Prepared in cooperation with Colorado Parks and Wildlife

Assessing the Use of Existing Data to Compare Plains 

Fish Assemblages Collected from Random and Fixed

Sites in Colorado

By Robert E. Zuellig and Harry J. Crockett

Open-File Report 2013–1115

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2013

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Suggested citation:

Zuellig, R.E., and Crockett, H.J, 2013, Assessing the use of existing data to compare plains fish assemblages 

collected from random and fixed sites in Colorado: U.S. Geological Survey Open-File Report 2013–1115, 9 p., 

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iii

Contents

Abstract ......................................................................................................................................................................... 1

Introduction.................................................................................................................................................................... 1

Purpose and Scope.................................................................................................................................................... 2

Study Area ................................................................................................................................................................. 3

Study Methods............................................................................................................................................................... 4

Fish Sampling Methods Used for the 1990s Inventories ............................................................................................ 4

Fish Sampling Methods Used Between 2007 and 2010............................................................................................. 4

Data Preparation........................................................................................................................................................ 5

Data Analysis ............................................................................................................................................................. 6

Testing for Differences Between Time Periods (1990s and 2000s)........................................................................ 7

Assessing the Use of Existing Data to Compare Fish Assemblages from Random and Fixed Sites—Differences 

Between Time Periods...................................................................................................................................... 7

Moving Forward............................................................................................................................................................. 8

Summary ....................................................................................................................................................................... 8

Acknowledgments.......................................................................................................................................................... 9

References Cited........................................................................................................................................................... 9

Figure

1. Distribution of selected sites compiled from the Colorado Parks and Wildlife aquatics database with fishassemblage data collected from the Interior Plains portion of the South Platte and Arkansas River Basins, 

Colorado, from 1992 to 1996 and 2007 to 2010 ............................................................................................................ 3

Table

1. Description and location of selected sites compiled from the Colorado Parks and Wildlife aquatics database

with fish-assemblage data collected from the Interior Plains portion of the South Platte and Arkansas River Basins, 

Colorado, from 1992 to 1996 and 2007 to 2010 ............................................................................................................ 6
iv

Conversion Factors and Datum

Multiply By To obtain

Length

foot (ft)

mile (mi)

millimeter (mm)

0.3048

1.609

0.03937

meter

kilometer (km) 

inch

Horizontal coordinate information is North American Datum of 1983 (NAD 83)”

Initialisms Used in This Report

ANOSIM analysis of similarity

CPW Colorado Parks and Wildlife
1 

Assessing the Use of Existing Data to Compare Plains 

Fish Assemblages Collected from Random and Fixed

Sites in Colorado

By Robert E. Zuellig1 and Harry J. Crockett2

Abstract

The U.S. Geological Survey, in cooperation with Colorado Parks and Wildlife, assessed the 

potential use of combining recently (2007 to 2010) and formerly (1992 to 1996) collected data to 

compare plains fish assemblages sampled from random and fixed sites located in the South Platte and 

Arkansas River Basins in Colorado. The first step was to determine if fish assemblages collected 

between 1992 and 1996 were comparable to samples collected at the same sites between 2007 and 2010. 

If samples from the two time periods were comparable, then it was considered reasonable that the 

combined time-period data could be used to make comparisons between random and fixed sites. In 

contrast, if differences were found between the two time periods, then it was considered unreasonable to 

use these data to make comparisons between random and fixed sites. One-hundred samples collected 

during the 1990s and 2000s from 50 sites dispersed among 19 streams in both basins were compiled 

from a database maintained by Colorado Parks and Wildlife. Nonparametric multivariate two-way 

analysis of similarities was used to test for fish-assemblage differences between time periods while 

accounting for stream-to-stream differences. Results indicated relatively weak but significant timeperiod differences in fish assemblages. Weak time-period differences in this case possibly were related 

to changes in fish assemblages associated with environmental factors; however, it is difficult to separate 

other possible explanations such as limited replication of paired time-period samples in many of the 

streams or perhaps differences in sampling efficiency and effort between the time periods. Regardless, 

using the 1990s data to fill data gaps to compare random and fixed-site fish-assemblage data is ill 

advised based on the significant separation in fish assemblages between time periods and the inability to 

determine conclusive explanations for these results. These findings indicated that additional sampling 

will be necessary before unbiased comparisons can be made between fish assemblages collected from 

random and fixed sites in the South Platte and Arkansas River Basins.

