U.S. Department of the Interior August 2022
Technical Memorandum No. ENV-2022-086
Rio Grande Silvery Minnow
October Monitoring: 2020
Rio Grande Delta Channel, Middle Rio Grande, New Mexico
Upper Colorado Basin Region
Mission Statements
The U.S. Department of the Interior protects and manages the
Nation’s natural resources and cultural heritage; provides scientific
and other information about those resources; honors its trust
responsibilities or special commitments to American Indians,
Alaska Natives, and affiliated Island Communities.
The mission of the Bureau of Reclamation is to manage, develop,
and protect water and related resources in an environmentally and
economically sound manner in the interest of the American public.
Cover Photo – Shoreline of the Rio Grande River (Bureau of Reclamation/Eric Best).
U.S. Department of the Interior August 2022
Technical Memorandum No. ENV-2022-086
Rio Grande Silvery Minnow
October Monitoring: 2020
Rio Grande Delta Channel, Middle Rio Grande, New Mexico
Upper Colorado Basin Region
Prepared by:
Eric Best, Fisheries Biologist
Matthew Bullard, Fisheries Biologist
Fisheries and Wildlife Resources Group
Bureau of Reclamation, Technical Service Center
Denver, Colorado
Prepared for:
Albuquerque Area Office
555 Broadway NE, Suite 100
Albuquerque, NM 87102
Acronyms and Abbreviations
CPUE catch per unit effort
FR Federal Register
LFCC Low Flow Conveyance Channel Outflow
m meter(s)
n sample size
Reclamation Bureau of Reclamation
RM River Mile
silvery minnow Rio Grande silvery minnow (Hybognathus amarus)
USFWS U.S. Fish and Wildlife Service
VIE visible implant elastomer
Symbols
≈ approximately
°C degrees Celsius
> greater than
< less than
µs/cm microsiemens per centimeter
% percent
i
Contents
Page
Executive Summary ...................................................................................................................ES-1
1.0 Introduction ..........................................................................................................................1
2.0 Methods................................................................................................................................3
2.1 Study Area ...............................................................................................................3
2.2 Seine Nets ................................................................................................................4
3.0 Results ..................................................................................................................................5
3.1 Observations During the 2020 Fish Surveys ...........................................................5
3.1.1 2020 Rio Grande Flows and Water Quality Parameters ..............................5
3.1.2 2020 Rio Grande Silvery Minnows Captured and Catch
per Unit Effort ..........................................................................................6
3.2 Sampling Site Results ..............................................................................................8
3.2.1 San Acacia (River Mile 112) ........................................................................8
3.2.2 Upper Escondida (River Mile 104) ............................................................11
3.2.3 Lower Escondida (River Miles 102–103) ..................................................13
3.2.4 Rhodes Canyon (River Mile 92) ................................................................15
3.2.5 LFCC (River Mile 54) ................................................................................17
3.2.6 Nogal Canyon (River Mile 51)...................................................................19
3.2.7 Pete Well (River Miles 47–48)...................................................................21
3.2.8 Rockhouse (River Mile 44) ........................................................................23
3.2.9 North Monticello (River Miles 38–39) ......................................................25
3.2.10 South Monticello (River Mile 37) ..............................................................27
3.3 Mesohabitat Utilization ..........................................................................................29
4.0 Discussion ..........................................................................................................................30
5.0 References ..........................................................................................................................33
ii
Tables
Table Page
1 Mesohabitat classification types on the Middle Rio Grande, New Mexico ........................4
2 Water quality measurements for all survey locations ......................................................... 6
3 Number of silvery minnows captured at each sampling site on the Middle
Rio Grande, New Mexico, October 2020 ....................................................................... 7
4 Species composition at the San Acacia site (Totals are > 100% due to rounding.) ............ 9
5 Species composition at the Upper Escondida site............................................................. 11
6 Species composition at the Lower Escondida site ............................................................ 13
7 Species composition at the Rhodes Canyon site ............................................................... 15
8 Species composition at the LFCC site (Totals are > 100% due to rounding.) .................. 17
9 Species composition at the Nogal Canyon site ................................................................. 19
10 Species composition at the Pete Well site (Totals are > 100% due to rounding.) ............ 21
11 Species composition at the Rockhouse site ...................................................................... 23
12 Species composition at the North Monticello site ............................................................ 25
