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
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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
The federally endangered Rio Grande silvery minnow (USFWS, 1994) was historically the most abundant fish in the Rio
Grande Basin (Bestgen and Platania, 1991); however, H. amarus
populations have been declining to the point of being listed under
the Endangered Species Act (USFWS, 1994). From its headwaters in the San Juan Mountains of Colorado, the Rio Grande
flows through a series of structural basins, where the alluvial
valley is very wide, separated by intervening canyons where the
valley is narrow (Schmidt et al., 2003). The occurrence of wide
alluvial valleys and intervening narrow canyons is important in
analyzing channel adjustment to the regulation of stream flow
and sediment flux (Schmidt et al., 2003). Historically, the Rio
Grande had a mobile bed and erodible banks, and the channel
changed from year to year. Today’s channel is smaller, more
stable, changes less from year to year, and infrequently inundates its former floodplain (Schmidt et al., 2003). The species’
This article is not subject to U.S. copyright law.
Address correspondence to Dr. Hugo A. Magana, USDA Forest Service,
Rocky Mountain Research Station, 333 Broadway Blvd #115, Albuquerque,
NM 87102. E-mail: hmagana@fs.fed.us
steady decline coincided with flood-control and river channelization projects that began in the 1940s and eventually converted
much of the Rio Grande from a wide, shallow, meandering river
to a narrow channel fragmented by dams (Ikenson, 2002). Extensive recovery efforts such as artificial propagation facilities,
habitat restoration projects, and minnow refugium have been
ongoing, yet little research has been performed on H. amarus
food resources. This work investigates food awareness, diatom
and substrate preference, and conditioning response of the Rio
Grande silvery minnow (Hybognathus amarus).
Diatoms are a desirable food source over other members of
the primary production community through storage of photosynthetically produced sugars in the form of lipids rather than starch
(Julius et al., 2007). Therefore, many members of higher trophic
levels selectively feed on diatoms when present with other primary producers (Julius et al., 2007). Minnows are generalists
and forage on diatoms found in benthic and planktonic communities (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
the genus Hybognathus are thought to be obligate herbivores
because they lack a defined stomach and have a long, narrow,
and coiled alimentary tract (Hlohowskyj et al., 1989; Etnier
468
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FEEDING PREFERENCE OF THE RIO GRANDE SILVERY MINNOW
and Starnes, 1993; Ross, 2001). Shirey (2004) quantified gut
contents of historical H. amarus specimens collected in 1874.
Examination of the 1874 specimens indicates that H. amarus
fed on 30 genera and 70 species of diatoms as well as cyanobacteria (Anabaena sp. and Merismopedia sp.), detritus, and pine
pollen. The gut contents from H. amarus collected in 1874 revealed that Nitzschia palea and N. paleacaea were the 4th and
5th most dominant diatom taxa (Shirey, 2004). In a pilot study
during 2004, I captured 247 H. amarus larvae from a restored
floodplain in Los Lunas, NM, and analyzed gut contents for a
subset. The H. amarus larvae collected in 2004 revealed that
diatoms were the main component of their diet. A total of 13
genera and 15 species of diatoms were identified from the 2004
H. amarus larvae.
Typically, 95% of hatchery-raised fish die from predation
or starvation in the first few weeks following stocking when
they are released into their natural environment (Suboski and
Templeton, 1989; Brown and Laland, 2001). Suboski and Templeton (1989) suggest that hatchery fish die of starvation because
they do not recognize natural food sources. Wiley et al. (1993)
evaluated the potential to train hatchery-raised trout to increase
post-stocking survival in streams by simulating natural conditions, feeding them natural foods, and raising them at moderate densities. I propose that hatchery-reared H. amarus can be
trained en masse to recognize and feed on natural food resources
(diatoms) prior to release, which may help to increase their survival in the wild. This study is unique in that I visually recorded
H. amarus feeding habits using natural food sources (unialgal
diatom cultures) on agar amended substrates to establish diatom
preferences among 15 diatom species.
