Use of internal reference population for growth rate comparison of tilapia strains. I. In a crowded environment

This paper reports an experimental design in which reference fish are included in each replicate to provide internal statistical control over environmental variation. Growth of 10 full-sib families from each of 3 strains of Oreochromis niloticus were compared in a crowded environment. A third strain of red tilapia was mass-spawned to provide reference fish. From each family, 25 equal-sized tilapia fry were matched with 25 equal-sized red tilapia fry and reared for 3 weeks in small floating cages inside a bigger tank. Growth of test strains were positively correlated with growth of the reference population. The 3 strains differed significantly in their growth rates. The reference fish technique is useful in experiments in ponds or cages, where variable environmental factors induce positive correlations between reference and test strains.