Blood refractive index and whole-body lipid content as indicators of nutritional condition for penaeid prawns

To assess potential indices of nutritional condition, we examined changes in lipid content and the blood refractive index (BRI) of postlarval, juvenile and adult prawns (Penaeus spp.) that had been starved or fed to different levels of satiation. The whole-body lipid content (WBL) of small juvenile P. esculentus (20–120 mg wet weight) decreased significantly after 12 h of starvation. The WBL of postlarval P. monodon (4–10 mg wet weight) also decreased during starvation, with WBL almost halving after 5 days. Juvenile P. monodon (0.5–1.3 g wet weight) fed at 25 or 50% of the satiation ration for 6 days had significantly lower WBL than satiated prawns. The lowest concentrations of WBL were recorded for starved prawns and those fed to 25% of satiation. In larger juvenile P. semisulcatus (0.3–1.2 g wet weight) the WBL was also sensitive to starvation, declining significantly after only 1 day. The blood lipid content and digestive gland lipid of adult P. semisulcatus (28–43 g wet weight) decreased significantly after 4 days of starvation. The BRI of starved prawns and those fed to satiation correlated well with the WBL in the case of smaller prawns (r . 0.96) and with the blood lipid of adult P. semisulcatus (28–43 g wet weight). However, there was a weaker correlation between BRI and WBL in prawns whose feed allocations were between starvation and satiation (r 5 0.86). These results from several prawn species and different size classes show that both the BRI and WBL have potential as indices of nutritional condition. Although BRI is much easier and cheaper to measure than WBL and can be measured in the field, it is suitable only for prawns from which sufficient blood can be obtained — generally those larger than 300 mg wet weight (7 mm carapace length). In contrast, WBL can be measured with a good level of precision in individual prawns as small as 20 mg wet weight (3 mm carapace length) using relatively simple laboratory equipment.  2000 Elsevier Science B.V. All rights reserved.