An International Periodical Promoting Conservation and Biodiversity Southwestern United States—Mexico—Central America Una Revista Internacional para Fomentar la Conservación y Biodiversidad El Suroeste de USA—México—Centroamérica FOODS OF AGE-0 RIO GRANDE SILVERY MINNOWS (HYBOGNATHUS AMARUS) REARED IN HATCHERY PONDS

We examined contents of alimentary canal from pond-reared Rio Grande silvery minnows (Hybognathus amarus; n 5 183) to determine frequency of occurrence for foods consumed among small (9–20 mm), medium (21–35 mm), and large fish (36–75 mm). Among 183 fish, 19% (n 5 34) of alimentary canals were empty. Of those with food items (n 5 149), insects were the most common and were present in 66% of fish, followed by formulated feed (60%), diatoms (40%), cladocerans (36%), rotifers (35%), filamentous algae (32%), bryozoan statoblasts (19%), copepods (11%), protozoa (9%), plant material (9%), ostracods (6%), detritus (5%), and sand (4%). Among size groups, small and medium fish consumed a greater variety of foods than large fish. Information on contents of alimentary canal of pond-reared Rio Grande silvery minnows will aid in refining culture techniques for this federally listed endangered species. RESUMEN—Examinamos los contenidos de los estómagos de los peces Hybognathus amarus (n 5 183) provenientes de criaderos para determinar la frecuencia de ocurrencia de tipos de comida consumidos por peces pequeños (9–20 mm), medianos (21–35 mm), y grandes (36–75 mm). Entre 183 peces, 19% (n 5 34) de los estómagos estuvieron vacı́os. Entre los que tenı́an comida (n 5 149), insectos fueron más comunes, encontrándose en 66% de los peces, seguidos por alimento comercial (60%), diatomas (40%), cladóceros (36%), rotı́feros (35%), algas filamentosas (32%), briozoos estatoblastos (19%), copépodos (11%), protozoarios (9%), material de plantas (9%), ostrácodos (6%), detritus (5%), y arena (4%). Entre grupos de tamaño, los peces pequeños y medianos consumieron una variedad mayor de comida que los peces grandes. Información sobre contenidos de estómagos de H. amarus provenientes de criaderos ayudará a refinar técnicas de cultivo para esta especie listada federalmente como en peligro de extinción. The Rio Grande silvery minnow Hybognathus amarus historically occurred in the Rio Grande from Espanola, New Mexico, to the Gulf of Mexico, and in the Pecos River from Santa Rosa, New Mexico, to the Rio Grande (Bestgen and Platania, 1991). Local and widespread fragmentation of rivers by diversions of water and reservoirs, dewatering of mainstem reaches, introductions of congener plains minnow H. placitus, and nutrient enrichment are likely causes of range-wide reductions in occurrence and abundance (Bestgen and Propst, 1996). Consequently, this once widespread and abundant fish was extirpated from the Pecos River and the Rio Grande in Texas by the 1960s (Bestgen and Platania, 1991; Hubbs et al., 1991), prompting listing of the fish as an endangered species (United States Department of the Interior, 1994). Currently, the Rio Grande silvery minnow occurs only in a 280-km section of the Rio Grande between Cochiti Reservoir and Elephant Butte Reservoir in New Mexico (United States Fish and Wildlife Service, 1999, 2007). However, persistence of Rio Grande silvery minnows in this section of the Rio Grande is tenuous because of introductions of exotic fish and continued degradation of water quantity and quality (Bestgen and Propst, 1996). As such, wild stocks are supplemented by hatchery-reared fish. The primary goal of the Rio Grande silvery minnow recovery plan (United States Fish and Wildlife Service, 1999, 2007) is to prevent extinction of the Rio Grande silvery minnow in the middle Rio Grande; captive propagation is THE SOUTHWESTERN NATURALIST 54(4):475–479 DECEMBER 2009