Introduction

Colorado Parks and Wildlife (CPW) conducted fish inventories of the South Platte and Arkansas 

River Basins during the early to mid-1990s (Nesler and others, 1997; Nesler and others, 1999) to 

evaluate the status of plains fishes in Colorado and identify species in need of conservation and 

protective status. Site selection for these inventories was based on the ease of property access 

 

1

U.S. Geological Survey, Denver Federal Center, MS 415, Denver CO 80225

2

Colorado Division of Wildlife, 317 W. Prospect Street, Fort Collins, CO 80526
2 

(convenience) and representativeness (provincial judgment) determined by CPW fisheries-management 

biologists. Sites with water at road crossings and easily accessible areas such as State-owned properties

were targeted. This process resulted in 1,381 and 2,128 site visits in the South Platte and Arkansas River 

Basins, respectively. Less than 30 percent of the visited sites had water resulting in 534 sites in the 

South Platte and 416 sites in the Arkansas River Basins. Spatially, sites were sampled roughly from 1.6- 

to 8-km intervals (0.99- to 4.97-mi) in the South Platte River Basin (Nesler and others, 1997). The linear 

distance between sites with fish varied considerably in the Arkansas River Basin because of the 

ephemeral nature of most streams (Nesler and others, 1999). 

CPW is considering more statistically rigorous sampling methods accompanied with a 

statistically based selection of sites to update and monitor the status of plains fishes in Colorado. 

Statistically based designs use a probabilistic approach to randomly select sampling locations from a 

targeted population of sites. Additionally, CPW is interested in exploring the possibility of using 

multivariate methods to monitor changes in fish assemblages in addition to using other methods to 

monitor changes in individual species occurrence, abundance, and distribution. Although each of these 

goals and interests could entail different sampling methods or analyses, all require spatially extensive 

sampling. Incorporating a probabilistic study design will allow managers to extend statistical inference 

to unsampled locations within the plains portions of the South Platte and Arkansas River Basins 

(sampling frame) in Colorado. Although extending statistical inference to unsampled locations is 

desirable, CPW has a strong interest in continuing to sample many of the sites visited during the 1990s 

inventories for several reasons. First, some State protected species have limited distributions and are 

reliably found at a few known sites. Second, many CPW fisheries-management biologists have longterm data from sites sampled during the 1990s inventories and want to continue sampling those sites. 

Third, less logistical effort is needed before and during field sampling because landowner permission is 

already established at sites sampled during the 1990s inventories. Establishing landowner permission at 

random sites and gaining stream access can be labor intensive and costly. So before a probabilistic 

sampling design is adopted, CPW wants to determine the comparability of fish assemblages collected 

from random and paired nonrandom sites (fixed sites, hereafter) utilizing available data collected during 

two distinct time periods (1992 to 1996 and 2007 to 2010). At the present time (2012), in order to make 

this comparison, many of the random sites sampled between 2007 and 2010 will need to be paired with 

data from a nearby fixed site that was sampled during the 1990s; however, doing so could bias the 

analysis if fish assemblages differ between the two time periods. To this end, the U.S. Geological 

Survey, in cooperation with CPW, used nonparametric multivariate analysis to test if plains fish 

assemblages differed at sites that were sampled during both time periods (1992 to 1996 and 2007 to 

2010) in the South Platte and Arkansas River Basins in Colorado. 