13 Species composition at the South Monticello site ............................................................ 27
14 Number of each mesohabitat sampled and corresponding silvery minnow
captures ......................................................................................................................... 29
iii
Figures
Figure Page
1 Study area overview for fall 2020 Rio Grande silvery minnow survey sites. .....................2
2 Daily mean discharge for U.S. Geological Survey Station 08355050
Rio Grande near Escondida, New Mexico, from January 1 to October 30, 2020. ......... 5
3 Total number of silvery minnows captured in each sampling year on the
Middle Rio Grande, New Mexico. .................................................................................. 7
4 CPUE (fish/100 square meters) for silvery minnows during October seining
on the Middle Rio Grande, New Mexico. ....................................................................... 8
5 Silvery minnow length-frequency histogram from the San Acacia site
(RM 112) on the Middle Rio Grande, New Mexico, October 2020. ............................. 9
6 San Acacia seining locations and silvery minnow capture locations
(RM 112), Middle Rio Grande, New Mexico, October 2020. ...................................... 10
7 Silvery minnow length-frequency histogram from the Upper Escondida site
(RM 104) on the Middle Rio Grande, New Mexico, October 2020. ............................ 11
8 Upper Escondida seining locations and silvery minnow capture locations
(RM 104), Middle Rio Grande, New Mexico, October 2020. ...................................... 12
9 Silvery minnow length-frequency histogram from the Lower Escondida site
(RMs 102–103) on the Middle Rio Grande, New Mexico, October 2020. .................. 13
10 Lower Escondida seining locations and silvery minnow capture locations
(RMs 102–103), Middle Rio Grande, New Mexico, October 2020. ............................ 14
11 Rhodes Canyon seining locations and silvery minnow capture locations
(RM 92), Middle Rio Grande, New Mexico, October 2020. ........................................ 16
12 LFCC seining locations and silvery minnow capture locations (RM 54),
Middle Rio Grande, New Mexico, October 2020. ........................................................ 18
13 Silvery minnow length-frequency histogram from the Nogal Canyon site
(RM 51) on the Middle Rio Grande, New Mexico, October 2020. .............................. 19
14 Nogal Canyon seining locations and silvery minnow capture locations
(RM 51), Middle Rio Grande, New Mexico, October 2020. ........................................ 20
15 Pete Well seining locations and silvery minnow capture locations
(RMs 47–48), Middle Rio Grande, New Mexico, October 2020. ................................ 22
16 Rockhouse seining locations and silvery minnow capture locations
(RM 44), Middle Rio Grande, New Mexico, October 2020. ........................................ 24
17 North Monticello seining locations and silvery minnow capture locations
(RMs 38–39), Middle Rio Grande, New Mexico, October 2020. ................................ 26
18 Silvery minnow length-frequency histogram from the South Monticello site
(RM 37) on the Middle Rio Grande, New Mexico, October 2020. .............................. 27
19 South Monticello seining locations and silvery minnow capture locations
(RM 37), Middle Rio Grande, New Mexico, October 2020. ........................................ 28
20 Silvery minnow captures according to mesohabitat classification during
October seining on the Middle Rio Grande, New Mexico, 2020. ................................ 30
ES-1
Executive Summary
In late October 2020, the Bureau of Reclamation (Reclamation) conducted surveys for the
federally endangered Rio Grande silvery minnow (Hybognathus amarus) at select sites along the
Rio Grande below the San Acacia Diversion Dam to the South Monticello boat ramp in Elephant
Butte Reservoir. Reclamation and U.S. Fish and Wildlife Service biologists sampled 10 sites
using seine nets. Surveys were conducted to maintain compliance with the Endangered Species
Act requirements for Reclamation’s river maintenance and water management activities and to
evaluate potential habitat restoration projects.
During the fall seining efforts, 17 Rio Grande silvery minnows were captured: 9 at the South
Monticello site, 3 at the Upper Escondida site, 2 at the Lower Escondida site, 2 at the San Acacia
site, and 1 at the Nogal Canyon site. The length-frequency histograms indicated that roughly
half of the silvery minnows captured in 2020 were young-of-year, which suggests a lack of
recruitment associated with the low flows that attributed to the lower numbers of age-0 silvery
minnows observed in 2020, and it also informs us that there was some recruitment experienced.
1
1.0 Introduction
The Rio Grande silvery minnow (silvery minnow; Hybognathus amarus)) is a State and federally
listed endangered species. It had historically been found throughout the Rio Grande mainstem
stretching from the confluence of the Rio Chama in northern New Mexico downstream to the
Gulf of Mexico (Bestgen and Platania 1991). The population began to decline with the
destruction and modification of its habitat due to dewatering and water diversions, water
impoundments, and river modifications. The silvery minnow has been extirpated from about
93 percent of its native range—currently persisting in only one 280-kilometer reach of the
Rio Grande in New Mexico, downstream from Cochiti Dam to the headwaters of Elephant Butte
Reservoir and a portion of the Rio Grande in Big Bend National Park in Texas (U.S. Fish
and Wildlife Service [USFWS] 2010). The USFWS federally listed the species in 1994
(59 FR 36988–36995) and designated critical habitat in February 2003 (68 FR 8087). See the
USFWS recovery plan (USFWS 2010).
