Reviews in Fisheries Science, 17(4):468–477, 2009 ISSN: 1064-1262 print DOI: 10.1080/10641260902985096 Feeding Preference of the Rio Grande Silvery Minnow (Hybognathus amarus) HUGO A. MAGANA USDA Forest Service, Rocky Mountain Research Station, Albuquerque, New Mexico, USA The Rio Grande silvery minnow (Hybognathus amarus) was historically the most abundant fish in the Rio Grande Basin. However, populations have been declining to the point of being listed under the Endangered Species Act. Potential causes for the decline have been studied, yet little attention has been paid to food resources. This study had three objectives: (1) Determine whether larval fish show a substrate preference when foraging. (2) Determine whether larval fish have a diatom preference when presented with 15 diatom species over six feeding trials. (3) I investigated the possibility of training/conditioning H. amarus to feed on natural food sources (diatoms) and observe conditioning response (reaction time to feeding). I found no difference between substrate preference (p = 0.26). Results for feeding trials 1, 2, and 3 revealed a preference for Nitzschia palea (p < 0.01). Trial 4 revealed a preference for N. paleaformis (p < 0.01). Navicula veneta was the preferred diatom species in feeding trial 5. Nitzschia cf. intermedia was preferred in trial 6 (p < 0.03). Results from these feeding trials proved that H. amarus larvae learn quickly and can be trained to feed on diatom cultures after only one 30-min exposure. Pre-conditioned H. amarus arrived at diatoms cultures in 49 sec ± 39 sec compared to non-conditioned H. amarus, which arrived at diatom cultures at 250 sec ± 550 sec. Keywords diatoms, feeding preference, Rio Grande silvery minnow, Hybognathus amarus INTRODUCTION steady decline coincided with flood-control and river channel- ization projects that began in the 1940s and eventually converted The federally endangered Rio Grande silvery minnow (US- much of the Rio Grande from a wide, shallow, meandering river Downloaded By: [MAGANA, HUGO A.] At: 16:23 17 July 2009 FWS, 1994) was historically the most abundant fish in the Rio to a narrow channel fragmented by dams (Ikenson, 2002). Ex- Grande Basin (Bestgen and Platania, 1991); however, H. amarus tensive recovery efforts such as artificial propagation facilities, populations have been declining to the point of being listed under habitat restoration projects, and minnow refugium have been the Endangered Species Act (USFWS, 1994). From its headwa- ongoing, yet little research has been performed on H. amarus ters in the San Juan Mountains of Colorado, the Rio Grande food resources. This work investigates food awareness, diatom flows through a series of structural basins, where the alluvial and substrate preference, and conditioning response of the Rio valley is very wide, separated by intervening canyons where the Grande silvery minnow (Hybognathus amarus). valley is narrow (Schmidt et al., 2003). The occurrence of wide Diatoms are a desirable food source over other members of alluvial valleys and intervening narrow canyons is important in the primary production community through storage of photosyn- analyzing channel adjustment to the regulation of stream flow thetically produced sugars in the form of lipids rather than starch and sediment flux (Schmidt et al., 2003). Historically, the Rio (Julius et al., 2007). Therefore, many members of higher trophic Grande had a mobile bed and erodible banks, and the channel levels selectively feed on diatoms when present with other pri- changed from year to year. Today’s channel is smaller, more mary producers (Julius et al., 2007). Minnows are generalists stable, changes less from year to year, and infrequently inun- and forage on diatoms found in benthic and planktonic com- dates its former floodplain (Schmidt et al., 2003). The species’ munities (Sray, 1998). Other species of the genus Hybognathus feed on “diatoms, algae, larval insect exuvia, and plant material scraped from bottom sediments” (Whitaker, 1977). Adults of This article is not subject to U.S. copyright law. Address correspondence to Dr. Hugo A. Magana, USDA Forest Service, the genus Hybognathus are thought to be obligate herbivores Rocky Mountain Research Station, 333 Broadway Blvd #115, Albuquerque, because they lack a defined stomach and have a long, narrow, NM 87102. E-mail: [email protected] and coiled alimentary tract (Hlohowskyj et al., 1989; Etnier 468 FEEDING PREFERENCE OF THE RIO GRANDE SILVERY MINNOW 469 and Starnes, 1993; Ross, 2001). Shirey (2004) quantified gut and south of Albuquerque, NM. Samples were collected in trip- contents of historical H. amarus specimens collected in 1874. licate using three connected, bottomless, five-gallon buckets Examination of the 1874 specimens indicates that H. amarus placed at the river margin. The use of bottomless buckets al- fed on 30 genera and 70 species of diatoms as well as cyanobac- lowed for isolation