Vaughn Et Al: Timed Searches Vs

A Comparison of the Effectiveness of Timed Searches vs. Quadrat Sampling in Mussel Surveys

Caryn C. Vaughn, Christopher M. Taylor', and Kelly J. Eberhard Oklahoma Biological Survey and Department of Zoology, University of Oklahoma, Norman

Abstract. We compared data collected from qualitative timed searches with that from quantitative quadrat samples for 31 sites in small- to medium-sized streams in the Red River drainage. At each site, the area within 15 0.25-m^ quadrats was sampled to a depth of 15 cm. The entire mussel bed was then searched by hand by at least two experienced surveyors for at least 1 hour. Species richness in quadrat samples and species richness in timed searches were significantly correlated (r = 0.46, P = .009); however, more species were found in timed searches than in quadrat searches at 25 (80%) of the 31 sites. The mean confidence level for estimating species richness using 15 quadrats was 79%, and the mean number of quadrats required to achieve a 95% confidence level was 368. The mean confidence level for estimating density using 15 quadrats was 97%. Significant differences were found in individual species occurrences in timed searches vs. quadrats (Fried man test statistic = 10.62, P < .001). In general, most species were more likely to be found in timed searches, but some smaller species occurred more often in quadrats. Relative abundances of individual mussel species collected through timed searches were significantly different than relative densities of the same species collected with quadrats for only four species: Actinonaias ligamentina,Amblema plicata plicata, Obovaria jacksoniana, and Potamihis purpnratiis. Amblema plicata collected in timed searches tended to be larger than those collected with quadrats. Our results indicate that timed searches tend to overestimate large species, highly sculptured species, and those that protrude from the substratum, and underestimate buried, small, and smooth-shelled species. Quadrats will underestimate rare species and the total number of species, unless a very large number of samples are taken. We suggest that qualitative and quantitative surveying methods be used in combination whenever possible.

Introduction

Those of us conducting mussel surveys are often Oklahoma (Figure 1). (Dur survey efforts were faced with difficult choices regarding sampling restricted to areas that contained concentrations of methods. Quantitative sampling with quadrats or mussels that could be defined as beds. At each site transects are necessary to estimate mussel densities we used snorkeling or SCUBA to determine the (Miller and Payne 1988; Cawley 1993). However, edges of the mussel bed. Average width and length the amount of replication required to find rare of each mussel bed were measured in meters and species using quantitative techniques can greatly used to calculate mussel bed area. Sampling was exceed the time and budget of many projects. done by hand while wading, snorkeling, or with the Therefore, those conducting surveys for rare, aid of SCUBA in deeper areas (>.75 m). Mussels threatened, and endangered species often rely on were placed in a canvas bag imderwater and re qualitative techniques (Kovalak et al. 1986). In this moved to shore. Individual mussels were identified, study we compare two common methods of collect measured (total length), and returned to the mussel ing mussels, qualitative sampling of mussels for a bed after all sampling was completed. specific period of time and quantitative sampling of For each mussel bed, we first collected the mussels with 0.25-m^ quadrats. quadrat samples and then conducted timed searches. Quadrat sampling was done with 0.25-m^ PVC pipe quadrats. Fifteen randomly selected Methods quadrats were sampled in each mussel bed. All microhabitats (e.g., pools vs. riffles), including areas We sampled mussels from 31 sites in small- to where mussels were relatively sparse or absent as medium-sized streams in the Red River drainage of well as areas where they were dense, were sampled

'Current address: Department of Biological Sciences, Mississippi State University, Mississippi State, Mississippi 158 Conservation and Management of Freshwater Mussels II: Proceedings of a UMRCC Symposium

Figure 1. Locations of sites sampled in the Red River drainage, south-central U.S.A.

(see Downing and Downing 1992; Cawley 1993). from quadrat searches with the Friedman two-way Quadrats were searched until all mussels had been test for each species occurring at a minimum of four recovered to a depth of 15 cm. Timed surveys were sites. then conducted by searching the entire mussel bed Finally, we compared the size-frequency for at least 1 hour by two experienced surveyors. distribution of the most common species, Amblema During timed surveys mussels were located by sight plicata plicata, from timed searches vs. quadrat and feel. When a patch of mussels was located, the searches, using Replicated Goodness of Fit tests (G^) surveyor dug in the substratum for buried mussels. (Sokal and Rohlf 1981). We did this on a site-by-site For a subset of the sites (11-31) we used the basis and with a pooled data set from one river following formula (Southwood 1978) to (1) estimate (Little River; Figure 1). the relative error of our 15-quadrat sample size in determining mean species richness and density and (2) determine the number of quadrat samples Results and Discussion needed to estimate mean species richness with 95%