Purpose and Scope

This report presents the strength of similarity between plains fish assemblages collected during 

two distinct time periods from the South Platte and Arkansas River Basins in Colorado. Data consisting 

of 100 samples collected from 50 sites dispersed among 19 streams during the 1990s and 2000s (1992 to 

1996 and 2007 to 2010) were compiled from the CPW aquatics database (unpublished data, Aquatics 

Wildlife Research Group, Colorado Parks and Wildlife, June 2011). These data were used to assess the 

potential for bias when using existing CPW data to make fish-assemblage comparisons between random 

and fixed sites by determining the strength of similarity between plains fish assemblages collected 

during two distinct time periods. Results will help CPW move forward in developing a plan for 

monitoring the status of plains fish assemblages in Colorado. 
3 

Study Area

The study area consists of the Interior Plains (Fenneman and Johnson, 1946) portion of the 

South Platte and Arkansas River Basins in Colorado (fig. 1). Detailed environmental characteristics of 

this region as they relate to river systems are described elsewhere (Dennehy and others, 1993; Fausch 

and Bestgen, 1997; Nadler and Schumm, 1981; Eschner and others, 1983). 

Figure 1. Distribution of selected sites compiled from the Colorado Parks and Wildlife aquatics database with fishassemblage data collected from the Interior Plains portion of the South Platte and Arkansas River Basins, 

Colorado, from 1992 to 1996 and 2007 to 2010.
4 

Study Methods

Plains fish community data from the South Platte and Arkansas River Basins were queried from 

the CPW aquatics database (unpublished data, Aquatics Wildlife Research Group, Colorado Parks and 

Wildlife, June 2011) for sites that were sampled during two time periods (1992 to 1997 and 2007 to 

2010). These data were explored to assess the potential for bias from using existing CPW data to 

compare fish assemblages collected from random and fixed sites. This process focused on determining 

if fish assemblages collected during 1992 to1996 were comparable to those collected at the same sites 

during 2007 to 2010 using nonparametric multivariate methods (PRIMER-E ver. 6.1, Plymouth, United 

Kingdom; Clarke and Gorley, 2006). If samples from the two time periods were comparable 

(statistically indistinguishable), then it was considered reasonable that the combined time-period data 

could later be used to make comparisons between random and fixed sites. In contrast, if statistical 

differences were found in fish assemblages between the two time periods, then it was considered 

unreasonable to use these data to make comparisons between random and fixed sites. 

Fish Sampling Methods Used for the 1990s Inventories

Nesler and others (1997, 1999) briefly described sampling methods used during the 1990s native 

fish inventories of the South Platte and Arkansas River Basins. In general, fish were sampled from all 

habitats present at each site using seining, backpack electrofishing, bank-mounted electrofishing, or a 

combination of these methods. Electrofishing was not used in the lower Arkansas River due to high 

specific electrical conductivities. Site selection for these inventories was based on the ease of property 

access and representativeness as determined by CPW fisheries-management biologists. Sites with water 

at road crossings and easily accessible areas such as State owned properties were targeted. Site length 

varied from 165 to 985 feet in order to capture all habitat features present at each site. Very little 

quantitative information was available regarding sampling efficiency or site-specific effort. 

Fish Sampling Methods Used Between 2007 and 2010

A combination of electrofishing, seining, and multipass removal sampling along a predefined 

reach was used at sites visited between 2007 and 2010. Targeted reach length was 20 times the mean 

stream width to include multiple riffle, run, and pool sequences (Fitzpatrick and others, 1998; Stanfield 

and others, 1997) but was constrained to 150 and 500 feet for efficiency and logistical purposes. For 

most streams (width <30 feet), electrofishing was conducted using two Smith-Root LR-24 backpack 

electrofishers and a total crew of four. For the largest steams (width >30 feet), a three-electrode bargeelectrofishing system was used and supplemented as necessary by one or two backpack electrofishers 

with the crew expanded as needed. Specific electrical conductivity and temperature were measured prior 

to sampling and used to determine electrofisher settings in accordance with power transfer theory (Kolz, 

1989; Kolz, 2006); effectiveness of calculated settings was confirmed by electrofishing outside the 

sample reach. Sampling crews were experienced and care was taken to ensure thorough coverage of the 

entire sample reach and all present habitat types. 