Since 1992, the Bureau of Reclamation (Reclamation) has funded fish community surveys
(Silvery Minnow Population Monitoring Program) of the Middle Rio Grande (Dudley and
Platania 1993, 1997). Starting in 2010, Reclamation initiated additional annual surveys for the
silvery minnow during fall and winter throughout the Middle Rio Grande to obtain silvery
minnow data from areas not evaluated during the Silvery Minnow Population Monitoring
program and during winter periods when such surveys were rare, and to document the fish
community at certain habitat restoration sites.
Survey efforts for this study were conducted to determine the distribution and relative abundance
of the silvery minnow within and downstream from designated critical habitat (Figure 1). From
2010 through 2013, survey site locations were not standardized; starting in 2014, the Rockhouse,
North Monticello, and South Monticello sites were randomly selected and have been sampled on
an annual basis. Additional survey sites have been added over the years in order to monitor areas
for possible restoration projects. These sites can best be described as “presence only” rather than
“presence/absence” because the survey locations within sites were not randomly selected, and
not all of the habitat types were surveyed equally.
Reclamation biologists surveyed 10 sites in October 2020. The Low Flow Conveyance Channel
Outflow (LFCC), Nogal Canyon, and Pete Well sites have been surveyed each year during fall
seining efforts. The Upper Escondida and Lower Escondida sites were added in 2017. The
San Acacia site was added in 2018. The Rhodes Canyon site was incorporated in 2020 to include
baseline data for proposed Reclamation restoration projects.
Technical Memorandum No. ENV-2022-086
Rio Grande Silvery Minnow October Monitoring: 2020
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Figu
re 1.—Study area overview for fall 2020 Rio Grande silvery minnow survey sites.
Technical Memorandum No. ENV-2022-086
Rio Grande Silvery Minnow October Monitoring: 2020
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2.0 Methods
2.1 Study Area
Survey sites were selected within and downstream from silvery minnow critical habitat
throughout the Middle Rio Grande. Ten sites were sampled in October 2020. These sites have a
multitude of mesohabitats with varying depths and velocities. The sampling sites stretched from
roughly 3 miles downstream from the San Acacia Diversion Dam to the delta of Elephant Butte
Reservoir: San Acacia (River Mile [RM] 112), Upper Escondida (RM 104), Lower Escondida
(RMs 102–103), Rhodes Canyon (RM 92), LFCC (RM 54), Nogal Canyon (RM 51), Pete Well
(RMs 47–48), Rockhouse (RM 44), North Monticello (RMs 38–39) and South Monticello (RM
37). With the continued recession of the reservoir, more of the reservoir pool has been exposed,
making additional surveys possible (see Figure 1). Surveys took place from October 19 through
23 generally between 9 a.m. and 5 p.m. These sites were selected in an effort to continue the
long-term data collection associated with each site and river reach (2010 to 2020). Site selection
was based on the need for the collection of baseline or monitoring data from Reclamation
operations that may include habitat restoration projects or channel maintenance activities.
Rio Grande flows fluctuate regularly and can change the dynamics of its geomorphology and
habitats. With lower flows, previously inundated sandbars and side channels can become more
prevalent and form shallow areas of low-velocity habitats used by silvery minnows up until flow
intermittency. With increased flows, inundation of some of these sandbars and side channels
become deeper with higher velocities, and the higher elevation flood plain becomes inundated,
possibly forming shallow, low-velocity habitats. During our survey efforts, we focused
on locations likely occupied by silvery minnows, including shallow, low-velocity shorelines
with emergent vegetation rather than locations with a high gradient bank and swift laminar flow.
The mesohabitat classification was recorded at each seine haul location. Each area surveyed was
classified as one of five habitat types (Table 1). Mesohabitats may be further described by habitat
features if present (e.g., shoreline, mid-channel, debris pile, vegetation, and undercut bank).
Substrates at the seine haul locations were also recorded (e.g., silt, sand, gravel, or cobble) as
was the relative velocity (e.g., slack water, slow, medium, or fast). To ensure crew safety, all
seine hauls were conducted in water no more than waist deep (≈3.5 feet).
Technical Memorandum No. ENV-2022-086
Rio Grande Silvery Minnow October Monitoring: 2020
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Table 1.—Mesohabitat classification types on the Middle Rio Grande, New Mexico
Mesohabitat type
Description
Run
Swiftly flowing reaches with little surface agitation and no major flow
obstructions. Mesohabitat often appears as flooded riffles.
Pool
The portion of the river within the channel that is deep and has relatively little
velocity compared to the rest of the channel; usually created by debris or a
sandbar.
Backwater
A small body of water that is connected to the main channel, with no appreciable
flow; often created by a drop in flow that partially isolates a former channel.
Riffle
A shallow, high-velocity habitat where the water surface is irregular and broken
by waves; generally indicates gravel-cobble substrates.
Plunge pool
A pool created by water spilling over a riffle; substrate particle size is variable
depending upon velocity.
