STATISTICAL INTERRELATION OF LENGTH, GROWTH, AND SCALE CIRCULUS SPACING - USE OF OSSIFICATION TO DETECT NONGROWING FISH

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 n ongrowing 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 r esults indicated that response of the osseoid layer to growth rates oc curred in 1 wk or less. Discriminant analysis based solely on osseoid variables classified over 84% of all fish correctly on the basis of th e feeding treatment after 8 d of growth. This technique was field-test ed in cage-cultured tilapia in Java, Indonesia. We were able to classi fy 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 f ish.