Removal sampling consisted of a minimum of three passes but included additional passes when 

a new species was encountered on the third pass. In these cases additional passes were made until no 

new species were collected. Typically, reaches were electrofished on the first pass and seined on the 

second, whereas the third and subsequent passes employed whichever gear type proved most effective 

from the first two passes. All nets and seines were equipped with 3/16- or 1/4-inch mesh. After each 

pass, fish were identified to species and individually measured (total length to nearest millimeter). For 

exceptionally large catches, only the first 50 individuals of each species were measured, but all were 

identified and enumerated. Except for voucher specimens, all fish were held in live cages upstream of 
5 

the sample reach and released after all passes were completed. Infrequently, it was necessary to deviate 

from this protocol in order to effectively sample the site (for example, if ambient conductivity 

prohibited effective electrofishing); however, these data were excluded from analysis so related 

sampling details are not included herein. 

Data Preparation

After reviewing the methods in Nesler and others (1997, 1999), available information in the 

database, and speaking with biologists involved in the 1990s inventories, it was apparent that sampling 

effort varied within the inventory data by necessity and that the inventory methods differed from the 

methods used between 2007 and 2010. Unfortunately, there was not enough available information 

within the 1990s inventories data to make adjustments to account for unequal sampling effort or 

sampling efficiency. For these reasons, data were reduced to presence-absence of species for all 

analyses to help account for unequal sampling effort among samples and sites. The consequences of 

using presence-absence data are that all information about abundance is lost, and rare species are given 

equal weight with common species thereby losing information about species’ prevalence and fishcommunity structure. Additionally, only samples from first-pass electrofishing collected between June 

and November were considered, and where gear type was unknown, it was assumed that first-pass data 

were collected using electrofishing equipment. This assumption mostly applied to 1990s inventory data 

and only accounted for a few samples. Finally, samples collected during 1992 to 1996 were paired with 

samples collected on the next closest month and day during 2007 to 2010 for each site. This resulted in 

most samples (45 of 50) being collected within a 90-day index period between June and November. 

Analysis was completed with and without the remaining five-paired samples collected outside of 90 

days. It was found that these samples had little influence on the results, so they were retained to 

maximize the number of sample pairs. The final dataset consisted of 100 samples collected during the 

1990s and 2000s from 50 sites dispersed among 19 streams in both basins (table 1, fig. 1). 
6 

Table 1. Description and location of selected sites compiled from the Colorado Parks and Wildlife aquatics 

database with fish-assemblage data collected from the Interior Plains portion of the South Platte and Arkansas 

River Basins, Colorado, from 1992 to 1996 and 2007 to 2010.

[ID, identifier; latitude and longitude in degrees, minutes, and seconds]

Site ID 

(fig. 1)

Station 

ID1 Station name Latitude Longitude

South Platte River

1 SP0006 West Plum Creek above highway 67 39 ̊ 25' 43" 104 ̊ 58' 02"

2 SP0627 South Platte River above 31st Avenue 39 ̊ 46' 06" 104 ̊ 59' 02"

3 SP1181 Clear Creek below McIntyre Street above Coors effluent 39 ̊ 46' 17" 105 ̊ 10' 22"

4 SP0631 South Platte River above 124th Avenue at Henderson 39 ̊ 55' 19" 104 ̊ 52' 07"

5 SP1663 South Boulder Creek 250 feet above South Boulder Road 39 ̊ 59' 07" 105 ̊ 13' 17"

6 SP0961 South Boulder Creek 0.38 miles below Baseline Road 40 ̊ 00' 21" 105 ̊ 12' 59"

7 SP0766 Left Hand Creek 400 feet above Main Street 40 ̊ 08' 48" 105 ̊ 06' 11"

8 SP0771 Saint Vrain Creek above 75th Street 40 ̊ 10' 10" 105 ̊ 10' 43"

9 SP0773 Saint Vrain Creek 0.60 miles above North 85th Street 0.68 miles below North 

75th Street

40 ̊ 10' 30" 105 ̊ 09' 54"