The objectives of this study include the following: (1) determine substrate preference of H. amarus when foraging, (2)
ascertain diatom preference of H. amarus among 15 species
presented over six feeding trials, (3) determine conditioning response (reaction time to feeding) of pre-conditioned (trained)
H. amarus metalarvae.
METHODS
Diatom Culturing
Multi-species periphyton samples were collected from five
sites located adjacent to the Middle Rio Grande (MRG) north
469
and south of Albuquerque, NM. Samples were collected in triplicate using three connected, bottomless, five-gallon buckets
placed at the river margin. The use of bottomless buckets allowed for isolation of benthic samples from the scouring effect
of current flow. Episammic and epipelic algal samples were
collected within each bucket using a 100 × 15-mm Petri dish
and removed with a spatula (Moulton et al., 2002) and transported to the U.S. Department of Agriculture Forest Service,
Rocky Mountain Research Station, Albuquerque, NM. Samples
were washed into 1/2 -dram glass vials with Bozniak community growth media (Bozniak, 1969) and placed in environmental
growth chambers (10◦ , 15◦ , and 22◦ C at 10:14, 12:12, and 14:10
light/dark photoperiods).
A single 20-μL sample was placed onto a microscope
slide and examined at 1,000× magnification. Standard Pasteur
pipettes (133 mm) were flamed and pulled to a thickness of 0.3
mm. The desired diatom was drawn up into the pipette via capillary action and deposited into a separate sterile drop of water. A
new pipette was used to re-isolate the diatom and deposited into
a new drop of water. The serial wash process was continued for
six drops or until only the chosen diatom remained (Hoshaw and
Rosowski, 1969). Individual diatoms were transferred to 50-ml
Erlenmeyer flasks containing approximately 3 mm of #30 silica
sand as a substrate and 20 ml of Bozniak growth media. After
visible algal growth was observed (40–60 days), culture samples with <5% (determined by cell counts) of non-target diatom
species were saved as inoculum for feeding trials. Repeated
attempts to culture all identified diatoms (38 genera and 120
species) collected from the MRG proved unsuccessful regardless of modifications to growth media. Seven genera and 15 diatom species were successfully cultured at RMRS. Nine species
of the genera Nitzschia were cultured, while only one species
each of the other six genera was cultured. It is unclear why the
Nitzschia species grew well, but not other genera (Table 1).
Diatom cultures were processed and permanently mounted
(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
differences experimentally, fine-grained sediment and coarsegrained sand (0.44–1.24 mm ± 0.05 mm SE ) were obtained
from the margins of the MRG and prepared for diatom culturing. Sediment and sand were autoclaved and prepared by
heating 250 ml of Bozniak media in a 1,000-ml beaker and
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
Nitzschia palea
N. linearis
N. paleacea
FT #2
FT #3
FT #4
FT #5
N. palea
N. linearis
N. paleacea
N. palea
Fragilaria crotonensis
Synedra ulna
Surirella angusta
Cyclotella meneghiniana
Nitzschia paleaeformis
Navicula veneta
Achnanthes suchlandtii
Nitzschia palea
N. cf. palea
Navicula veneta
Navicula sp.
Nitzschia palea
N. capitellata
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FT #6
N. paleaeformis
N. cf. intermedia
N. palea
N. cf. palea
Navicula veneta
Nitzschia molestiformis
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H. A. MAGANA
boiling, an equal volume of fine-grained sediment or sand was
added, mixed thoroughly, and poured into 100 × 15-mm Petri
dishes until half full, covered, and sealed with Parafilm (American Can Co., Greenwich, CT, USA). Substrates were allowed
to acclimate in environmental growth chambers for three days
prior to inoculation. Substrates were inoculated with 1 ml of the
diatom inoculum, resealed, and allowed to grow until visible
diatom growth was evident (40–60 days).
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Feeding Trials
Eight 37.5-L aquaria were maintained at room temperature
(23.3–26.5◦ C). Six aquaria were used for feeding trials and two
for holding fish. A total of six feeding trials (FT) were performed
with six replicates each. All H. amarus larvae used in FT were
obtained from the Albuquerque BioPark, Albuquerque, NM.
Hybognathus amarus larvae were randomly selected from a
single aquarium and placed into one of six aquaria until each
aquarium contained 10 fish each for FT 1–3 and six fish each
for FT 4–6. Three to six algal cultures were randomly selected
in various combinations for each FT. One short cylinder (in the
shape of a hockey puck and herein referred to as a “puck”), 21
mm in diameter × 10 mm in height, 346 mm2 ) was removed
from each diatom culture Petri dish with a cork borer. Diatom
pucks were evenly spaced approximately 30 mm apart on a
PlexiglasTM table in a 2 × 3 configuration and placed into each
replicate aquarium for presentation and video recording.
