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2014 Northeastern Naturalist Notes Vol. 21, No. 1
D.F. McAlpine and M.C. Sollows
A Quadrat-sieve System for Sampling Freshwater Mussels Using
SCUBA
Donald F. McAlpine1,2,* and Mary C. Sollows1,2,3
Abstract - We describe a simple combined-quadrat-sieve system, suitable for in-river surveys of
unionoid populations where SCUBA is required. The apparatus has allowed us to survey and excavate
0.25-m2 quadrats with mean search times of less than 7 min quadrat-1 over 4-hour dives. We have successfully
detected mussels near the minimum size, more often retrieved by sieving at the surface, while
minimizing habitat disturbance.
Unionoid mussels (Margaratiferidae and Unionidae) are a diverse group of freshwater
organisms that often contribute significantly to the invertebrate biomass and the functional
ecology of rivers, lakes, and other wetlands (Strayer 2008, Vaughn and Hakencamp 2001).
Many North American unionid species are now of conservation concern (Williams et al.
1993); thus, there is considerable interest in methodologies for monitoring and sampling
unionoid populations (Strayer and Smith 2003 and references cited therein).
Sampling within quadrats of varying size, most commonly 0.25 m2, is a frequent and
useful means for quantifying abundance of freshwater mussel populations (Strayer and
Smith 2003). Simply counting mussels visible at the substrate surface has limitations
because up to nearly 80% of a population, especially for smaller-sized mussels, may be
buried out of sight in sand or gravel (Smith et al 2000). Lack of excavation during sampling
can lead to dramatic misrepresentations of population age-structure (Hornbach and
Deneka 1996). While Smith et al. (2000) found that excavation in itself did not appear to
increase mussel mortality, the method often requires that researchers move the substrate to
the surface or to a laboratory for sieving, which greatly disturbs the habitat. Also, Smith
et al. (2000) found that excavation can be 3–12 times more time-consuming than surface
counts alone, which significantly increases survey costs. It is clear that quantitative mussel
sampling requires at least some, and sometimes considerable, excavation of the substrate
from quadrats. Here we describe a simple combined quadrat-sieve apparatus suitable for
in-river use that we have used with some success when surveying unionoid populations.
Figure 1 illustrates the quadrat-sieve. A 0.25-m2 frame (inside dimensions) of 25.4 x
25.4 x 3.2-mm (1 x 1 x ⅛-inch) welded aluminum angle bounds the quadrat. We welded a
screened stage (55 x 25 mm) constructed of the same aluminum angle along one side of the
frame. The stage floor, elevated ≈80 mm above the bottom edge of the quadrat, is supported
by the quadrat frame and a 50-mm leg on each corner. Screened walls, 145 mm in height,
enclose the stage on three sides, leaving it open into the quadrat space. The three walls and
floor of the stage are enclosed with 16-gauge stainless steel, 5-mm (¼-inch) screening,
riveted in place. A 12-cm-diameter circular aluminium plate (Fig. 1A) is welded to a corner
of the quadrat opposite the sieve and is intended to provide a hard surface on which a dive
notebook can be placed.
We tied a length of orange surveyor’s tape to a 190.5-mm (7.5-in) nylon cable-tie
threaded through a hole in the frame (Fig. 1B) opposite the sieve to prevent loss of the
1New Brunswick Museum, 277 Douglas Avenue, Saint John, NB, Canada E2K 1E5.2Department of
Biology University of New Brunswick, Saint John, NB, Canada E2L 4L5. 3Canadian Rivers Institute,
c/o University of New Brunswick, Saint John, NB, Canada E2L 4L5. *Corresponding author - Donald.
mcalpine@nbm-mnb.ca.
Manuscript Editor: Melisa Wong
Notes of the Northeastern Naturalist, Issue 21/1, 2014
2014 Northeastern Naturalist Notes Vol. 21, No. 1
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D.F. McAlpine and M.C. Sollows
quadrat-sieve in murky waters. We have found use of a dive light helpful in waters >1 m
deep. The total weight of the apparatus is 4 kg. The cost of construction at a local metalworking
shop was $320 (Canadian) per quadrat-sieve.
When using the quadrat-sieve, a diver lies prone on the bottom, either to one side of the
sieve or directly behind it, with the sieve orientated to the downstream side of the quadrat.
Mussels visible at the substrate surface are removed from the quadrat first and placed in a
mesh bag. The diver then excavates the quadrat to a depth of ≈15 cm by scooping and pulling
the substrate into the sieve. The area under the sieve may become congested with fine
sediment before the quadrat is fully excavated, especially on fine sand bottoms, but a sweep
of the hand under the sieve easily clears the area. Individual pieces of gravel or cobble >5
mm in diameter can be removed by hand from the surface of the stage as excavation continues.
