Surber Sampler Research Articles

Detecting North American signal crayfish (Pacifastacus leniusculus) in riffles

Abstract The spread of the invasive signal crayfish (Pacifastacus leniusculus) outside its natural range is of widespread concern due to the threats posed to native biodiversity. To date, there is no standard protocol for determining signal crayfish presence or absence in a watercourse. For the purposes of this investigation, the crayfish detection ability of active sampling methods — hand‐netting, electrofishing (one, two and three runs), kick sampling and Surber sampling — was tested at 30 sites along the River Clyde, southern central Scotland. No single technique was successful in detecting crayfish in 100% of the sites known to contain crayfish and so the application of combinations of techniques was considered. The combination of techniques that resulted in a 100% detection rate was electrofishing (three runs) together with kick sampling. These results suggest that three‐run electrofishing and kick sampling are the best candidates for incorporation into a crayfish detection protocol. The mean time taken to apply electrofishing (three runs) was significantly greater than the mean time to apply kick sampling. Given the lower effort required for its application, kick sampling is recommended as the preliminary technique: if kick sampling yields a negative result, the application of electrofishing will decrease the chance of recording a false negative presence. If both kick sampling and electrofishing fail to detect crayfish, trapping may further decrease the risk of a false negative result. These findings have assisted in the development of a crayfish detection protocol, which will be applied across Scotland to determine the current distribution of signal crayfish. Copyright © 2010 John Wiley & Sons, Ltd.

Invertebrate Composition and Abundance Associated with Didymosphenia geminata in a Montane Stream

ABSTRACT Didymosphenia geminata, a relatively new aquatic nuisance species that can form extensive, mucilaginous mats on stream substrates, was reported from Rapid Creek, South Dakota in 2002. To examine the association between D. geminata and the invertebrate community in Rapid Creek, macroinvertebrates were quantified using three gear types in the fall of 2006. D. geminata was present at two of four sites sampled (range = 5.53 to 809.68 g m−2 dry mass). At each site, invertebrates were collected using dip nets, Surber samplers, and drift nets. The combined percentage of Ephemeroptera, Plecoptera, and Trichoptera in areas with D. geminata was lower (41%) than in areas without D. geminata (76%). Diptera abundance was higher at sites with D. geminata than in sites where D. geminata was absent.

Destructive effect of quarry effluent on life in a mountain stream

Quarrying is a widespread method for acquiring construction material. The studies of quarrying effects to date have been conducted mostly in the fields of geology, (hydro)geochemistry and landscape management while ecological studies on effects of quarrying are surprisingly few. The goal of this study was to assess some ecological effects of quarry mining on mountain stream habitats. The study was performed at Bistra Stream on Medvednica Mountain in NW Croatia. The quarry is located 3 km downstream from the spring. Samples were taken at four sites on four dates during the spring of 2006. Standard physico chemical parameters were measured and triplicate benthos samples were taken using a 30 × 30 cm Surber sampler. Turbidity, pH and temperature increased significantly downstream of the quarry. All biocenotic descriptors decreased significantly downstream of the quarry including total taxa (by 60%), total number of individuals (by 85%), diversity index (by 56%). The most important cause of such changes in the macroinvertebrate assemblage structure was the change in pH and turbidity. The magnitude of changes in the macroinvertebrate assemblage structure was due to the extremely long duration of disturbance. However, we believe that the recovery of aquatic assemblages, upon closure of the quarry, would be fast and successful because of nearby streams that may serve as a recolonizing source.

ارزیابی کیفی رودخانه سبزکوه در استان چهارمحال و بختیاری با استفاده از شاخص های زیستی کفزیان