10 SP0774 Saint Vrain Creek above Hover Road 40 ̊ 09' 59" 105 ̊ 07' 52"

11 SP0800 Saint Vrain Creek below South main Street 40 ̊ 09' 17" 105 ̊ 06' 02"

12 SP1435 Big Thompson River 0.25 miles above Wilson Avenue between gravel pit 

ponds

40 ̊ 23' 52" 105 ̊ 06' 65"

13 SP0805 Big Thompson River 0.17 miles below Taft Avenue, Centennial Park 40 ̊ 23' 42" 105 ̊ 05' 35"

14 SP3310 Big Thompson River below River Street bridge 40 ̊ 23' 01" 105 ̊ 03' 04"

15 SP1497 Big Thompson River 0.25 miles above County Road 9e 40 ̊ 22' 59" 105 ̊ 02' 04"

16 SP1546 Big Thompson River above County Road 3c 40 ̊ 23' 34" 104 ̊ 57' 58"

17 SP0362 South Platte River above 37th Street Brower State Wildlife Area 40 ̊ 22' 43" 104 ̊ 40' 24"

18 SP3234 Fossil Creek 0.25 miles above South College Avenue at Redtail Grove Open 

Space

40 ̊ 30' 35" 105 ̊ 04' 45"

19 SP0636 Fossil Creek above South County Road 3 at River Bluff Open Space 40 ̊ 29' 07" 104 ̊ 57' 50"

20 SP0881 Spring Creek 250 feet above Drake Street 40 ̊ 33' 09" 105 ̊ 06' 31"

21 SP1158 Cache La Poudre River above North College Avenue 40 ̊ 35' 44" 105 ̊ 04' 41"

22 SP0786 Cache La Poudre River above Riverbend Ponds off Mulberry Road 40 ̊ 34' 35" 105 ̊ 02' 06"

23 SP0794 Cache La Poudre River Environmental Learning Center 0.87 miles above 

Boxelder Creek

40 ̊ 33' 08" 105 ̊ 00' 57"

24 SP1511 Cache La Poudre River at sewage disposal ponds 40 ̊ 33' 02" 105 ̊ 00' 29"

25 SP0790 Cache La Poudre River above North County Line Road 40 ̊ 28' 23" 104 ̊ 56' 37"

26 SP1580 Cache La Poudre River below highway 257 at Windsor sewage outfall 40 ̊ 26' 14" 104 ̊ 52' 23"

27 SP3279 Cache La Poudre River above Sheep Draw 40 ̊ 26' 42" 104 ̊ 46' 34"

28 SP0317 Willow Creek 0.12 miles above Willow Creek Ponds Pawnee National 

Grasslands

40 ̊ 48' 15" 104 ̊ 27' 58"

Arkansas River

39 AR0168 Fountain Creek below Janitell Road below Colorado Springs, Colo. at USGS 

stream gage 07105500

38 ̊ 48' 08" 104 ̊ 47' 39"