2.2 Seine Nets
Biologists used seine nets measuring 3 meters (m) wide by 1 m high with 4.8-millimeter
knotless mesh. The nets had floats on top and a weighted lead line on the bottom. Each crew
member held one end of the net perpendicular to the current and then hauled it downstream for
a distance of 1–13 m. The length of the seine haul was measured with a distance tape, and the
average depth of the seine haul was recorded with a wading rod. Where possible, crews seined
20–30 mesohabitats at each of the 10 sites. Waypoints, defined as Universal Transverse Mercator
points, were recorded at the beginning of each seine haul location.
All fishes captured were placed into a 5-gallon bucket after each seine haul. Endangered fishes
(e.g., silvery minnow) were processed first in order to reduce stress from capture and handling.
Each fish was identified to species and measured for standard length as well as total length. Any
silvery minnows caught were examined for any visible implant elastomer (VIE) tags, and tag
color and location were verified and recorded.
Catch per unit effort (CPUE) was calculated for each site by dividing the number of silvery
minnows captured by the area sampled and multiplying by 100 to provide fish per 100 square
meters (m²). Area was calculated by multiplying the length of the seine haul by the width of
the seine (3 m). Length data were rounded to the nearest 10-millimeter bin category for length-
frequency histograms. All data on silvery minnow captures are provided in appendix 1.
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Rio Grande Silvery Minnow October Monitoring: 2020
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3.0 Results
3.1 Observations During the 2020 Fish Surveys
3.1.1 2020 Rio Grande Flows and Water Quality Parameters
Flows in 2020 were variable and below average, tapering quickly in spring to early summer,
leading to subsequent drying of over 37 miles of the Rio Grande in the San Acacia Reach in
three different segments (Figure 2) (E. Kandl 2020, personal communication). In late July, the
Middle Rio Grande experienced a large rain event, and the river was continuous; however, by
early August, portions of the Middle Rio Grande flows were once again discontinuous.
Figure 2.—Daily mean discharge for U.S. Geological Survey Station 08355050 Rio Grande near
Escondida, New Mexico, from January 1 to October 30, 2020.
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Rio Grande Silvery Minnow October Monitoring: 2020
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Water quality parameters, such as temperature and dissolved oxygen, are summarized in
Table 2. Water quality was found to be within ranges determined to be adequate for aquatic life
(New Mexico Environment Department 2020) and at an acceptable level for silvery minnow
survival (Hutson et al. 2012).
Table 2.—Water quality measurements for all survey locations
Site
Date
DO
(mg/L)
Oxygen
saturation
(%)
Temperature
(°C)
Specific
conductivity
µs/cm
San Acacia
10/23/2020
8.2
92.8
12.2
955
Upper Escondida
10/22/2020
10.3
114.6
11.9
965
Lower Escondida
10/22/2020
8.2
99.9
10.3
955
Rhodes Canyon
10/22/2020
9.6
129.6
21.0
897
LFCC
10/21/2020
8.9
103.6
14.2
849
Nogal Canyon
10/20/2020
9.4
105.3
12.5
854
Pete Well
10/20/2020
10.1
128.2
18.4
853
Rock House
10/21/2020
9.7
104.2
10.7
839
North Monticello
10/19/2020
8.3
103.1
17.3
888
South Monticello
10/19/2020
8.1
94.5
13.9
711
3.1.2 2020 Rio Grande Silvery Minnows Captured and Catch per Unit
Effort
Seventeen silvery minnows were captured in 2020 (Figure 3); a majority (9; 53%) were captured
at the South Monticello site (Table 3). The corresponding CPUEs (fish/100 square meters) for
each sampling site were:
• South Monticello: 1.57
• Upper Escondida: 0.82
• San Acacia: 0.52
• Lower Escondida: 0.42
• Nogal Canyon: 0.12 (Figure 4).
The silvery minnows were primarily captured in slow to medium flows with a sandy substrate.
The depth in which the silvery minnows were caught ranged from 0.1 to 0.3 m. A variety of
fish size classes were caught throughout the sampling sites. Length-frequency histograms are
provided in the subsections below to illustrate the size classes of silvery minnows caught at each
sampling site. No silvery minnows collected were observed to have VIE tags.
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Rio Grande Silvery Minnow October Monitoring: 2020
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0
50
100
150
200
250
300
350
400
2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020
Number of Silvery Minnows
Year
Number of Silvery Minnows
Figu
re 3.—Total number of silvery minnows captured in each sampling year on the Middle Rio Grande,
New Mexico.
Table 3.—Number of silvery minnows captured at each
sampling site on the Middle Rio Grande, New Mexico,
October 2020
Sampling site
Silvery minnow
South Monticello
9
Upper Escondida
3
San Acacia
2
Lower Escondida
2
Nogal Canyon
1
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Rio Grande Silvery Minnow October Monitoring: 2020
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0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
San Acacia Upper Escondida Lower Escondida Nogal S. Monticello
CPUE (Silvery minnow/ 100 m
2
)
Sampling Location
Silvery Minnow CPUE
Fig
ure 4.—CPUE (fish/100 square meters) for silvery minnows during October seining on the Middle
Rio Grande, New Mexico.
