Effects of variable ration levels on direct and indirect measures of growth in juvenile red drum (Sciaenops ocellatus)

Abstract

Otolith growth is often used as a proxy for fish growth; however, for red drum (Sciaenops ocellatus) no experimental studies have demonstrated a coupling of somatic and otolith growth under variable environmental conditions. My objective was to examine relationships between somatic growth (length and weight) and two indirect measures of growth: otolith growth (sagittal and lapillar) and RNA:DNA ratio. Juvenile red drum were exposed to different feeding regimes (0%,2.5%,5%,10%,15%, and 20% body weight/day) for 30 days to produce salient differences in somatic growth. Collections were taken every 5 days for the 0% treatment and every 10 days for all others during the course of the trial. Length and weight measures were taken on individuals sub sampled during the feeding trial. In addition, otoliths and white muscle tissue were sampled for microstructure and biochemical analysis, respectively. Positive correlations were observed between feeding levels and somatic growth. Growth rates (g/day) and mean weight (g) were more sensitive indicators of condition prior to day 20; at day 30, mean length (mm standard length) was a useful indicator of condition. In addition, somatic growth was closely coupled with otolith growth and RNA:DNA ratio. Rostrum growth of the sagittal otolith showed a delay in growth compared to somatic growth, but was a sensitive measure of condition by day 30. Overall, mean otolith size as a percentage of body weight was the best descriptor of condition. RNA:DNA ratio was a good indicator of well fed v. starved conditions, but failed to distinguish between a large number of ration levels. Findings from this study suggest not only that otolith growth and somatic growth do not react to variable ration levels in the same manner, but that otolith growth measurements and RNA:DNA ratios are suitable measures of relative growth.