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Considerations to evaluate culvert-passage of aquatic organisms
with mark and recapture/relocation
Introduction
The simplest way to evaluate aquatic organism passage is to monitor movements of individuals
(Figure 1). Here we outline some basic considerations in applying this method for evaluating
movement of organisms through culverts in wadeable streams. The steps in the process are 1)
selection an appropriate study design (Figure 2); 2) capture, mark, and recapture or resighting of
organisms to assess movement; 3) evaluation of results in relation to assumptions of this
approach.
The basic approach discussed here involves simple marking of individuals at some time (e.g., fin
clips of fish; Figure 1) upstream and/or downstream of culverts or control reaches (Figure 1, 2).
Investigators return at some later time in an attempt to recapture marked individuals to
determine if any passed through the culvert (Figure 1).
Figure 1. Simple illustration of mark and recapture (or resight) of individual movement relative
to a culvert in a stream.
before-capture and marking
upstream
upstream
downstream
after-recapture
downstream
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We assume here that investigators are familiar with basic procedures for capture and handling of
organisms of interest. This topic is outside of the scope of this discussion. Our focus is on selected
study designs and key assumptions associated with simple mark and recapture/resight and how to
address them in designing an effort to monitor aquatic organism passage through a culvert.
Key assumptions
1. Lack of detection of movement through a culvert is not due to low expected probability
of recapture. The probability of recapturing individuals depends on several factors
described below (Figure 3).
2. Movement is constrained by the culvert only. Application of this method assumes all
individuals must move through a culvert. If an individual is marked up or down-stream of
a culvert and found in a different location (e.g., upstream marked individual is detected
downstream of a culvert). Individuals may also be recaptured or relocated within the
culvert itself, providing some evidence of passage. This assumption may not be valid for
species that can move outside of streams (e.g., amphibians, crayfish).
3. Marking of individuals does not influence their behavior. Handling and marking of
organisms may change their behavior and resulting pattern of movement. If these effects
are substantial, observed movement through mark-recapture may not accurately reflect
movement that would occur naturally.
4. Other characteristics of individuals do not confound assessment of movement behavior.
Not all individuals have the same propensity or ability to move. Comparisons of marked
samples of individuals assume they are similar, unless the analysis of movement explicitly
accounts for individual characteristics.
Methods
Study design
There are three common study designs, designated as follows: A. stream-culvert-stream, B.
stream-culvert-main stream, and C. reference-culvert (Figure 2). When possible, we recommend
including a reference site (Figure 2 C.). This allows for a comparison of presence or absence of
movement through a culvert, as well as an assessment of the relative degree of movement in sites
with and without culverts. In other words, designs A and B will answer the question of “Did
organisms move through the pipe?”
Adding C as a design element can also answer “How much do culverts constrain movement
relative to comparable sites without culverts?” Control sites (Figure 2 C.) can be added to
monitoring designs A or B. Without a control site for reference, it is only possible to address the
question “Did organisms move through the pipe?”
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Figure 2. Three common study designs for mark-recapture/resight studies of individual
movement relative to culverts.
A. “Stream-culvert-stream” design. This includes cases where individuals are marked for a
given distance upstream and downstream of a culvert.
sampling area
upstream
downstream
B. Stream-culvert-main stream. In this case a small stream with a culvert drains directly into
a larger main stream, thus a downstream site is not available as above (A). In this case,
sampling within the larger stream is substituted for the downstream tributary location.
main stream
upstream
downstream
C. Reference or “false" culvert. It may be desirable in some cases to compare movement in
the vicinity of a culvert to a comparable location lacking a culvert, as diagrammed below. The
sampled location depicted below can be compared to other sites with culverts (e.g., A. or B.
above) to assess the degree of movement in sites with and without culverts. Comparing sites
that are similar with respect to factors believed to influence movement is a key design
element. The dashed outline of a culvert indicates a “control” or “false culvert” site.
Study reaches for mark-recapture/resight
upstream
“false culvert” or control site
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Working with marked individuals
Here we assume the simplest of marking methods is used, and that identification of specific
individuals is not of interest. This would include simple “batch” marks such as fin clips on fish, toe
or tail clips on amphibians, and so on, depending on the species of interest. Protocols for how to
batch-mark a particular species are beyond the scope of this document. Here we discuss how to
address each of the major assumptions listed above for study of marked individuals in relation to
passage through culverts.
Assumption 1. Lack of detection of movement through a culvert is not due to low expected
probability of recapture.