3.2 Sampling Site Results
The number of silvery minnows caught and species composition at each sampling site are
described below. (Note: The aerial imagery used in the figures might not be contemporaneous).
3.2.1 San Acacia (River Mile 112)
Twenty-one seine hauls were completed at this site. A total of 1,438 fishes were captured,
comprised of 5 different species (Table 4). Red shiners (Cyprinella lutrensis) were the
most abundant species caught (99%). Two silvery minnows were captured at this sampling site
(Figure 5). Figure 6 illustrates the 21 seining locations and silvery minnow capture locations at
the San Acacia site.
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Rio Grande Silvery Minnow October Monitoring: 2020
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Table 4.—Species composition at the San Acacia
site (Totals are > 100% due to rounding.)
Species
1
Percent
composition
Red shiner
99
Gambusia
1
Channel catfish
< 1
Silvery minnow
< 1
Flathead chub
< 1
1
Scientific names for fish species are included in
appendix B.
0
1
2
3
50 60 70
Frequency
San Acacia
Size Classes (mm)
Figure 5.—Silvery minnow length-frequency histogram from the San Acacia site (RM 112) on the
Middle Rio Grande, New Mexico, October 2020.
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Rio Grande Silvery Minnow October Monitoring: 2020
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Fig
ure 6.—San Acacia seining locations and silvery minnow capture locations (RM 112), Middle
Rio Grande, New Mexico, October 2020.
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3.2.2 Upper Escondida (River Mile 104)
Twenty seine hauls were completed at this site. A total of 646 fishes were captured, comprised of
7 different species (Table 5). Red shiners were the most abundant species caught (98%). Three
silvery minnows were caught at this sampling site (Figure 7). Figure 8 illustrates the 20 seining
locations and silvery minnow capture locations at the Upper Escondida site.
Table 5.—Species composition at the Upper Escondida site
Species
1
Percent
composition
Red shiner
98
Gambusia
1
Common carp
< 1
River carpsucker
< 1
Flathead chub
< 1
Silvery minnow
< 1
Channel catfish
< 1
0
1
2
3
4
70 80 90
Frequency
Upper Escondida
Size Class (mm)
Figure 7.—Silvery minnow length-frequency histogram from the Upper Escondida site (RM 104)
on the Middle Rio Grande, New Mexico, October 2020.
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Fig
ure 8.—Upper Escondida seining locations and silvery minnow capture locations (RM 104), Middle
Rio Grande, New Mexico, October 2020.
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3.2.3 Lower Escondida (River Miles 102–103)
Twenty-two seine hauls were completed at this site. A total of 1,549 fishes were captured,
comprised of 6 different species (Table 6). Red shiners were the most abundant species caught
(99%). Two silvery minnows were captured at this sampling site (Figure 9). Figure 10 illustrates
the 22 seining locations and silvery minnow capture locations at the Lower Escondida site.
Table 6.—Species composition at the Lower Escondida
site
Species
Percent
composition
Red shiner
99
Common carp
< 1
Flathead chub
< 1
Gambusia
< 1
Silvery minnow
< 1
River carpsucker
< 1
0
1
2
3
60 70 80
Frequency
Lower Escondida
Size Class (mm)
Figure 9.—Silvery minnow length-frequency histogram from the Lower Escondida site
(RMs 102–103) on the Middle Rio Grande, New Mexico, October 2020.
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Fig
ure 10.—Lower Escondida seining locations and silvery minnow capture locations (RMs 102–103),
Middle Rio Grande, New Mexico, October 2020.
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3.2.4 Rhodes Canyon (River Mile 92)
Twenty seine hauls were completed at this site; all were above a sediment plug that was forming
above an irrigation pump. Only one red shiner was captured at this site (Table 7). No silvery
minnows were captured at this sampling site. Figure 11 illustrates the 20 seining locations and
silvery minnow capture locations at the Rhodes Canyon site.
Table 7.—Species composition at the Rhodes Canyon
site
Species
Percent
composition
Red shiner
100
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Fig
ure 11.—Rhodes Canyon seining locations and silvery minnow capture locations (RM 92), Middle
Rio Grande, New Mexico, October 2020.
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3.2.5 LFCC (River Mile 54)
Thirty seine hauls were completed at this site. A total of 1,642 fishes were captured, comprised
of 7 different species (Table 8). Red shiners were the most abundant fish caught (93%). No
silvery minnows were captured at this sampling site. Figure 12 shows the 30 seining locations
and silvery minnow capture locations at the Pete Well site.
Table 8.—Species composition at the LFCC site
(Totals are > 100% due to rounding.)