of benthic samples from the scouring effect teria (Anabaena sp. and Merismopedia sp.), detritus, and pine of current flow. Episammic and epipelic algal samples were pollen. The gut contents from H. amarus collected in 1874 re- collected within each bucket using a 100 × 15-mm Petri dish vealed that Nitzschia palea and N. paleacaea were the 4th and and removed with a spatula (Moulton et al., 2002) and trans- 5th most dominant diatom taxa (Shirey, 2004). In a pilot study ported to the U.S. Department of Agriculture Forest Service, during 2004, I captured 247 H. amarus larvae from a restored Rocky Mountain Research Station, Albuquerque, NM. Samples floodplain in Los Lunas, NM, and analyzed gut contents for a were washed into 1/2 -dram glass vials with Bozniak commu- subset. The H. amarus larvae collected in 2004 revealed that nity growth media (Bozniak, 1969) and placed in environmental diatoms were the main component of their diet. A total of 13 growth chambers (10◦,15◦, and 22◦C at 10:14, 12:12, and 14:10 genera and 15 species of diatoms were identified from the 2004 light/dark photoperiods). H. amarus larvae. A single 20-μL sample was placed onto a microscope Typically, 95% of hatchery-raised fish die from predation slide and examined at 1,000× magnification. Standard Pasteur or starvation in the first few weeks following stocking when pipettes (133 mm) were flamed and pulled to a thickness of 0.3 they are released into their natural environment (Suboski and mm. The desired diatom was drawn up into the pipette via capil- Templeton, 1989; Brown and Laland, 2001). Suboski and Tem- lary action and deposited into a separate sterile drop of water. A pleton (1989) suggest that hatchery fish die of starvation because new pipette was used to re-isolate the diatom and deposited into they do not recognize natural food sources. Wiley et al. (1993) a new drop of water. The serial wash process was continued for evaluated the potential to train hatchery-raised trout to increase six drops or until only the chosen diatom remained (Hoshaw and post-stocking survival in streams by simulating natural condi- Rosowski, 1969). Individual diatoms were transferred to 50-ml tions, feeding them natural foods, and raising them at moder- Erlenmeyer flasks containing approximately 3 mm of #30 silica ate densities. I propose that hatchery-reared H. amarus can be sand as a substrate and 20 ml of Bozniak growth media. After trained en masse to recognize and feed on natural food resources visible algal growth was observed (40–60 days), culture sam- (diatoms) prior to release, which may help to increase their sur- ples with <5% (determined by cell counts) of non-target diatom vival in the wild. This study is unique in that I visually recorded species were saved as inoculum for feeding trials. Repeated H. amarus feeding habits using natural food sources (unialgal attempts to culture all identified diatoms (38 genera and 120 diatom cultures) on agar amended substrates to establish diatom species) collected from the MRG proved unsuccessful regard- preferences among 15 diatom species. less of modifications to growth media. Seven genera and 15 di- The objectives of this study include the following: (1) de- atom species were successfully cultured at RMRS. Nine species termine substrate preference of H. amarus when foraging, (2) of the genera Nitzschia were cultured, while only one species ascertain diatom preference of H. amarus among 15 species each of the other six genera was cultured. It is unclear why the presented over six feeding trials, (3) determine conditioning re- Nitzschia species grew well, but not other genera (Table 1). sponse (reaction time to feeding) of pre-conditioned (trained) Diatom cultures were processed and permanently mounted Downloaded By: [MAGANA, HUGO A.] At: 16:23 17 July 2009 H. amarus metalarvae. (Julius et al., 1997) and microscopically identified using keys by Krammer and Lange-Bertalot (1999). Substrate characteristics may influence diatom growth and foraging preference of fish (Webster and Hart, 2004). To test for METHODS differences experimentally, fine-grained sediment and coarse- grained sand (0.44–1.24 mm ± 0.05 mm SE ) were obtained Diatom Culturing from the margins of the MRG and prepared for diatom cul- turing. Sediment and sand were autoclaved and prepared by Multi-species periphyton samples were collected from five heating 250 ml of Bozniak media in a 1,000-ml beaker and sites located adjacent to the Middle Rio Grande (MRG) north adding 3.75 g of noble agar (Patrick and Wallace, 1953). After Table 1 Fifteen diatom species used in six individual feeding trials (FT) with six replicates each FT #1 FT #2 FT #3 FT #4 FT #5 FT #6 Nitzschia palea N. palea N. palea Nitzschia paleaeformis Navicula veneta N. paleaeformis N. linearis N. linearis Fragilaria crotonensis Navicula veneta Navicula sp. N. cf. intermedia N. paleacea N. paleacea Synedra ulna Achnanthes suchlandtii Nitzschia palea N.