The three objectives for this study were: (1) determine substrate preference by summing total sampling time per diatom
puck over 20 min, (2) determine diatom preference in the same
fashion as substrate preference, (3) determine conditioning response. Conditioning response was reported as the time between
the introductions of food stimuli to the time of first sampling. I
used recently hatched H. amarus protolarvae (4.7–6.7 mm standard length) for feeding trials 1 and 2 to evaluate food awareness.
I define food awareness as the time required to locate and sample a diatom puck from the time of introduction. To facilitate
documentation of H. amarus food awareness, the front panel of
each aquarium was divided into equal quadrants to identify x
and y position of each fish in relationship to food stimuli that
was placed in the same lower left quadrant for each trial. Food
awareness was determined from the quadrant location of each
fish in 5-min intervals over the 30-min feeding trial.
I used pre-conditioned H. amarus from prior feeding trials
(FT 1, 2, and 5) to assess conditioning response to food stimuli.
In this study, “conditioned” means that H. amarus larvae were
exposed to food stimuli for 30 min in a previous FT and used in
the subsequent FT. The time between one FT and a subsequent
FT ranged between 5 and 17 days. Larval fish were maintained
R
on aquarium flake food (TetraMin tropical flakes) between
trials.
Feeding trials were digitally videotaped (Sony DCRVX2100, 33 frames/sec) for 30 min each, but analysis was
limited to the first 20 min since feeding ceased after 20 min.
Video was transferred to a computer where substrate and diatom preference and conditioning response were examined in
5-min intervals using video editing software (Cyberlink PowerDirector 4, Cyberlink Corp., Fremont, CA, USA).
Statistical Analyses
Data that was not normally distributed were log10 transformed to meet the assumptions of ANOVA. Diatom and substrate preference was determined by recording the number of
contacts and sampling time of each diatom puck and summing
total sampling time recorded per diatom puck over 20 min, and
then analyzed using randomized block rank test and Friedman’s
statistic testing.
Friedman’s test is a nonparametric analysis that may be performed on data from a randomized block experiment and compares the medians of three or more dependent groups. It tests the
null hypothesis that the different samples were drawn from distributions with the same median. Alternative hypothesis states
that at least one median is different from the rest. Friedman’s
test uses ranks instead of original data (1 is assigned to lowest value, etc.). Prior to testing for conditioning response, I
assessed food awareness of naive (non-conditioned) larvae. I
examined whether H. amarus larvae were randomly distributed
among quadrants or preferred one quadrant over another. A
multi-response permutation procedure (MRPP) was conducted
to test whether there were significant differences between two
or more groups of sampling units. Following the completion of
the MRPP, principal components (PCA) were estimated from
the data, and the first two components were plotted by group
to provide a visual description of separation among the groups.
This analysis was conducted because total summed frequencies
differed among tanks. Component one of the PCA analysis was
quadrant 3 compared with quadrants 1, 2, and 4. Component
two of the PCA was quadrant 1, and quadrant 2 compared with
quadrant 4. The variables analyzed were the summed frequency
of fish presence in each quadrant over time interval for feeding
trials 1–3. Conditioning response was evaluated using multiple comparison analysis (Dunnet, 1980) based on control trials
(non-conditioned). Significance was determined using an alpha
= 0.05. Statistical analyses for analysis of variance (ANOVA)
R
and Tukey’s multiple comparisons were performed with SAS
9.1 statistical programs (SAS Institute Inc. Cary, NC).