Species
Percent
composition
Red shiner
93
Gambusia
6
Threadfin shad
1
Channel catfish
< 1
Gizzard shad
< 1
River carpsucker
< 1
Bullhead minnow
< 1
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Figure 12.—LFCC seining locations and silvery minnow capture locations (RM 54), Middle Rio Grande,
New Mexico, October 2020.
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3.2.6 Nogal Canyon (River Mile 51)
Thirty seine hauls were completed at this site. Among all the sampling locations, the highest
number of fishes captured (all species) was at this location. A total of 1,961 fishes were caught,
comprised of 7 different species (Table 9). Red shiners were the most abundant species captured
(93%). One silvery minnow was captured in this sampling site (Figure 13). Figure 15 illustrates
the seining locations and silvery minnow capture locations at the Nogal Canyon site.
Table 9.—Species composition at the Nogal Canyon
site
Species
Percent
composition
Red shiner
93
Gambusia
6
Threadfin shad
< 1
Channel catfish
< 1
Common carp
< 1
Freshwater drum
< 1
Silvery minnow
< 1
0
1
2
3
50 60 70
Frequency
Nogal Canyon
Size Class (mm)
Figure 13.—Silvery minnow length-frequency histogram from the Nogal Canyon site
(RM 51) on the Middle Rio Grande, New Mexico, October 2020.
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Fig
ure 14.—Nogal Canyon seining locations and silvery minnow capture locations (RM 51), Middle
Rio Grande, New Mexico, October 2020.
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3.2.7 Pete Well (River Miles 47–48)
Twenty-one seine hauls were completed at this site. A total of 602 fishes were captured,
comprised of 4 different species (Table 10). Red shiners were the most abundant species
captured (90%). No silvery minnows were captured at this sampling site. Figure 15 illustrates
the 21 seining locations and silvery minnow capture locations at the Pete Well site.
Table 10.—Species composition at the Pete Well
site (Totals are > 100% due to rounding.)
Species
Percent
composition
Red shiner
90
Gambusia
8
Threadfin shad
2
Channel catfish
< 1
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Fig
ure 15.—Pete Well seining locations and silvery minnow capture locations (RMs 47–48), Middle
Rio Grande, New Mexico, October 2020.
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3.2.8 Rockhouse (River Mile 44)
Twenty seine hauls were completed at this site. A total of 1,541 fishes were captured, comprised
of 4 different species (Table 11). Red shiners were the most abundant species captured (76%).
No silvery minnows were captured at this sampling site. Figure 16 illustrates the seining
locations and silvery minnow capture locations at the Rockhouse site.
Table 11.—Species composition at the Rockhouse site
Species
Percent
composition
Red shiner
76
Gambusia
22
Threadfin shad
1
Blue catfish
< 1
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Fig
ure 16.—Rockhouse seining locations and silvery minnow capture locations (RM 44), Middle
Rio Grande, New Mexico, October 2020.
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Rio Grande Silvery Minnow October Monitoring: 2020
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3.2.9 North Monticello (River Miles 38–39)
Twenty-six seine hauls were completed at this site. A sediment plug had formed in the channel,
and seine hauls were conducted upstream of and downstream from this plug: 5 were upstream,
and 21 were downstream. A total of 1,373 fishes were caught, comprised of 8 different species
(Table 12). No silvery minnows were captured at this sampling site. Figure 17 illustrates the
26 seining locations and silvery minnow capture locations at the North Monticello site.
Table 12.—Species composition at the North Monticello
site
Species
Percent
composition
Red shiner
87
Gambusia
8
Bullhead minnow
4
Channel catfish
< 1
Gizzard shad
< 1
Blue catfish
< 1
Common carp
< 1
Bluegill
< 1
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Fig
ure 17.—North Monticello seining locations and silvery minnow capture locations (RMs 38–39),
Middle Rio Grande, New Mexico, October 2020.
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Rio Grande Silvery Minnow October Monitoring: 2020
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3.2.10 South Monticello (River Mile 37)
Twenty seine hauls were completed at this site. A total of 441 fishes were caught, comprised of
6 different species (Table 13). Red shiners were the most abundant species caught (70%). Nine
silvery minnows were captured at this sampling site; this was the highest concentration of silvery
minnows captured at a single location (Figure 18). Figure 19 illustrates the seining locations and
silvery minnow capture locations at the South Monticello site.
Table 13.—Species composition at the South Monticello
site
Species
Percent
composition
Red shiner
70
Gambusia
27
Silvery minnow
2
Bigscale logperch
< 1
Blue catfish
< 1
Threadfin shad
< 1
0
1
2
3
4
5
6
7
8
9
30 40 50
Frequency
South Monticello
Size Class (mm)
Figure 18.—Silvery minnow length-frequency histogram from the South
Monticello site (RM 37) on the Middle Rio Grande, New Mexico, October 2020.
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Fig
ure 19.—South Monticello seining locations and silvery minnow capture locations (RM 37),
Middle Rio Grande, New Mexico, October 2020.