In especially fast waters, a second diver may be necessary to hold the quadrat in
place. Mussels can be moved to one side of the sieve until the quadrat is excavated or can
be placed in a mesh bag as detected. We have frequently used two bags of different colors,
placing surface mussels in one bag and excavated mussels in the other so that the proportion
of each type can be calculated later. After the quadrat is cleared of mussels, they can
be brought to the surface for counting and measuring. The substrate can be immediately
returned to the depression created upon completion of excavation, thereby minimizing habitat
disturbance. A metal scoop may prove necessary for excavation where gravels overlay
or are interspersed with cobble. In murkier water on sand bottoms, we found that a scoop
Figure 1. Aluminum quadrat-seive apparatus used for sampling freshwater mussel populations by
SCUBA where substrate excavation is required. It consists of a 0.25-m2 frame (inside dimensions) with
a 55-mm x 25-mm screened stage on one side. A. circular disc that provides a surface on which a divebook
can be rested. B. provides attachment for flagging that reduces loss when diving in murky waters.
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2014 Northeastern Naturalist Notes Vol. 21, No. 1
D.F. McAlpine and M.C. Sollows
was often lost or forgotten when not attached to the quadrat by a line. However, when we
screwed an eye-bolt to the frame and tied a line between the bolt and a scoop, the line invariably
became entangled in the SCUBA regulator or other gear. In these circumstances,
excavation by hand proved the most efficient.
Strayer et al. (1997) estimated search times of wadeable streams at 1.2–10 (mean = 4.1)
min quadrat-1 without excavation; Smith et al. (2000) suggested an average search time of
7 min quadrat-1 (1 min for the surface count, 6 minutes for excavation) when wading. Underwater
sampling tends to be much more time-consuming than sampling where diving is
not required, but we are aware of no reported estimates of quadrat search-times by divers.
We have used the apparatus described to survey mussels in waters 1 to >5 m deep in medium-
sized to large rivers where SCUBA was required; in both slow-moving and relatively
fast-moving (0.28 m s-1) waters; and on fine sand, gravel, and cobble substrates (Martel et
al. 2010). Although one should expect search times to vary greatly depending on mussel
density, visibility, vegetation cover, and substrate type, we have searched and excavated
quadrats up to 10–15 cm deep using the quadrat-sieve during 4-hr dives where mean search
times were less than 7 min quadrat-1. Conditions in these circumstances included fine-medium sand,
little vegetation, low currents, moderate visibility (high tannin concentrations), and mussel
densities of ≈0–8 m-2.
In particular, we have found the quadrat-sieve useful when sampling Lampsilis cariosa
(Say) (Yellow Lampmussel), where we located >40% of mussels below the substrate
surface (D.F. McAlpine, unpubl. data; Sabine et al. 2004) and in studies of Margaritifera
margaritifera L. (Freshwater Pearl Mussel; Sollows et al. 2013). When sampling the latter,
we detected mussels as small as 11.5 mm total length (TL) and determined that all Freshwater
Pearl Mussels less than 30 mm TL at our study sites were below the substrate surface. Cawley
(1993) reviewed freshwater mussel surveys that used 2–6-mm sieve sizes and concluded
that 5-mm mesh was adequate for sampling juvenile mussels. However, sieving in situ may
fail to capture the smallest juveniles present; Hastie et al. (2000) and Young et al. (2001)
report that when sieving for Freshwater Pearl Mussels, juveniles less than 10 mm TL may not be
found. Cawley (1993) reviewed 16 studies and found that the smallest mussel detected was
6 mm TL (using a 5-mm sieve). These results from previous studies suggest that our apparatus
allowed us to find mussels near the minimum size normall y detectable.
The device described here has proven effective in the environments where we have
used it, including on fine sand-gravel and gravel-cobble substrates and in water of varying
clarity, depth, and current. However, we have only used the quadrat-sieve to sample
6 of the 10 unionoid species present in New Brunswick (see Martel et al. 2010). We
recommend that the device should be tested to determine its effectiveness in detecting
sub-surface juvenile mussels of a wider array of mussel species. It would also be useful to
compare data collected systematically using the device under a variety of field conditions.
We have not compared the time required for sampling using the quadrat-sieve versus the
hydraulic siphon used by Koch (1990, cited in Cawley 1993), although the latter would
seem to be much more likely to cause substrate disturbance. Finally, given the constraints
of operating in SCUBA gear where visibility may be reduced, direct comparison between
surface- and bottom-sieving would be useful to confirm that the latter is an equally effective
method to detect juveniles.
Acknowledgments. Our field studies on the freshwater mussels of Maritime Canada have
been supported by grants to D.F. McAlpine from the New Brunswick Wildlife Trust Fund,
New Brunswick Environmental Trust Fund, and the Fish and Wildlife Branch of the New
Brunswick Department of Natural Resources.
2014 Northeastern Naturalist Notes Vol. 21, No. 1
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D.F. McAlpine and M.C. Sollows
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