Abstract
Estimates of growth over a short period are often difficult to obtain from fish unless individuals are tagged or experimental conditions are highly controlled. We demonstrate that the outer edge of scales (the osseoid layer) in tilapia (Oreochromis sp.) can be used to recognize nongrowing fish. We measured the width of the osseoid tissue at various positions of the anterior edge of the scale as well as inward to the first circulus on the edge of the scale in fed and starved fish. Our results indicated that response of the osseoid layer to growth rates occurred in 1 wk or less. Discriminant analysis based solely on osseoid variables classified over 84% of ail fish correctly on the basis of the feeding treatment after 8 d of growth. This technique was field-tested in cage-cultured tilapia in Java, Indonesia. We were able to classify correctly 86% of the growing and nongrowing fish in the population after a 21-d period. These measurements can supplement the information collected from the spacing of circuli on the calcified portion of the scale to completely specify the recent growth history of individual fish.
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