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Rio Grande Silvery Minnow October Monitoring: 2020
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3.3 Mesohabitat Utilization
Seine hauls were completed in seven different mesohabitats and further categorized based on
their location within the river channel (Table 14). Silvery minnows were captured in:
• 50 percent of the mid-channel pools sampled (n = 2)
• 10 percent of riffle habitats (n = 10)
• 5 percent of shoreline run habitats (n = 121)
• 13 percent of mid-channel run habitats (n = 71)
Table 14.—Number of each mesohabitat sampled and corresponding
silvery minnow captures
Mesohabitat type Number seined
Silvery minnow
captured
Mid-channel pool
2
1
Shoreline pool
8
0
Backwater
9
0
Riffle
10
1
Plunge pool
15
0
Shoreline run
121
6
Mid-channel run
71
9
No silvery minnows were collected in plunge pools (n = 15), backwaters (n = 9) or shoreline
pools (n = 8) in 2020. Figure 20 illustrates the mesohabitat types where silvery minnows were
captured.
Technical Memorandum No. ENV-2022-086
Rio Grande Silvery Minnow October Monitoring: 2020
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Figu
re 20.—Silvery minnow captures according to mesohabitat classification during October
seining on the Middle Rio Grande, New Mexico, 2020.
4.0 Discussion
Sampling in the San Acacia reach remains a high priority because of the large morphological
changes to the Middle Rio Grande (e.g., the formation of sediment plugs in the historic
pool of Elephant Butte Reservoir) and the higher levels of operational activities (such as channel
realignment) in these locations. Reclamation’s annual October seine surveys in the Middle
Rio Grande contribute to a long-term monitoring program that began in 2010. Since 2010, the
number of silvery minnows captured during the fall seining efforts have greatly fluctuated with
the annual variation in water levels throughout the Rio Grande. Higher densities of silvery
minnows are linked to extended high flows in spring that allow for dispersal of eggs and larvae
into suitable nursery habitats and provide suitable rearing conditions, leading to recruitment and
increased survival (Dudley and Platania 2011).
The low flows and drying of portions of the Middle Rio Grande experienced in 2020 negatively
impacted silvery minnows, and our low catch appeared to reflect that. Some of the drier years
were from 2012 through 2015, which coincided with some of the lowest numbers of silvery
minnows collected. The highest silvery minnow capture rates were in 2017, a year when the
Rio Grande had increased surface water and greater flood plain inundation. The river did not dry
out throughout most of summer 2017, likely attributing to the increased densities (Archdeacon
et al. 2019). In 2020, stretches throughout the Middle Rio Grande were dry due to reduced water
flow. Drying of portions of the channel occurred as early as May and may have interfered with
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spawning or recruitment dynamics. The numbers of silvery minnows captured dropped
dramatically this year compared to 2019 (17 versus 207 in 2020 versus 2019, respectively).
Given the magnitude of the difference between years, or when only comparing sites that were
sampled with similar effort and collection gears, it can be reasonably assumed that the higher
flows in 2019 were directly correlated to the numbers of silvery minnows collected that fall.
The South Monticello site had the highest number of silvery minnows captured. This stretch of
river did not dry out throughout the year. Conversely, sites upstream of South Monticello all
experienced drying to some extent, and the lower numbers, or absence of minnows, are most
likely attributed to both low silvery minnow densities and that they may not have repopulated
these recently rewetted stretches of river by the time our surveys were conducted.
The length-frequency histograms indicated that roughly half of the silvery minnows captured in
2020 were young-of-year, which suggests a lack of recruitment associated with the low flows
that attributed to the lower numbers of age-0 silvery minnows observed in 2020, and it also
informs us that there was some recruitment experienced.
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5.0 References
Archdeacon, T.P., S.A. Mullner, and J.D. Grant. 2019. Rio Grande Silvery Minnow Fish Rescue
2018, Annual Report. U.S. Fish and Wildlife Service, New Mexico Fish and Wildlife
Conservation Office, Albuquerque, New Mexico.
Bestgen, K.R. and S.P. Platania. 1991. Status and conservation of the Rio Grande silvery
minnow, Hybognathus amarus. The Southwestern Naturalist 36:225–232.
Dudley, R.K. and S.P. Platania. 1993. 1993 Population Monitoring of Rio Grande Silvery
Minnow, Final Report. University of New Mexico, Albuquerque.
_____. 1997. 1996 Population Monitoring of Rio Grande Silvery Minnow. Submitted to
the Bureau of Reclamation and the U.S. Army Corps of Engineers, Albuquerque,
New Mexico, USA. DOI: 10.13140/RG.2.1.4533.1362.
_____. 2011. Rio Grande Silvery Minnow Population Monitoring Program Results from
September 2009 to October 2010. Submitted by the American Southwest Ichthyological
Researchers, LLC, for the Middle Rio Grande Endangered Species Act Collaborative
Program, Bureau of Reclamation, Albuquerque, New Mexico.
