Echo-Hawk, Patricia Diana (2015-12). Performance of Different Diet Types on Larval Rearing of the Threatened Devils River Minnow (Dionda Diaboli). Master's Thesis.
Dionda diaboli is a threatened species of algivorous minnow endemic to spring-fed creeks and rivers of the Rio Grande drainage in south central Texas and adjacent regions of Mexico. Populations of D. diaboli are decreasing due to drought, habitat degradation (including the introduction of invasive piscivorous fishes and over- pumping of water). Therefore, this species is the focus of a federal captive rearing and maintenance program. Development of controlled larval rearing protocols is crucial for successful captive rearing and ultimately wild stock enhancement and species survival. This study explores the utility of four different diets for use in captive rearing of D. diaboli. Sixteen-day-old post-hatchings were stocked for 130 days in 20, 7.8-L flow-through tanks and fed four different diets, including two live feed diets (Artemia nauplii or mixed zooplankton) and two prepared feed diets (protein flakes or algal gel). During 16-46 days post hatch (dph), specific growth rate (SGR) for length (SGRL) and weight (SGRW) of individuals was highest for fish fed a diet of Artemia nauplii (2.71 mm/d SGRL, 2.84 mg/d SGRW) and algal gel (2.21 mm/d SGRL, 2.41 mg/d SGRW). The SGRL of zooplankton-fed fish during this time was 2.04 mm/d followed by protein flakes at 1.96 mm/d. SGRW for individuals fed protein flakes was 2.19 mg/d and was followed by individuals fed zooplankton at 1.96 mg/d. For successive time periods, there was a gradual shifting in highest to lowest grow rates per diet for both length and weight, with protein-flake-fed fish achieving the highest SGR for each successive sampling period beginning with the 46-76 dph time period for length (0.56 mm/d), and the 76-106 dph time period for weight (0.75 mg/d). Zooplankton fed fish achieved the poorest SGR for both length and weight for all time periods after 46 dph, and never developed external or internal morphology beyond that equivalent to 64 dph in normal development. Overall survival was highest for fish fed a diet of algal gel (100%) followed by fish fed Artemia nauplii and protein flake (99.7%) and lowest for fish fed zooplankton (77%). The number and length of intestinal coils was considered "normal" for 136 dph juveniles fed on a diet of Artemia nauplii, algal gel and protein flakes, while number and length of intestinal coils for 136 dph juveniles fed on a diet of zooplankton were comparable to those of "typical" 64 dph larvae. Juveniles at 136 dph fed algal gel and protein flakes exhibited higher quantities of visceral fat than those juveniles fed on a diet of either Artemia nauplii or zooplankton. Though overall SGR for both length and weight was greatest with the algal gel diet (1.05 mm/d SGRL, 1.65 mg/d SGRW), observed growth trends throughout the study suggest that nutritional requirements may change continually throughout the development of D. diaboli, and an optimal diet should satisfy the physiological and metabolic demands of different ontogenetic stages to ensure optimum growth.