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 for the recovery of the species. A starter diet was initially developed for this cyprinid to support rapid growth through the larval stage. The diet, however, was too costly to support the grower phase of the species. A series of trials was conducted to determine whether a grower diet could be developed that would be cost effective while maintaining fish performance. Quadruplicate lots of Rio Grande silvery minnow (approximately 90 d posthatch, 0.39-0.42 g) were fed a reference diet or one of five experimental diets until just before gonadal maturation at two facilities in New Mexico (New Mexico State University [NMSU] and Dexter National Fish Hatchery and Technology Center [Dexter]). The diets varied in percent composition of ingredients to account for a range in feed costs. Cyclop-eeze (Argent, Redmond, Washington), krill, liver, and fish meals were present in the reference diet and sequentially altered in the remaining test diets to maintain high protein levels in the diets (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% per day). The highest growth rate occurred across all diets within the first 30 d of both feed trials and then diminished by the end of the trials. There were no detectable differences in critical swimming speed or fat scores in fish fed diets with and without expensive feed ingredients or increasing fish meal. The most consistent growth was observed when fish were fed a diet low in fish meal, and our results clearly identified a less-expensive grower formulation for the production of this imperiled cyprinid.