0.5 DIET 



Exploitotjon roles per 3 weeks 



0.25 



33 



50 



28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 



WEEK NUMBER 



FiGUBE 7. — Yield per three weeks of each population during period of exploitation. 



Figure 8. — Mean yields for three 14- or 1.5-\veek periods. 

 Location of vertical lines along horizontal scale indicates 

 center of each period. 



free from the irregtilarities in removals and mor- 

 talities that occurred at the 1.5 diet level during 

 weeks 37 to 57 (table 7, footnotes). 



RELATION BETWEEN YIELD AND FOOD LEVEL 



Data in the preceding section showed that 

 yields at the 1.0 and 1.5 diets were related to 

 exploitation rate, but that no such relation was 

 detectable at the 0.5 diet. The available data may 

 now be brought together in an attempt to answer 

 such questions as that posed in the introduction: 

 "Might it be possible, for instance, to harvest a 

 greater percentage of the stock when food supply 

 and abundance are high than when they are low?" 

 It is instructive here to relate the yields to the 

 average total weight or biomass of the popula- 

 tions (table 10). Because the populations were 

 allowed to reach asymptotic size or a close ap- 

 proach to it, an additional point for each yield- 

 biomass curve is available — that at zero rate of 

 exploitation. If small deviations are charged to 

 random variability, the appropriate curves (fig. 9) 

 reveal a regular relation among exploitation rate, 

 biomass, and yield at each diet level (curves fitted 

 by inspection). 



The curves suggested that the relation of 

 jdeld to exploitation rate tends to be independent 

 of diet level. Absolute yields were obviously 

 dependent on aniotmt of food available, but the 

 greatest yield at each diet level occurred at or 



FOOD LEVEL AND EXPLOITATION IN FISH POPULATIONS 



435 