هدف از این تحقیق بررسی تغییرات جمعیتی و گروه­های شاخص فون بنتیک و نیز طبقه ­بندی کیفی آب رودخانه سبزکوه می­باشد که تحت تاثیر مستقیم فعالیت چندین سایت پرورش قزل آلای رنگین­ کمان و تخلیه پساب­ های حاصله در مسیر مورد مطالعه بوده است. این بررسی به­ مدت یک­سال از آغاز دوره پرورشی کارگاه­ها در تیر ماه 1385، با انتخاب 7 ایستگاه که 4 مجتمع پرورش ماهی قزل آلای رنگین­ کمان را پوشش دادند، در یک مسافت 35 کیلومتری رودخانه سبزکوه واقع در استان چهارمحال و بختیاری انجام گردید. درشت بی­ مهره ­گان کفزی رودخانه به­ کمک یک دستگاه نمونه بردار سوربر به مساحت 1600 سانتیمتر مربع (40×40 سانتیمتر) جمع ­آوری و بررسی شد. در مدت بررسی کلا 34 گروه (خانواده و جنس) از درشت بی­ مهره ­گان کفزی شناسایی شدند که لارو حشرات آبزی بیش از 70 درصد از آنها را تشکیل می­دادند. مقادیر شاخص زیستی هیلسنهوف در ایستگاه­های مطالعاتی بین 3/9 (ایستگاه 1) و 5/5 (ایستگاه­های 4 و6) متغیر بوده است که بر اساس طبقه­ بندی هیلسنهوف نشانگر کیفیت آبی به ترتیب خیلی خوب و نسبتا خوب می­باشد. بر اساس ساختار جمعیت کفزیان و شاخص زیستی هیلسنهوف، کیفیت آب در ایستگاه­های پایین دست خروجی مزارع نسبت به ایستگاه­های بالادست و ورودی کیفیت پایین­ تری داشته­اند.

Structure and distribution of Hydrachnidia (Parasitengona-Acari) in the sub-basin of the Grande River (Superior Basin of Quinto River. San Luis-Argentina)

AIM: The purpose of this work was to determine genera diversity, structural attributes and distribution of the Hydrachnidia assemblages and their possible influences with certain abiotic parameters along the sub-basin of the Grande River, a 6th order stream in the province of San Luis; METHODS: four sampling sites (C1, E1, E2 and E3) were established in two different hydrological periods: high waters (HW) and low waters (LW). Twenty-four samples were obtained using Surber sampler, and the hydraulic, physical and chemical characterization of the sampling sites was carried out. Density (ind.m-2), genus richness, Shannon-Wiener diversity index and Kownacki's dominance index were calculated; RESULTS: sixteen Hydrachnidia genera were observed. No significant differences were found between the abiotic parameters of each site. A positive correlation of Sperchon, Torrenticola and Neoatractides was observed in relation to flow and current velocity whereas Hygrobates showed a negative correlation. No significant differences were observed in Hydrachnidia abundances among the sites in HW and LW and between the pairs of each site in HW and LW. The genera with the highest densities were Hygrobates, Limnesia and Atractides, of which the first two were widely distributed. The rest only appeared sporadically. According to Kownacki's index Hygrobates and Limnesia were dominant, Atractides were subdominant and the rest were non dominant; CONCLUSIONS: The found genera were distanced systematically and philogenetically. However, the specimens belonging to these genera exhibited very similar morphological characteristics, which were adaptations to rheophilous habitats. The representativity of Hygrobates and Limnesia might be due to the great abundance of their hosts and preys in the sub-basin. The decrease in the densities and the changes in the Hydrachnidia composition in E3 (post-dam site) might be explained by the biotic interactions previously mentioned.

Inventory of caddisflies (Trichoptera: Insecta) of the Campos do Jordão State Park, São Paulo State, Brazil

Due to the incipient state of knowledge of the caddisflies of Brazil, faunistic inventories are extremely important. An inventory of the Trichoptera fauna of Campos do Jordão State Park (CJSP), São Paulo State, based on adults and larvae, is presented here, as well as comments on its taxonomic composition and the implications this poses for the value of conserving the Araucaria Forest. The samples were taken between August 2005 and February 2007. Larvae were collected with a Surber sampler in several kinds of mesohabitats, in 1st to 4th order streams throughout CJSP. Adults were sampled with light and Malaise traps, and also entomological nets. In all, 40 genera of Trichoptera belonging to 15 families were recorded. The CJSP fauna include 60% of the genera and 93% of the families reported from Brazil. Despite the high richness recorded, the cumulative curve for genera was still rising, indicating the possibility of a further increase with additional sampling effort. The high caddisflies richness, as well as the records of rare South American genera, may be related to peculiarities of the Serra da Mantiqueira, which seems to have maintained the habitat and climatic characteristics necessary to allow the survival of a Gondwanan relict of Araucaria moist forest, a hypothesis reinforced by the finding of relictual genera with a Gondwanan distribution, such as Neoatriplectides and Tolhuaca.

