Relative RNA Content as a Measure of Condition in Larval and Juvenile Fish

Abstract

The RNA:DNA ratio for first-feeding larvae (12 days after hatching, 4-6 mm standard length, SL) of Australian bass (Percichthyidae, Macquaria novemaculeata) exposed to four different feeding regimes over 8 days was found to be insensitive to the level of starvation. An alternative condition index based on residuals derived from the regression of RNA on SL showed significant differences over the course of the experiment; these reflected the four feeding treatments. Field collections of juvenile monacanthids (Paramonacanthus otisensis, 10-30 mm SL) from a local estuary revealed no significant difference in RNA: DNA ratio at three sites over six weeks. A residual-based index (RNA on dry weight) showed parallel fluctuations at all sites; they were positively correlated with water temperature. The RNA : DNA ratio depends on the difference in fluorescence between total nucleic acids (TNA, using thiazole orange) and DNA (using Hoechst 33258) to calculate RNA, as there is no RNA-specific fluorescent dye. The numerator is thus dependent on the denominator, and measurement error may be compounded in the ratio, exacerbating potential variability in the index. Ratios may also be variably correlated with age or size and consequently may erroneously indicate condition or growth in larger and faster-growing fish.