confidence limits: Richness n = (sîEx)y Species richness in quadrat samples and species where: richness in timed searches were significantly corre n = sample size (number of quadrats to sample) lated (r = 0.46, P = .009); however, more species were s = standard deviation (of richness or density) found in timed searches than in quadrat searches at £ = predetermined standard error as a decimal 25 (80%) of the 31 sites (Figure 2). This pattern X = mean (of richness or density) shows that, in general, more individuals are col lected in timed searches than in quadrat searches, We compared species richness in timed and the number of species found increases with the searches vs. quadrat searches across all 31 sites using number of individuals sampled. The relationship in Pearson correlation. We used the Friedman two- Figure 2 plateaus after large numbers of individuals way test (Sokal and Rohlf 1981) to compare across have been collected because the number of species in all species the number of times a species was present a mussel community is finite (Miller and Payne in timed searches but not in quadrats with the 1993). number of times it was present in quadrats but not Confidence levels for estimating mean species in timed searches. We compared the relative richness (number of species per square meter) using abundance of individual species from timed 15 quadrats ranged from 32 to 91% with a mean searches vs. the relative density of individual species confidence level of 79% (+/- 14%) (Table 1). The Vaughn et al: Timed Searches vs. Quadrat Sampling in Mussel Surveys 159

are biased towards surface-dwelling species, large species, and those with distinctive shell sculpture (Miller and Payne 1993), and tend to underestimate small species, smooth-shelled species, and deeply buried species. For the small- to medium-sized streams exam ined here, sampling a moderate number of quadrats provided very precise density estimates. Confi dence levels for estimating mean density using 15 quadrats ranged from 94 to 99% with a mean confidence level of 97% {+/-1.2%) (Table 2). Relative abundances of individual mussel species collected through timed searches were not significantly different than relative densities of the same species collected with quadrats (Table 3), 0 1 0 2 0 except for four species; Actinonaias ligamentina, Amblema plicata plicata, Obovaria jacksoniana, and Species richness from timed search Potamiliis piirpiiratus (Table 3). When P values were adjusted with a sequential Bonferroni test (Rice Figure 2. Relationship between species richness 1989), only O. jacksoniana remained significant (Table determined from timed searches vs. species richness 3). Amblema plicata and P. purpuratus were more determined from quadrat samples for the 31 sites (r = abundant in timed searches. Both of these species 0.46, P = .009). have large adults that tend to protrude above the substratum, and A. plicata is highly sculptured and easy to differentiate from stones. Actinonaias number of quadrat samples needed to achieve a 95% ligamentina and O. jacksoniana were more abundant confidence level ranged from 43 to 2,786 quadrats in quadrats. Actinonaias ligamentina tend to be with a mean of 368 quadrats (+/- 603 quadrats). The deeply buried and have smooth shells. Adult actual area that would need to be sampled to O. jacksoniana are very small. achieve a 95% confidence level ranged from 10.75 to 696.5 m^ with a mean of 92 m^ (+/- 151 m^). The Size proportion of area in the actual mussel bed at sites Amblema plicata collected in timed searches were that would need to be sampled to achieve this significantly larger than those collected with quad confidence level ranged from 0.33 to 612 %, with a rats in the Little River (Figure 3). Differences in mean of 46% (+/-136%) (Table 1). A. plicata size distributions, however, were generally These data indicate that, except in very small not significant when compared on a site-by-site mussel beds, sampling with quadrats alone will basis, probably because of smaller sample size. All underestimate species richness imless an extremely of our data were collected in mid- to late summer large number of quadrats are sampled. Further when juveniles of some species of mussels descend more, sampling with quadrats alone will underesti deeper into the substratum (Amyot and Downing mate the distribution of rare species and has the 1991). Quadrat sampling is more effective than potential to completely miss a rare species. For qualitative sampling for finding juveniles. Our data example, 10 of the sites used in this study are sites in support the conclusions of Miller and Payne (1988): the Kiamichi River that were surveyed for Arkansia quantitatively sampling mussels within the substra wheeleri, an extremely rare, federally endangered tum is necessary to obtain demographic data. species. Arkansia wheeleri were collected in quadrats at only 14% of the sites where the species was found Conclusions in timed searches (Vaughn and Pyron 1995). Thus, Whether to do a qualitative timed search or take sampling with quadrats alone would have drasti quantitative quadrat samples depends on the cally underestimated the distribution of this species. information being sought. If the objectives of a survey are to locate new mussel beds (including a Distribution and Abundance complete species list) or determine the presence of a Species that were found in timed searches but not in rare species, some type of qualitative sampling will quadrats were significantly different than species probably be necessary. If density or demographic found in quadrats but not timed searches (Friedman data are required, then quantitative techniques test statistic = 10.618,1 df, P = .001). Timed searches should be used. In this study, both techniques 160 Conservation and Management of Freshwater Mussels II: Proceedings of a UMRCC Symposium

provided comparable abundance estimates for most species, but if future monitoring is planned, one should include both techniques to assure adequate data on all species. In the small- to medium-sized streams in which we are working, combining the two methods allows us to obtain a large amount of data in a cost-effective manner.