RESULTS
Substrate Preference
Results of randomized block design ANOVA indicate no
apparent variation associated with diatoms and fine-grained
sediment or coarse-grained sand substrate (p = 0.26). When
looked at individually, each of the three Nitzschia species (palea,
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FEEDING PREFERENCE OF THE RIO GRANDE SILVERY MINNOW
471
Feeding Duration(Log10 Seconds)
3
2
1
0
-1
linearis
palea
paleaceae
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substrate
linearis
Sand
palea
paleaceae
Sedi
Figure 1 Notched box plots depicting diatom preference on two different substrates amended with agar (coarse-grained sand vs fine-grained sediment) and
growth media for feeding trials 1 and 2. No significant difference was observed (p = 0.26). Notched box-whisker plots were used to present median species
responses to the various treatment combinations at each location. “Boxes” are bounded by the first (25%) and third (75%) quartiles. The second quartile, which
is the median, is represented as a line within the box. The notched-in (indented) portion of each box represents the 95% confidence interval about the median.
Overlapping confidence intervals indicate similar responses between treatments. The “whiskers” show the extent of the rest of the data beyond the box. Whiskers
extend to data minimum or maximum. Outliers are defined as data that occur beyond the whiskers.
paleacae, and linearis) used in feeding trials 1 and 2 were fed
upon equally, regardless of substrate. Results are depicted using
notched box plots (Figure 1).
Diatom Preference
Results from all feeding trials suggest that H. amarus do
not feed equally on diatom species from the MRG. The use of
video playback in the present study elucidated diatom feeding
selectivity of H. amarus in a laboratory setting. Feeding trial
videos revealed that H. amarus moved quickly from diatom puck
to diatom puck, touching and tasting until selecting a preferred
diatom species and feeding.
For FT 1–3, there were 3,000 seconds possible (5 min × 60
sec × 10 fish) of feeding time per time interval if all fish feed
immediately and continuously. For FT 4–6, there were 1,800
seconds possible (5 min × 60 sec × 6 fish).
No significance was indicated for FT one; however, H.
amarus protolarvae (4.7–6.7 mm SL) fed more on Nitzschia
linearis during the first 10 min, but fed most on N. palea during
the last 10 min. Contact time of H. amarus was greater for N.
palea (2,665 sec) compared to N. linearis (1,091 sec) and N.
paleacae (577 sec) (Figure 2a and Table 2).
Results for FT 2 show no significance between diatom
species. H. amarus fed equally on the three diatom species
during the first 10 min, but increased feeding on N. palea and N.
paleacae during the next 5 min. Hybognathus amarus increased
feeding on N. linearis eight-fold during the last 5 min, while
feeding on N. palea increased slightly and feeding on N. palea-
cae decreased by half. Contact time of H. amarus was greater
for N. palea (770 sec) compared to N. linearis (657 sec) and N.
paleacae (452 sec) (Figure 2b and Table 2).
Hybognathus amarus protolarvae from FT 1 and 2 were held
over and used as conditioned mesolarvae (6.8–9.2 mm SL)
in FT 3. Results for FT 3 indicate a significant difference in
diatom preference. Nitzschia palea was the preferred diatom
species among the 5 species presented (p < 0.001). Hybognathus amarus fed on N. palea for 1,475 sec during the first
5 min then declined gradually for the remainder of the feeding trial. Contact time of H. amarus was greatest for N. palea
(4,399 sec) compared to Synedra ulna (1,070 sec), Fragilaria
crotonensis (921 sec), Surirella angusta (766 sec), or Cyclotella
meneghiniana (172 sec) (Figure 2c and Table 2).
Results from FT 4 using non-conditioned metalarvae (10.2–
12.3 mm SL) indicate a significant difference in diatom species.
Nitzschia paleaformis was preferred over other diatoms (p <
0.001) during the entire feeding trial. Contact time was greater
for N. paleaformis (2,255 sec) than N. palea (453 sec), Achnanthes suchlandtii (383 sec), Navicula veneta (307 sec), or
Nitzschia cf. palea (167 sec) (Figure 2d and Table 2).
Conditioned H. amarus mesolarvae (6.8–9.2 mm SL) from
FT 4 were employed in FT 5 and preferred Navicula veneta
over other diatoms (p < 0.001). Contact time of H. amarus
was greatest for Navicula veneta (1,192 sec) than Navicula sp.
(601 sec), Nitzschia palea (507 sec), or N. capitellata (311 sec)
(Figure 2e and Table 2).
Results for FT 6 indicate a significant difference in diatom
preference. Nitzschia cf. intermedia was preferred over other
diatoms (p < 0.03). Hybognathus amarus sampled N. cf.
