SILLIMAN: EXPERIMENTAL POPULATIONS OF TILAPIA MOSSAMBICA 



Table 2.-Water condition on selected dates.' 



O, (ppm.) CO, (ppm.) 



pH 



Date 



Month Test2 Control 3 Testz Control Test^ ControP 



1968: 



9 Aug. 

 16 

 29 



6 

 13 

 20 

 27 



4 

 11 

 18 

 25 



8 

 15 

 28 



6 



13 



26 



1969: 



2 Jan. 

 17 Sept. 



2 Oct. 



9 



23 



30 



1971: 



25-26 Feb. 

 1972: 



23 Mar. 

 1973: 

 29 May 



Sept. 



Oct. 



Nov. 



Dec. 



24.3 

 24.5 

 24.9 

 25.2 

 25.4 

 25.7 

 25.9 

 26.1 

 26.4 

 26.6 

 26.8 

 27.3 

 27.5 

 27.9 

 28.2 

 28.4 

 28.8 



29.1 

 37.6 

 38.1 

 38.3 

 38.7 

 39.0 



54.9 



67.7 



81.9 



5.2 

 4.6 

 4.8 

 3.6 

 3.6 

 4.0 

 4.0 

 4.0 

 3.8 

 3.0 

 3.0 

 3.6 

 3.4 

 4.2 

 3.8 

 3.4 

 3.0 



3.4 

 5.2 

 5.4 

 5.0 

 5.0 

 5.4 



4.6 



3.4 



3.4 



5.0 

 4.4 

 4.8 

 3.8 

 3.6 

 4.4 

 4.0 

 4.4 

 4.0 

 3.8 

 4.0 

 4.0 

 4.0 

 4.4 

 4.0 

 4.0 

 3.4 



3.6 

 4.8 

 5.4 

 4.8 

 4.8 

 5.0 



4.4 



"2.6 



3.2 



10 



10 



10 



6.9 



6.9 



10 — 



10 



10 



6.5 



6.7 



20 

 20 

 15 



20 

 25 

 15 



6.0 

 6.5 

 7.0 



6.0 

 6.5 

 7.0 



'With the exception noted in footnote 4, all values were within 

 (or above for oxygen) the ranges stated to be suitable for warm- 

 water fishes by Lewis (1963). These were: oxygen, 3-5 ppm.; car- 

 bon dioxide, below 30 ppm.; pH, 5-9. 



^Selectively fished. 



^Unselectively fished. 



■•Aeration was increased and O^ had risen to 3.8 ppm. by the 

 next day. 



were anesthetized with MS-222 ^ (tricaine 

 methanesulfonate) in a 1:2,500 solution. Caught 

 fish were measured some time after removal. They 

 were usually alive or freshly dead, and rigor mortis 

 was rare. The group selected for growth study at 

 the end of the experiment was measured alive 

 without an anesthetic. 



COURSE OF POPULATIONS 



A single population was started on 15 August 

 1966, but this was divided into two populations as 

 nearly equal as possible after 2 mo. A period of 

 population growth then ensued (Table 3, Figures 

 1, 2). This growth was extensively discussed by 

 Silliman (1970), who found growth in biomass of 

 the two populations to be practically identical. He 

 therefore combined the two populations for 

 growth analysis. A Gompertz curve fitted to the 

 total biomass of both populations had the formula 



^Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service, NOAA. 



Wi = 1.337 exp[2.85 - 2.85exp(-0.2 (^-3.6) )], where 

 Wf is biomass in kilograms at the time t in months. 

 The asymptote of this curve was 23.1 kg or 11.55 kg 

 per population. An accidental interruption to 

 population growth (Figures 1, 2) resulted from 

 temporary relocation of the fish during refinishing 

 of the laboratory floor. After recovery and re- 

 equalization, the populations did not approach the 

 asymptote predicted by the Gompertz curve but 

 leveled off at about 10 kg each. 



Because the fish were descendants of an original 

 three males and three females, I felt that in- 



400 



300 



200 



100- 



O 



a: 



2 300- 



200- 



100- 



Control 



10 



20 



30 40 50 60 



MONTH 



70 80 



Figure l.-Population size and catch, numbers. Percentages in- 

 dicate target exploitation rates. Test population was selectively 

 fished; control, unselectively. 



Figure 2.-PopuIation size and catch, weight. Percentages in- 

 dicate target exploitation rates. Test population was selectively 

 fished; control, unselectively. 



497 



