作者： Ralph P. Silliman
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摘要: Two populations of Tilapia mossambica were grown under controlled conditions. After a period growth and stabilization at about 10 kg 200 fish, exploitation was started; 50 fish outside stock added to each population increase genetic variability. Initial 10% (later 20%) per 2 mo encompassed all sizes above fry in the unselectively fished population. In selectively population, practiced only on that could not pass through 25·mm 22·mm) vertical slots between glass hars. Recruitment estimated from data stock, mortality. catch. Parabolas fitted stock. recruitment relation suggested greater than unselec tiveiy one. Rectilinear thickness-length regressions calculated for immature male separately females. The exploitation·yield assessed by fitting Fox surplus-yield models both populations. These revealed maximum sustainable yield weight Efficiency food conversion 29·36%. To test effect selection, group 46 matched as closely possible size sex composition, selected Growth length over 150 days significantly among males unselectiveiy those females practically identical groups. total distinctly As applied commercial fisheries, experimental results fishing wide range possible. If economic gains selection are indicated, they should be balanced against costs reduced retarded rate. Controlled selective breeding desirable at tributes plants animals is well-recognized technique agriculture. This has also had limited application culture, particularly with trout. Claimed achievements have included increased earlier age maturity. Fishery biologists speculated whether reverse process, attainment undesirable attributes, may occurred some because inadvertent imposition fishery. Although gill nets trawls perhaps most obvious gear elements causing phenomenon probably present extent gears. It thus becomes matter considerable importance determine if selectivity adversely affected stocks. general subject slow briefly reviewed Miller (1957). He adduced no data, however, drew firm