been made due to a general inability to endure theshort-term economic and social costs of reduced fishing. Indeed, historical analysisreveals that resources are consistently and inevitably over-exploited. There is aneed for models to examine the dynamics of this complex fishery issues and designpolicies for sustainable fishery development (Dudley 2008).The model developed here is based on the biomass dynamic model (Graham1935; Schaefer 1954, 1957), in which the population biomass increased due to bothgrowth and the addition of new fish and decreases due to natural mortality andcatch. Growth is equal to biomass fractional growth times existing biomass, and thenatural decrease is growth multiplied by the ratio of biomass population to maximum biomass population possible. Catch depends on the current level of populationbiomass, number of gear units and fishing gear efficiency. The fleet entering orleaving depends on catch per unit effort and vessel replacement. In some fisheries,fishing activity decreases ecosystem carrying capacity.
Draw the causal loopdiagram of the fishery management system.