40 AR0189 Fountain Creek above highway 16 38 ̊ 43' 33" 104 ̊ 43' 50"

41 AR0319 Fountain Creek below diversion dam at Clear Spring Ranch Park 38 ̊ 38' 52" 104 ̊ 41' 30"

42 AR0147 Fountain Creek above Pinon Road 38 ̊ 26' 19" 104 ̊ 35' 39"

43 AR0077 Greenhorn Creek above interstate 25 37 ̊ 57' 01" 104 ̊ 48' 13"

44 AR0093 Chico Creek below Peyton highway 38 ̊ 27' 31" 104 ̊ 25' 24"

45 AR0094 Chico Creek below Black Squirrel Creek 38 ̊ 24' 05" 104 ̊ 24' 23"

46 AR0149 Chico Creek 0.50 miles below DOT Road 38 ̊ 21' 21" 104 ̊ 23' 03"

47 AR0084 Huerfano River below Fields Road 38 ̊ 08' 45" 104 ̊ 20' 41"

48 AR0023 Wolf Creek above highway 50 38 ̊ 03' 52" 102 ̊ 20' 12"

49 AR0018 Buffalo Creek above highway 385 38 ̊ 08' 55" 102 ̊ 18' 43"

50 AR0013 Wild Horse Creek above County Road LL 38 ̊ 07' 46" 102 ̊ 08' 13"

1

Station IDs are from the Colorado Parks and Wildlife aquatics database.
7 

Data Analysis

Most multivariate analysis of ecological communities is based on some measure of community 

similarity among samples. Legendre and Legendre (1998) define several similarity measures used in 

ecology. In this case, Bray-Curtis similarity (Bray and Curtis, 1957) calculated from presence-absence 

data were used for all analyses. 

Analysis of similarity (ANOSIM; PRIMER-E ver. 6.1, Plymouth, United Kingdom) was used 

following details in Clarke and Warwick (2001) and Clarke and Gorley (2006). ANOSIM is based on a 

multivariate nonparametric-permutation procedure that compares the degree of separation between 

predefined groups of samples based on the ranks of community similarities underlying a nonparametric 

multidimensional scaling ordination (Clarke and Warwick, 2001). This procedure does not make 

assumptions about the distributional properties of the data, variance structure among groups, or about 

the balance of replicate samples within groups. The degree of separation among predefined groups is 

determined with the test statistic R, first as a global test to determine if differences among groups exist, 

then as pair-wise comparisons to determine which groups differ. Values of R near 0 indicate no 

distinguishable separation between groups, whereas values near 1 indicate complete separation. 

Statistical significance was determined by a general randomization procedure based on Monte Carlo 

significance tests described by Hope (1968). Analyses were determined statistically significant when 

less than 5 percent of the 9,999 permutated values were greater than the global R value. 

Testing for Differences Between Time Periods (1990s and 2000s) 

Fifty sites dispersed among 19 streams in both river basins (table 1, fig. 1) with fish data 

collected during both time periods (1992 to 1996 and 2007 to 2010) were used to determine if fish 

assemblages differed between the two time periods using a two-way ANOSIM analysis for a crossed 

design. The data structure fit the two-way crossed layout as all levels of factor A (stream) occurred in 

combination with every level of factor B (time period). The utility for two-way ANOSIM for a crossed 

design in this effort is that the analysis removes the effects of one factor (stream) while considering the 

significance of the other (time period). This is important because fish assemblages naturally vary among 

sites, and it was important to remove stream effects in order to reveal differences, if present, between 

the two time periods. 

Assessing the Use of Existing Data to Compare Fish Assemblages from 

Random and Fixed Sites—Differences Between Time Periods

Results of two-way ANOSIM indicated significant differences among steams and time periods. 

Differences in fish assemblages among streams were moderately strong (R = 0.61; significance level = 

0.01 percent). However, pairwise comparisons were not investigated as this result was expected because 

of the spatial extent of the dataset and what is known about the longitudinal distribution of plains fishes 

in Colorado (Fausch and Bestgen, 1997; Nesler and others, 1997; Nesler and others, 1999). The 

important part of the analysis was to remove stream effects to determine differences among time 

periods. After doing so, significant time period differences in fish assemblages were observed, but they 

were much weaker than differences among streams (R = 0.20; significance level = 0.01 percent). 

