Dietary protein requirement of juvenile Mexican Silverside (Menidia estor Jordan 1879), a stomachless zooplanktophagous fish

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 prepared as flakes using jack (Caranx sp.) and red snapper (Lutjanus 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 analysis of individual weight gain against protein level showed a protein requirement of 409 g kg−1 for juveniles of M. estor.