Population 



150 

 100 



50 







150 - 



100 - 



50 







150 



100 



50- 



WEEK 32 



y^^A 



WEEK 33 



WEEK 34 P 



l^^^'i 



^TTTPi 



m 



M 



M 



^L 



rzTzi 



m 



Fry Im Ad Fry Im. Ad 

 SIZE CATEGORY 



Fry Im Ad 



Figure o. — Composition accorrling to categories "fry," 

 "immature," aiul "adult" (defined in section "Experi- 

 mental diet and procedures") of populations at the 1..') 

 diet level, immediately before I'xploitation. 



the other six populations. I decided to proceed 

 with exploitation of the 1.5 level group because 

 of the substantial amount of time and effort 

 already invested and the desire to have yields 

 comparable with those for the other populations. 

 This decision was supported by the fact that com- 

 jiositions were fairly stable within populations 

 (fig. 5) even though discrepant between them. 



Response of the populations to exploitation is 

 indicated by the mean numbers and weights for 

 each chet level (tables 6 to 9 and fig. 6). The 

 saw-tooth pattern of reduction by removals and 

 subsequent recovery is characteristic. As pointed 

 out by SilUman and Outsell (195S), this kind of 

 variation reflects the resihence of natural popu- 

 lations as long as exploitation rates are not high 

 enough to cause extinction. 



As was also mentioned by Silliman and Outsell, 

 jjopulation weights are more stable than popula- 

 tion numbers, since the latter are affected more 

 by entrance and mortality of broods of fry. The 

 weights reveal the typical decline in population 

 size after the incejition of exploitation, followed 

 by near stabihty during the final weeks of the 

 experiment. Although populations at all three diet 

 levels decreased in biomass under exploitation, 

 they maintained the preexploitation rank, wliich 

 was the same as that of tlie diet levels. 



200 



150 = 



m 100 

 2 



2 50 

 < 





 40- 



3 30 

 5 20 



s 



2 10- 



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



WEEK NUMBER 



68 70 72 



Figure 6. — Response of populations to exploitation. Data are means for each diet level. 

 FOOD LEVEL AND EXPLOITATION IN FISH POPULATIONS 



433 



