RNA–DNA Ratio in Scales from Juvenile Cod Provides a Nonlethal Measure of Feeding Condition

We removed scales from juvenile Atlantic cod Gadus morhua, extracted RNA and DNA from tissue associated with the scales, and found that the ratio of RNA to DNA (R/D) reflects the feeding condition and growth of the fish. Moreover, R/D was found to be a more sensitive indicator of feeding condition than either weight or length. We performed a series of experiments in which the diets of juvenile cod (mean total lengths, 10.4–18.8 cm) were manipulated. At regular intervals, the fish were weighed and measured, and scales were removed from each individually marked fish. In fish that were either fed or starved for 49 d, R/D was significantly correlated with growth in length for individual fish (r2 = 0.27, P < 0.0001) and tank means (r2 = 0.59, P < 0.0001). The ratio of RNA to DNA was also sensitive to changes in feeding condition. Fish that were initially starved had significantly (P < 0.05) higher R/D after 2 d of feeding, and fish that were initially fed had significantly (P < 0.05) lower R/D after 7 d of no food. Finally, we found that the R/D of epidermal scrapings reflected the feeding condition of small fish (mean (±SD) total length, 2.76 ± 0.18 cm) that had not yet developed identifiable scales. This technique of measuring RNA and DNA in extracts of scale‐associated tissue provided a sensitive, nonlethal means of estimating feeding condition and growth.