Nonetheless, none of the permutated values exceeded the Global R value testing for time-period 

differences, which suggests only a 0.01-percent chance of drawing an incorrect conclusion (Type I error 

in statistical terms). It is possible the low R value in this case was related to there being too few paired 

time-period samples in nearly half of the stream groups (9 of 19 streams had only one pair of timeperiod samples). ANOSIM is appropriate for unbalanced designs, but replication within groups is 

beneficial in most cases (Clarke and Warwick, 2001; Clarke and Gorley, 2006). Collecting additional 
8 

samples to increase the number of replicates where they were limited and rerunning the analysis would 

increase confidence in these results. 

Although these results suggest some differences exist in fish assemblages between time periods, 

the difficulty here is revealing what contributed to these differences. For example, it is difficult to 

separate possible influential factors such as apparent differences in methods, effort, unknown sampling 

error, or actual decadal changes in fish assemblages in response to various natural or anthropogenic 

stressors. Regardless, using the 1990s data to fill data gaps to determine if fish assemblages differ 

between random and fixed sites is not warranted based on the significant separation between time 

periods, even though the strength of the separation was weak. 

Moving Forward

The initial purpose of this study was to use existing CPW data to make comparisons between 

random- and fixed-site locations to inform decisions about incorporating a probabilistic monitoring 

design. However, after preliminary efforts, it was determined that using these data could bias the 

analysis because data collected during the 1990s inventories were needed to compensate for data gaps 

when pairing recently (2007 to 2010) sampled random sites with nearby fixed-site locations. Some of 

the influential factors that could bias the analysis included differences in methods, effort, or fishassemblage response to changes in environmental conditions between time periods. As a result, the 

initial purpose was modified to determine if using available data to make comparisons between random 

and fixed sites could bias the analysis by testing for differences in fish assemblages collected during the 

two distinct time periods (1992 to 1996 and 2007 to 2010). The results indicated discrete separation

between time periods making it difficult to justify using existing data to evaluate differences between 

random and fixed sites. Therefore, additional sampling will be required to make unbiased comparisons 

between fish assemblages collected from random and fixed sites. This will require pairing contemporary 

(2007 to present) samples from each random site with a sample collected from a nearby fixed site

preferably on the same stream. However, doing so will require substantial effort because less than half 

of the random sites sampled between 2007 and 2010 (100 sites) could be paired with a nearby fixed-site 

sample (40 sites), and of those, less than half (13 sites) could be paired with contemporary data 

(unpublished data, Aquatics Wildlife Research Group, Colorado Parks and Wildlife, June 2011). 

Summary 

Colorado Parks and Wildlife (CPW) is considering more statistically rigorous sampling methods 

accompanied with a statistically based selection of sites (sampling design) to update and monitor the 

status of plains fishes in Colorado. Additionally, CPW is interested in the possibility of using 

multivariate statistics to monitor changes in fish assemblages. As part of this consideration, CPW seeks 

to understand the comparability of fish assemblages from random and fixed sites. To this end, the U.S. 

Geological Survey, in cooperation with CPW, used nonparametric multivariate statistics to test if plains

fish assemblages differed at sites that were sampled during two distinct time periods (1992 to 1996 and 

2007 to 2010) in the South Platte and Arkansas River Basins in Colorado. Results of two-way Analysis 

of Similarities indicated significant but relatively weak separation between the two time periods after 

removing the effects of stream group. Although the analysis was equipped for unbalanced designs, it is 

possible the weak separation was influenced by the limited replication within many of the streams, 

temporal change of fish assemblages associated with environmental factors, differences in sampling 

effort between the time periods, or other unknown factors. Options to further evaluate the comparability 

of fish-assemblage data from random and fixed sites will require additional sampling to make unbiased 

comparisons. 
9 

Acknowledgments

The authors thank the numerous State, Federal, university, and other biologists and technicians 

who have collected plains fish data in Colorado. James Bruce and Billy Justus, U.S. Geological Survey, 

provided useful comments that greatly improved earlier versions of this manuscript. 

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