188 Fishing and Natural Mortality 



fish of desirable 

 sizes 



stunted 

 fish 



fish of 

 desirable 

 sizes 



rapid 

 growth 



slow 

 growth 



Underfishing 

 With Low Natural 

 Predation and 

 Stable Water 

 Levels 



Planned 



Cropping 



or High Natural 



Predation with 



Fluctuating 



Water Levels 



Overfishing 

 With Annual Fishing 

 Pressure Exceeding 

 1000 Man-hours 

 Per Acre 



Figure 7.1. Diagrams representing theoretical fish populations 

 superimposed on segmented triangles proposing the first six year 

 classes of fish populations reduced ( space ) on a 50 per cent mortality 

 rate. Underjished population is characterized by high survival of all 

 year classes and stunting, with few fish attaining desirable sizes. 

 Planned cropping and/or high natural predation with fluctuating water 

 levels produces a population, the members of which grow rapidly to 

 large average sizes. Overpopulation is prevented, and, therefore, an 

 abundance of food is usually available. Overfished population is char- 

 acterized by over abundant year classes of very young fish showing 

 slow growth until they reach sizes large enough to interest anglers. 

 At this point, many are taken and the few that escape grow rapidly to 

 large average sizes. 



