SILLIMAN: EXPERIMENTAL EXPLOITATION OF FISH POPULATIONS 



280- 

 240 

 1200 



U- 



U.160 

 o 



£ 120 



CD 



I 80 



z 



40 



35 

 30- 



_25- 



E 

 |20H 



El5- 



i£ 



Ui 



glO- 

 5 



240 



WEEK 



3-weeh period 80 



Figure 3. -Course of independent swordtail population. 

 Numbers indicate target exploitation rates. 



Competing Populations 



The two mixed populations, each composed of 

 both guppies and swordtails, were started 24 Oc- 

 tober 1965 (week 0). Three-week period started 

 with week 1, since weights were not recorded in 

 week 0. 



In the control (unexploited) pair of populations 

 (Figure 4), both species grew for an initial period 

 of about 30 wk. The swordtail population then 

 began to decline and disappeared at week 129. This 

 will be discussed in the section on competitive 

 exclusion. Extinction of the swordtail was 

 followed by a large oscillatory fluctuation in the 

 guppy (about -39"^^ to +269f of the asymptotic 

 level), which will be discussed in the section on 

 such fluctuations. 



Initial growth in the test (exploited) pair was 

 similar to that in the control pair (Figures 4, 5). 

 Exploitation was started first on the swordtail 



360- 

 320 

 280- 



^ 240- 

 ^ 20O 



Ll. 



o 



cr 160H 



UJ 



m 



5 120- 



Z. 



80- 



40- 



0- 



WEEK 



3-week period 6 



—1 — 

 10 



— 1 — 

 20 



30 



40 



— I — 

 50 



I 

 60 



70 



Figure 4.— Course of competing populations, control pair. Solid line, guppy; broken line, swordtail. 



877 