A key consideration in using a mark-recapture or mark-resight approach relates to expected
sample sizes for recaptured or resighted individuals. Of the initial sample of marked individuals,
some will lose their marks (e.g., regrowth of fins) or marks will otherwise not be detectable after
initial marking. This reduces the number of individuals that will be possible to resight or
recapture. Other factors influencing this final expected number include survival, extent or
distribution of movements relative to the area sampled (Figure 2).
Figure 2. A hypothetical study area (downstream only, Figure 2A) for mark and recapture of
individuals. The bell curve depicts the distribution of movements of individuals, centered on
initial capture and marking of individuals. Some of these individuals may leave the sampled
area and thus be lost to the potential pool of recaptures (“Fish moved outside of study area”).
Others will be lost due to undetectable marks, death before the recapture event, or imperfect
probability of recapture (present but not captured). The top bell curve (solid line) depicts
available marked fish (for recapture) assuming no death and perfect detection. The lower bell
curve (dotted line) depicts impacts of mortality and imperfect detection. Note in the latter case
that fish entering or passing through the culvert may not be detected, even though there is a
low level of passage. This is due to a low expected number of marked individuals recaptured
(see text for how to calculate this).
Area studied for marked
individuals
culvert
Stream flow
Distribution of movements
100% survival or perfect detection
<100% survival or imperfect detection
Fish moved outside of
study area
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Based on these considerations (see also Figure 3) it is possible to estimate the expected number of
recaptured individuals. For example, if we assume that we initially tag a sample of 100 fish and
that the fraction of individuals retaining visible marks is 0.9, survival is 0.9, fraction of individuals
staying in the study reach is 0.9, and probability of recapture is 0.9, then we expect to recapture
about 66 individuals. This is a very optimistic scenario. If we sample with the same probabilities,
but reduce the probability of recapture to 0.3 (more realistic) the expected number of recaptures
is 22.
To calculate expected numbers of fish to recapture, multiply the initial number of fish marked by
the expected mark loss rate (a fraction between 0 and 1). Multiply this number by the expected
survival (0-1), multiply this number by the expected probability of leaving the study area (0-1), and
this number by the expected probability of recapture (0-1). The resulting number is the expected
number of recaptures. This is a useful and sometimes sobering exercise for thinking about
appropriate numbers of individuals to mark to ensure an adequate sample size of recaptures.
Even if estimates of these quantities are unknown, it can be useful consider different hypothetical
scenarios (e.g., hypothetical values of the parameters in a spreadsheet) for an evaluation.
Assumption 2. Movement is constrained by the culvert only. Some species may move overland,
or there may be cases where other factors constrain movement through a culvert. For example,
the presence of predators in a culvert outflow pool may constrain movement of some species.
These potential influences should be considered and sampling plans modified such that this
assumption is valid. For example, efforts to mark and recapture individuals in terrestrial habitats
might be necessary if the species studied is semi-aquatic. In cases where factors other than a
culvert may constrain movement, it might be prudent to remove the constraint or perhaps
consider a different location for study that lacks this confounding influence.
Assumption 3. Marking of individuals does not influence their behavior. This is a fundamental
assumption of all mark-recapture studies. Marking should not influence movement behavior in
particular. For example, overzealous fin-clipping or handling of fishes may reduce their ability to
move after marking.
Assumption 4. Other characteristics of individuals do not confound assessment of movement
behavior. There are a variety of factors that could be operating here that could be important, and
are best illustrated by examples. For example, it is well-known that size and developmental
characteristics of fish can affect their movement behavior. In some species, juveniles disperse and
adults are less mobile, whereas in others the opposite is true. The timing and time frame over
which a culvert is studied could also influence study outcomes. Factors such as flow, temperature,
and time of year could influence movement behavior.
Study designs with pre-planned analyses to account for these influences could be useful for
dealing with this assumption, but the simplest approach is to avoid the violations of this
assumption altogether by thinking through potential problems and minimizing them in the
resulting study design (e.g., species and life stages studied, types of culverts or other location
studied, times or timeframes of the study).
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It should go without saying that it is important to consider all of the factors outlined above and
that key factors potentially influencing the validity of these assumptions be measured and
documented as part of the monitoring effort. Better yet, if a study design can be formulated to
explicitly model the influences of these factors in relation to the influence of culverts, all the
better. In practice, this may be difficult without consulting a statistician, but at the least a list of
potentially important factors related to these assumptions should be documented, along with
measures taken to minimize their influence or account for them in some other way.
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