Diet Optimization of Juvenile Rio Grande Silvery Minnow

Captive propagation of the federally-endangered Rio Grande silvery minnow Hybognathus amarus has been identified as essential to its recovery.  A starter diet was initially developed for the cyprinid to support rapid growth through the larval stage.  This diet, however, was too costly to support the grower phase of the species.  A series of trials were conducted to determine if a grower diet could be developed that would be cost effective yet maintain fish performance.  Quadruplicate lots of  Rio Grande silvery minnow (approximately 90 days post-hatch; 0.39 – 0.42 g) were fed a reference diet or one of five experimental diets until just prior to gonadal maturation at two facilities in New Mexico (NMSU and Dexter).  The diets varied in percent composition of ingredients to account for a range in feed costs.  Cyclop-eeze®, krill, liver, and fish meals were present in the reference diet and sequentially altered in the remaining test diets to maintain diets high in protein (57.9-64.9% dry matter).  Total weight gains at the end of the 16-week trials were lower than expected (140-200%) with no differences among diets at NMSU.  Although total weight gain was less at Dexter (105-150%), there were differences among diets.  Specific growth rate was greater at both facilities in fish fed the reference diet and a diet lowest in fish meal (1.82%/d).  The highest growth rate occurred across all diets within the first 30 days of both feed trials and then diminished by the end of the trials.   There were no detectable differences in critical swimming speed (U_crit) or fat scores in fish fed diets with and without expensive feed ingredients or increasing fish meal.  The most consistent growth was observed when feeding a diet low in fish meal and clearly indentifies a less expensive grower formulation for the production of the imperiled cyprinid.