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year time horizon. The area of reserve required increased with fishing intensity. Furthermore, the greater the uncertainty in fishing mortality (which is equivalent to decreasing management control), the larger the reserves required.
Recommended maintaining exploited populations at 75% of their unexploited size in order to avoid recruitment overfishing.
Guénette et al., 2000:
Used a spatially explicit model to examine whether reserves could have prevented the collapse in 1992 of the migratory northern cod (Gadus morhua) population off eastern Canada. Found that, in the absence of other management measures, reserves covering 80% of the area would have been necessary, but that with temporal closures to trawls and gill nets, reserves covering 20% of the area would have been adequate.
Looked at the use of reserves as a tool to maintain fish populations above target levels. Found that if a stock was initially heavily fished (i.e., starts at 35% of its unfished size) reserves of 20 and 30% of the management area guaranteed persistence above this level for 20 and 100 years, respectively. The greater the level of population desired, the longer the planning horizon, and the higher the degree of variability in fishing mortality (= less control over fishing), the larger reserves are required to maintain target populations. Reserves increased cumulative yields from the fishery when populations were initially heavily exploited.
Used fishery models to estimate that maintaining fish populations above 20% of their unexploited size would avoid recruitment overfishing.
Mace and Sissenwine, 1993:
For 91 fish populations (representing 27 species) in North America and Europe, calculated that the average threshold replacement stock size corresponds to a 20% spawning potential ratio (one fifth of unexploited population size). Maintaining at least a 30% spawning potential ratio would avoid recruitment overfishing for 80% of these species; therefore, a 35% spawning potential ratio would be a conservative management target. However, safe minimum population levels ranged as high as 70% for some species.