Hutson, A.M., L.A. Toya, and D. Tave. 2012. Production of the endangered Rio Grande silvery
minnow, Hybognathus amarus, in the rearing facility at the Los Lunas Silvery Minnow
Refugium. Journal of the World Aquaculture Society 43:84–90.
Production of the Endangered Rio Grande Silvery Minnow, Hybognathus amarus, in the
Conservation Rearing Facility at the Los Lunas Silvery Minnow Refugium
(conanp.gob.mx)
Kandl, E. 2020. Bureau of Reclamation, Water Management Division, Albuquerque,
New Mexico, personal communication. December 1.
New Mexico Environment Department. 2020. New Mexico’s Water Quality Standards as
approved by the New Mexico’s Water Quality Control Commission.
https://www.env.nm.gov/surface-water-quality/wqs/ (accessed on November 16, 2021).
U.S. Fish and Wildlife Service (USFWS). 1994. Endangered and Threatened Wildlife and Plants:
Final rule to list the Rio Grande silvery minnow as an endangered species. Federal
Register 59: 36988-36995.
_____. 2003. Endangered and Threatened Wildlife and Plants; Final rule for the Designation of
Critical Habitat for the Rio Grande silvery minnow. Federal Register 68: 8087-8135.
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Rio Grande Silvery Minnow October Monitoring: 2020
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_____. 2010. Rio Grande Silvery Minnow (Hybognathus amarus) Recovery Plan, First Revision.
Albuquerque, New Mexico.
USFWS (see U.S. Fish and Wildlife Service).
Appendix A
Silvery Minnow (Hybognathus amarus) Collection Data:
October 2020
Technical Memorandum No. ENV-2022-086
Rio Grande Silvery Minnow October Monitoring: 2020 – Appendix A
A-1
Table A-1.—Date, location of capture, tag information, and standard length of silvery minnows captured during the October 2020 surveys in the Middle Rio Grande in
New Mexico
Scientific name
Survey date
Easting
Northing
Universal
Transverse
Mercator
zone
Site
Species
not found
Visible
implant
elastomer tag
information
Standard length
(millimeters)
Hybognathus amarus
10/19/2020
297547
3686246
13
South Monticello
No mark
32
Hybognathus amarus
10/19/2020
297547
3686246
13
South Monticello
No mark
39
Hybognathus amarus
10/19/2020
297547
3686246
13
South Monticello
No mark
40
Hybognathus amarus
10/19/2020
297547
3686246
13
South Monticello
No mark
40
Hybognathus amarus
10/19/2020
297547
3686246
13
South Monticello
No mark
40
Hybognathus amarus
10/19/2020
297547
3686246
13
South Monticello
No mark
43
Hybognathus amarus
10/19/2020
297547
3686246
13
South Monticello
No mark
43
Hybognathus amarus
10/19/2020
297566
3686217
South Monticello
No mark
40
Hybognathus amarus
10/19/2020
297569
3686168
South Monticello
No mark
40
Hybognathus amarus
10/19/2020
North Monticello (upstream
sediment plug)
X
Hybognathus amarus
10/19/2020
North Monticello (downstream
sediment plug)
X
Hybognathus amarus
10/20/2020
301862
3706172
Nogal
No mark
61
Hybognathus amarus
10/20/2020
Red Rock/Pete Well Road
X
Hybognathus amarus
10/21/2020
Rockhouse
X
Hybognathus amarus
10/21/2020
LFCC confluence
X
Hybognathus amarus
10/22/2020
326447
3775509
Lower Escondida
No mark
59
Hybognathus amarus
10/22/2020
326522
3774803
Lower Escondida
No mark
75
Hybognathus amarus
10/22/2020
326143
3778043
Upper Escondida
No mark
77
Hybognathus amarus
10/22/2020
326143
3778043
Upper Escondida
No mark
79
Hybognathus amarus
10/22/2020
326026
3777694
Upper Escondida
No mark
75
Hybognathus amarus
10/22/2020
Rhodes Canyon
X
Hybognathus amarus
10/23/2020
326012
3787936
San Acacia
No mark
66
Hybognathus amarus
10/23/2020
326214
3787788
San Acacia
No mark
52
Appendix B
Common and Scientific Names of Fish Species
B-1
Common name
Scientific name
Bigscale logperch
Percina macrolepida
Blue catfish
Ictalurus furcatus
Bluegill
Lepomis macrochirus
Bullhead minnow
Pimephales vigilax
Channel catfish
Ictalurus punctatus
Common carp
Cyprinus carpio
Flathead chub
Platygobio gracilis
Freshwater drum
Aplodinotus grunniens
Gambusia
Gambusia affinis
Gizzard shad
Dorosoma cepedianum
Red shiner
Cyprinella lutrensis
River carpsucker
Carpiodes carpio
Silvery minnow
Hybognathus amarus
Threadfin shad
Dorosoma petenense
