Validation of the periodicity of growth increment deposition in otoliths from the larval and early juvenile stages of two cyprinids from the Orange–Vaal river system, South Africa

Abstract

This study tested the hypothesis that growth increments in larval largemouth yellowfish Labeobarbus kimberleyensis and smallmouth yellowfish L. aeneus from the Orange–Vaal river system, South Africa, are deposited daily. The periodicity of increment formation was determined by regular sampling and counting of increments deposited in the otoliths collected from a total of 52 smallmouth and 41 largemouth laboratory-reared larvae. Increments were counted using a compound light microscope under transmitted light at 100–400 magnification. Linear regression models were fitted to the known age post-fertilisation and the age estimated using increment counts to test the correspondence between the two for each species. Regression models explained 89–99% of the variability in estimated age from increment counts, depending on the upper age cut-off applied to the data. The slopes of the regression models were significantly lower than 1.0, which indicated that increment counts were consistently lower than actual ages and this disparity increased for older individuals. It was, therefore, confirmed that larvae and juveniles of both largemouth and smallmouth yellowfish deposited increments on a near-daily basis, and that counts are a reliable estimate of age up to c. 100 d.