Table 1. Sampling error by site for estimating species richness.

Conﬁdence level Number of samples A r e a ( m ^ ) M u s s e l b e d Proportion of mussel with 15 quadrats needed to achieve represented by area (m^) bed represented by (%) 95% conf. level that many samples recommended sample size (%)

10.75 1.08 190.5 101.87 13.25 0.33 18 0.64 696.5 95.28 45.75 0.52 68.5 8.06 117.75 38.99 45.5 0.95 76.25 19 0.70 35 3.46 26.25 0.69 19.5 7.50 13.5 2.25 65.25 10.68 84.75 8.48 244.75 611.88 72.75 7.28 28.5 0.38 40.25 14.91

Table 2. Sampling error by site for estimating density.

Mean individuals Conﬁdence level SITE per m^ with 15 quadrats (%]

11 47 97 12 3.2 99 14 21.9 98 15 6.93 96 17 42 98 18 21.14 97 19 26 98 20 34 98 21 5.6 96 22 11 99 23 5.3 98 25 33 99 27 17.33 97 28 26.67 97 30 27.76 98 Vaughn et al.: Timed Searches vs. Quadrat Sampling in Mussel Surveys 161

Table 3. Comparison of the relative abundance of individual species from timed searches vs. the relative density of individual species from quadrat searches using the Friedman two-way test. N is the number of sites where both species occurred. Bonferoni corrected P value is .002 for 95% conﬁdence level and is given in parentheses * p = -05, ** p = .001. Nomenclature follows Williams et al. (1992).

Mean relative Mean relative P Species abundance (%) density (%) N

Actinouaias ligameiitina 12.37 16.32 23 0.022* Atublenm plicala plicata 39.00 30.96 33 0.009** Arkausia wheeleri 0.42 0.25 6 0.420 ElUpsaria Imeolata 1.18 5.49 15 0.440 Fuscomia ﬂava 10.35 12.89 31 0.590 lampsilis satura 2.33 2.56 25 0.320 Lnmpsilis radiata 2.66 3.43 11 0.370 Lampsilis teres 1.07 1.11 17 0.220 Lasmigom costata 0.36 0.54 6 1.000 0.180 Lcptodea fragilis 0.54 0.91 5 0.320 Megalonaias nervosa 6.12 4.26 16 Obliqiiaria reﬂexa 2.61 2.79 22 0.670 Obovaria jacksoniana 0.28 1.79 13 0.002** (*) Plectomerus dombeyanus 11.89 14.39 13 0.780 Pleurobema coccineinn 2.67 1.83 7 0.260 Potamilus piirpuratus 2.82 1.36 17 0.030* Pti/chobranchiis occidentalis 1.74 2.66 13 0.780 Qiiadriila cylindrica cylindrica 1.10 1.33 7 0.700 Quadriila pustulosa pusliilosa 16.68 19.05 33 0.223 Quadrula qiiadrula 1.66 1.93 17 0.470 Strophitus undulatiis 0.79 1.49 8 0.160 Toxolasma parvus 0.83 3.47 4 0.320 31 0.370 Tritogonia verrucosa 4.73 4.35 Truncilla donaciformis 0,60 0.94 6 0.410 Truncilla tnmcata 1.22 1.40 17 0.470 ViUosa arkansasensis 6.55 17.48 5 0.650 0.130 Villosa lienosa 1.75 1.26 11

>» o □ Timed c 20 Q) ■ Quadrats 3 CT 0)

<30 80 120 160 200 Mussel length (mm)

Figure 3. Size distribution of Amblema plicata plicata collected in timed surveys vs. quadrat sampling (G,, = 19.76, df = 10, P < .025). 162 Conservation and Management of Freshwater Mussels II: Proceedings of a UMRCC Symposium

Acknowledgments We thank Matthew Craig, Mark Pyron, and Mat thew Winston for help with ﬁeld work, and Sherri Johnson for commenting on the manuscript. This research was funded by grants to C.C.V. from the National Science Foundation (DEB-9306687), the U.S. Fish and Wildlife Service, the U.S. Forest Service, the Oklahoma Department of Wildlife Conservation (project E-12), and the University of Oklahoma.

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