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H. A. MAGANA
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472
Figure 2 Graph a corresponds to diatom preference of two-week-old H. amarus protolarvae. Graph b corresponds to diatom preference of four-week-old H.
amarus metalarvae. Graph c corresponds to conditioning response of pre-conditioned H. amarus from FT one and two. Graph d corresponds to diatom preference
of two-month-old H. amarus mesolarvae. Graphs e and f correspond to >3-month-old H. amarus mesolarvae.
intermedia (1,007 sec) more than Navicula veneta (938 sec),
Nitzschia palea (616 sec), N. molestiformis (421 sec), N. cf.
palea (357 sec), or N. paleaformis (237 sec) (Figure 2f and Table
2). To normalize the data for unequal number of fish per trial, I
calculated the number of seconds feeding per fish. Ranking for
all FT was in accordance to Friedman’s rank test (Table 3).
Conditioning Response
Results of PCA indicate that the distribution of summed
and proportion of summed frequencies among quadrants did
not differ between trials 1 and 2 (non-conditioned fish), but
differed for trial 3 (conditioned fish) compared to either trial 1
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FEEDING PREFERENCE OF THE RIO GRANDE SILVERY MINNOW
Table 2 Significance of diatom feeding preference for Rio Grande silvery
minnow
Table 3 Friedman’s rank test for preferred
diatoms in feeding trials
FT
1
2
3
4
5
6
Diatom preferred
Diatom most eaten
Diatom significance
Feeding trial
Diatom species
Nitzschia linearis
N. linearis
N. palea
N. paleaformis
Navicula veneta
Nitzschia cf. intermedia
N. palea
N. palea
N. palea
N. paleaformis
N. veneta
Nitzschia cf. intermedia
None
None
(p = 0.001)
(p = 0.001)
(p < 0.001–0.045).
(p = 0.001)
1
2
3
4
5
6
Nitzschia palea
Nitzschia palea
Nitzschia palea
Nitzschia paleaformis
Navicula veneta
Nitzschia cf. intermedia
or 2. Activity in trial 3 was concentrated in quadrant III (food
location), while activity was more dispersed in trials 1 and 2
(p < 0.001) (Figure 3). Results from feeding trials 1, 2, and 4
indicate that non-conditioned fish were often not immediately
attentive to the food stimuli presented. Mean time to first feeding
in non-conditioned trials was 250 sec ± 550 sec. Hybognathus
amarus used in feeding trials 3 and 5 were pre-conditioned from
their use in feeding trials 1, 2, and/or 4, and arrived at diatom
pucks in 49 sec ± 39 sec (p = 0.0014) (Figure 4), with some
fish commencing feeding within 4 sec of food presentation.
DISCUSSION
Video playback has been an important tool to ethologists
for the past decade, since it can provide important insights into
feeding behavior, mate courtship, and visual receptor sensitivity (Kodric-Brown, 1999; Nicoletto and Kodric-Brown, 1999;
Rowland, 1999). The use of video playback in the present study
allowed for the determination of substrate preference, diatom
feeding selectivity, and conditioning response of H. amarus in
a laboratory setting.
0.2
Component 2 (0.1514)
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473
Trial 1
Trial 2
Trial 3
0.15
0.1
0.05
-0.4
-0.2
0
-0.05 0
0.2
0.4
0.6
-0.1
-0.15
-0.2
Component 1 (0.7806)
Figure 3 Results of the principal components analysis (PCA) for fish location
among quadrants for feeding trials 1, 2, and 3. Feeding trials 1 and 2 (nonconditioned fish) compared with feeding trial 3 (conditioned fish). Food was
placed in quadrant 3 for all feeding trials. Seventy-eight percent of variation
in fish location is explained by component 1, which is quadrant 3 compared
with quadrants 1, 2, and 4. Fifteen percent of variation in fish location can be
explained by component 2, which is quadrant 1 and quadrant 2 vs quadrant 4.
Activity in trial 3 was concentrated in quadrant III, while more dispersed in
trials 1 and 2.
