SELECTIVE AND UNSELECTIVE EXPLOITATION OF 

 EXPERIMENTAL POPULATIONS OF TILAPIA MOSSAMBICA 



Ralph P. Silliman' 



ABSTRACT 



Two populations of Tilapia mossambica were grown under controlled conditions. After a period of 

 growth and stabilization at about 10 kg and 200 fish, exploitation was started; about 50 fish of outside 

 stock were added to each population to increase genetic variability. 



Initial exploitation at 10% (later 20%) per 2 mo encompassed all sizes above fry in the unselectively 

 fished population. In the selectively fished population, exploitation was practiced only on fish that could 

 not pass through 25-mm Oater 22-mm) vertical slots between glass bars. 



Recruitment was estimated from data of stock, mortality, and catch. Parabolas fitted to the stock- 

 recruitment relation suggested greater recruitment in the selectively fished stock than in the unselec- 

 tively fished one. 



Rectilinear thickness-length regressions were calculated for immature and male fish and separately 

 for females. 



The exploitation-yield relation was assessed by fitting Fox surplus-yield models to both populations. 

 These revealed a greater maximum sustainable yield in weight from the unselectively fished population 

 than from the selectively fished one. Efficiency of food conversion was 29-36%. 



To test for genetic effect of selection, a group of 46 fish, matched as closely as possible in size and sex 

 composition, was selected from each population. Growth in length over a period of 150 days was 

 significantly greater among males from the unselectively fished population than among those from the 

 selectively fished one. Growth in length of females was practically identical for both groups. Growth in 

 total weight was distinctly greater for the group from the unselectively fished population than from the 

 selectively fished one. 



As applied to commercial fisheries, the experimental results suggested fishing as wide a range of sizes 

 as possible. If economic gains from selection are indicated, they should be balanced against possible 

 costs in reduced yield and retarded growth rate. 



Controlled selective breeding for desirable at- 

 tributes in plants and animals is a well-recognized 

 technique in agriculture. This technique has also 

 had limited application in fish culture, particularly 

 with trout. Claimed achievements have included 

 increased size and earlier age at maturity. Fishery 

 biologists have speculated whether the reverse 

 process, attainment of undesirable attributes, 

 may have occurred in some fished populations 

 because of inadvertent imposition of selection by 

 the fishery. Although gill nets and trawls are 

 perhaps the most obvious gear elements causing 

 selection, the phenomenon is probably present to 

 some extent with practically all fishing gears. It 

 thus becomes a matter of considerable economic 

 importance to determine if gear selectivity has 

 adversely affected fish stocks. 



The general subject of selection for slow growth 

 by fishing was briefly reviewed by Miller (1957). 

 He adduced no data, however, and drew no firm 



'Northwest Fisheries Center, National Marine Fisheries Ser- 

 vice, NOAA, Seattle, WA 98112; present address: 4135 Baker 

 NW, Seattle, WA 98107. 



Manuscript accepted December 1974. 

 FISHERif BULLETIN: VOL. 73, NO. 3, 1975. 



conclusions, merely noting that he knew of no in- 

 stances where changed growth rates in fish could 

 not be attributed to some cause other than genetic 

 change. It seems entirely possible, nevertheless, 

 that such a change could occur, if selection were of 

 sufficient strength and continued during a 

 sufficient number of generations. Such a pos- 

 sibility is indicated by the success of artificial 

 selection in altering quantitative characters in a 

 wide variety of organisms. 



The purpose of the work reported herein was to 

 test experimentally whether selective fishing 

 could produce a genetic change in the growth pat- 

 tern of the fish in the fished stock. This problem' 

 was approached by growing two populations of 

 Mozambique tilapia, Tilapia mossambica, under 

 as nearly identical conditions as possible. One of 

 these was then fished selectively from only those 

 fish above a fixed thickness. The other was fished 

 over the entire range of sizes, except the small 

 "fry." 



A secondary purpose was to compare amount 

 and size composition of the yields under selective 



495 



