A study was conducted in order to determine the protein requirements of juvenile Mexican silverside (Menidia estor). Seven isoenergetic diets (� 19.9 MJ kg)1) with dietary protein levels of 250, 300, 350, 400, 450, 500 and 550 g kg)1 were pre- pared as flakes using jack (Caranx sp.) and red snapper (Lutj- anus sp.) fillets, tuna (Thunnus sp.) ovaries and California squid (Loligo sp.) as protein sources, and their effects on growth, survival and feed utilization of juvenile M. estor (69.24 ± 5.03 mg initial weight) were evaluated. Fish were fed by hand to apparent satiation, five times a day, for 8 weeks. Best growth and survival were obtained with diets with protein levels between 400 and 500 g kg)1, with no significant differences between them (P > 0.05). Specific growth rate and feed intake were also highest for these treatments (P < 0.05). There were no significant differences in feed conversion ratio and protein efficiency ratio between fish fed all the diets. Broken-line ana- lysis of individual weight gain against protein level showed a protein requirement of 409 g kg)1 for juveniles of M. estor.

The Mexican silverside, Menidia estor, is a species with great regional importance and with very high prices in local markets. Unfortunately, due to high ¢sheries pres- sure, environmental degradation and pollution, the species has become endangered. Recently, there has been much progress in the biotechnology of this spe- cies, aimed at its culture, and the present paper describes the advances in feeding and nutrition of those important ¢sh. M. estor is a stomachless, zoo- plantophagous ¢sh, which also occasionally feeds on small ¢sh and crustaceans in the adult stage. Studies on the digestive enzymatic activities show high proteo- lytic capacity and a late or di¡erent model of digestive maturation from that described for marine stomach ¢sh. Nutritional studies on M. estor have shown that juveniles have dietary requirements of about 400 g kg 1 protein and 80 mg kg1 vitamin C. The upper level of dietary carbohydrate for good growth and survival for juveniles is about 150 g kg1. Based on the high docosahexaenoic acid (DHA) content in £esh when fed on diets low in DHA, it is believed that the species has the capacity to biosynthetize420-car- bon fatty acids from 18-carbon fatty acids. The high le- vels of DHA in £esh makes this ¢sh a very signi¢cant potential component of human nutrition. Using these ¢ndings the ¢rst practical diets for commercial culture of the species have been developed.