The results from this study provide unequivocal evidence
that H. amarus eat diatoms and prefer certain diatom species
over others. Given a suite of diatom species presented, N. palea,
N. paleaformis, Navicula veneta, and Nitzschia cf. intermedia
were preferred by H. amarus in the feeding trials. Interestingly,
the results from the present study concur with results of Shirey
(2004), who reported that N. palea and N. paleacae were the
3rd and 5th most abundant diatoms found in digestive tracts of
H. amarus specimens collected in 1874. Nitzschia palea and N.
paleacae comprised 8% and 7%, respectively, out of 70 diatom
species found in 1874 H. amarus specimens.
One possible explanation why H. amarus prefer certain diatom species over others is growth form. Scanning electron
microscopy (SEM) images of the preferred diatoms reveal that
these diatom species have an erect growth form (Figure 5a,b)
compared to least preferred diatoms, which have prostrate
growth forms (Figure 5c,d).
In a concurrent study, I used high-speed photography (500
frames/sec) to elucidate the feeding biomechanics of H. amarus.
Many feeding behaviors, especially in larval and juvenile animals, occur at speeds that exceed the resolution of conventional
videography, but their visualization is crucial for an extensive
understanding of a species’ biology. For example, cycles of feeding behaviors in fishes often occur within a few milliseconds,
which would only be captured in 1 or 2 frames of a conventional
30-Hz video camera. However, when filmed at higher speeds
(250 or 500 frames/sec), it is possible to distinguish discreet
feeding behaviors. Three principal feeding behaviors have been
described for fishes, including suction feeding, in which small
food particles are moved toward the fish through buccal suction;
ram feeding, in which the fish overtakes the food item; and food
manipulation, in which the fish dislodges food from a substrate.
Hybognathus amarus uses the latter feeding behavior. The observed feeding behavior of H. amarus larvae was to touch the
substrate with the premaxilla, rise, and open and close its mouth
as if tasting. This process was repeated several times until the
larval fish opened its mouth and bit into the substrate, removing
the diatom layer and substrate. A scanning electron microscope
micrograph (SEM) image (230× magnification) of a preserved
two-week-old (6.8–9.2 mm SL) H. amarus larvae revealed putative developing taste papillae on the mandible (Figure 6a). The
SEM image (160× magnification) of a preserved six-month-old
(60 mm SL) H. amarus show putative developed taste papillae inside the mouth and on the premaxilla (Figure 6b). These
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H. A. MAGANA
Time to First Feeding (Seconds)
1000
750
500
250
0
-250
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1
2
3
4
5
6
Feeding Trial
Figure 4 Notched box plots depicting results of conditioning response of H. amarus to diatom pucks. Mean time to first feeding is recorded in seconds. The
letter “x” on each notched box plot represents the mean, and the line through the box plot represents the median. The small square at the top or bottom of feeding
trials 1, 2, and 4 represent the maximum or minimum observations. Non-conditioned fish were used in feeding trials 1, 2, 4, and 6. Pre-conditioned fish from
feeding trials 1 and/or 2 were randomly selected for use, as in feeding trial 3. Time to first feeding for feeding trials 3 and 5 were recorded as conditioning response.
Non-conditioned fish from feeding trial 4 were randomly selected for use as pre-conditioned fish in feeding trial 5. Feeding trial 6 measured feeding response of
older non-conditioned H. amarus.
similar structures have been identified as taste papillae of other
fish (Kortschal, 1992; Kortschal, 2000; Gomahr et al., 1992).
Other research groups have performed feeding studies with
H. amarus and improved survivability of post-hatch (4–20 day)
larvae, decreasing the mortality rates to less than 1% by providing live food (Artemia naupulii) and improved growth rates
by providing manufactured flake and pelleted feed (Caldwell,
2004). While these studies refined captive rearing methods, they
did not address naturally occurring food resources in the MRG.
With stress placed on our natural resources, many fisheries
increasingly rely on restocking from hatchery-reared sources in
an attempt to maintain commercially viable populations (Brown
and Laland, 2001). While restocking is widely used as a fisheries
management tool, it has also been used for the conservation and
management of threatened species (Flagg et al., 1995). Many
commercial fish species are not able to cope with the transition from life in captivity to life in the wild (Brown and Laland, 2001). On a global scale, 5 × 109 hatchery-reared salmon
are released annually, but less than 5% survive to adulthood
(McNeil, 1991). To survive to reproductive age, an animal that is
released from captivity must have, or quickly acquire, appropriate migratory, feeding, and anti-predator behaviors (Heggberget
et al., 1992).
The rearing protocol at the Albuquerque BioPark is to transfer two-week-old post-hatch Hybognathus amarus mesolarvae
to outdoor tanks filled with filtered city water and fed a mixture
of phytoplankton and zooplankton. Hybognathus amarus larvae
are then switched to live food (Artemia salina) and then sup-
plemented with manufactured flake feed (Silver Cup fish feed,
Nelson & Sons, Inc., Murray, UT, USA). While this protocol has
proven successful in hatchery settings, it does not mimic natural
conditions found in the MRG. Turbidity in the MRG is continually elevated (20–1,200 NTU) (personal communication, David
Van Horn, UNM) and light attenuates to zero at 40–50 cm. Low
light levels restrict primary productivity to the shallow margins
of the river and sand bars, where light penetration is higher
(Anderholm et al., 1995).
The literature reports that H. amarus larvae eat diatoms in
the wild, but the purpose of this study was to see if H. amarus
could be trained to feed on natural foods (diatoms) quicker than
they would without training, thus increasing probability of surviving in the wild to reproductive age. Using social learning
protocols prior to release may help to increase their survival in
the wild (Suboski and Templeton, 1989; Laland et al., 2003).
Social learning or “local enhancement” is the behavior (or simply the presence) of one individual attracting the attention of another individual to a particular location or stimulus, about which
the naive individual subsequently learns something (Brown and
Laland, 2003). Learned behavior can result from simple exposure of fish to conspecifics engaged in particular activities. Fish
that observe a conspecific consume a novel food later show an
increased tendency to eat that food themselves (Suboski and
Templeton, 1989). Large-scale training of foraging skills is feasible, relatively simple, and inexpensive to initiate, and could
enhance the viability of hatchery fish prior to their release into
the wild (Brown and Laland, 2001).
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475
Figure 5 Erect growth form of preferred diatoms Nitzschia palea (a) and N. paleaformis (b). Prostrate growth forms of least preferred diatoms Nitzschia
molestiformis (c) and Achnanthes suchlandtii (d).
Figure 6 SEM image (a) of a preserved two-week-old H. amarus (230×). Arrows point to developing putative taste papillae on premaxilla. SEM image (b) of
preserved six-month-old H. amarus (160×). Arrows point to putative taste papillae inside mouth and on premaxilla.
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H. A. MAGANA
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CONCLUSION
When presented with 15 diatom species in a variety of combinations, H. amarus preferred Nitzschia palea in three of the six
feeding trials (50%). There was no preference to substrate type
regardless of diatom species. Throughout this feeding study,
when one fish fed continuously on a diatom puck, cohorts were
observed to congregate and commence feeding on the same diatom puck. In making decisions, such as how to find food and
mates or avoid predators, many animals utilize information that
is produced by others (Brown and Laland, 2003). I have proven
that hatchery-reared H. amarus can be trained to recognize and
feed on natural food resources (diatoms) after only one 30-min
exposure. Using this information and applying it to the hatchery setting may improve survivability of H. amarus in the wild
by teaching them to recognize food sources and prevent the
extirpation of this fish species in the MRG.
This study greatly increases the knowledge base concerning
the feeding habits of H. amarus and elucidated some of the preferred diatoms available to the H. amarus in the MRG. Further
studies are required to determine if H. amarus cues on taste or
nutritive value of diatom species when selecting food sources.
This information can be advantageous to H. amarus propagation
managers, as well as propagation managers of other threatened
or endangered fish species that eat the same foods as H. amarus.
ACKNOWLEDGMENTS
I have many people to thank for their help during this research project. My research was partially funded by the USDA
Forest Service, Rocky Mountain Research Station, and the US
Bureau of Reclamation (Dr. Michael Porter) (Agreement 02-AI11221602-061). Thanks to Terina Perez and staff (Albuquerque
BioPark) for Rio Grande silvery minnow used in feeding trials.
Many thanks to Dr. Rudy King (USDAFS, RMRS, Fort Collins,
CO) and Dr. Darin Law (USDAFS, RMRS, Albuquerque, NM)
for assistance with experimental design and statistical analyses.
Rio Grande silvery minnow used in feeding trials were used
under USFWS permit TE097324-0 (H. A. Magana).
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