Phoretic association between larvae of Rheotanytarsus (Diptera: Chironomidae) and genera of Odonata in a first-order stream in an area of Atlantic Forest in southeastern Brazil

In this note, the occurrence of phoresy between larvae of Rheotanitarsus sp. (Diptera: Chironomidae) and larvae of Heteragrion sp. (Odonata: Megapodagrionidae) and of unidentified genera of Calopterygidae (Odonata) collected in a first-order stream in an area of Atlantic Forest in southeastern Brazil is reported. During the dry season of 2007 and the rainy season of 2008, with the aid of a Surber sampler, 15 samples of each of the following mesohabitats were collected: litter from riffle areas, litter from pool areas and sediment in pool areas. Eighty-five Odonata larvae were obtained, 10 (11.76%) with cases of phoresy by Rheotanytarsus sp.. These chironomids were associated with only one specimen of Megapodagrionidae, whereas the other larvae were recorded in association with Calopterygidae. Most of the Odonata with cases of phoresy by Rheotanytarsus sp. were recorded in the dry season. In the present study, the absence of the phoretic association with other potential hosts for Rheotanytarsus sp. found in the samples indicates a possible preference of these larvae for Odonata, which accounted for only 2.42% of the collected macroinvertebrates in litter and sediment.

The life history of Perla bipunctata Pictet, 1833 (Plecoptera: Perlidae) in the upper River Liffey, Ireland

Perla bipunctata is the most common species of Perlidae in Ireland occurring in fast flowing, clean rivers and small streams. Although a common component of freshwater biological studies, little is known about the autecology of this stonefly. Monthly kick and Surber samples were taken over a 1-year period in the upper reaches of the River Liffey (4th order) to determine the life history of larvae. Perla bipunctata has a merovoltine life cycle taking no less than three years to complete the immature stages. Life history plots suggest two periods of egg hatching, followed by two separate cohorts developing over different lengths of time. Mature larvae from both cohorts synchronise and emerge as adults over a short period in early summer. The merovoltine life cycles of long lived invertebrates like P. bipunctata emphasise the importance of such species in reflecting the ecological quality of freshwaters over a long period of time.

Life history and secondary production of Ephemera orientalis (Ephemeroptera: Ephemeridae) from the Han River in Seoul, Korea

In this study, we investigated the life history of Ephemera orientalis, a common lowland burrowing mayfly which resides in temperate East Asia and often causes a serious nuisance to people due to mass emergence from the Han River in Seoul, Korea. Larvae were sampled monthly (every two weeks during the emergence period) from April 2006 to June 2007 using a Surber sampler (50 × 50 cm, mesh 0.25 mm, two replicates). The mean density of E. orientalis was found to be 105.88±42.14 indiv/m2 during the study period. On the basis of the larval body size distribution and emergence time, we concluded that E. orientalis has a univoltine life cycle with two distinct cohort groups, S1-group emerging in May–June and S2-group in August–September. The estimated annual production of the larvae was 1350.84 mg DW/m2; the mean biomass was 325.17 mg DW/m2; the annual production to mean biomass ratio (P/B) was 4.15. Mean water temperature during the study period was 15.02±6.84°C. The accumulated degree days for the S1- and S2-groups were 2565.07 and 2,621.69 degree days, respectively. This study shows that the Han River population of E. orientalis lacks the fast growing cohort (F-group) in May–August, and demonstrates a relatively higher secondary production than that of other stream mayflies (e.g. Gapyeong stream population of E. orientalis).

Relating Values of Selected Benthic Macroinvertebrate Metrics from D-net to Surber Sampler in Two Southeast Wyoming Streams

Abstract Samples were taken from two southeast Wyoming streams to determine if values of selected benthic macroinvertebrate metrics derived from using a D-net could be related to values derived from using a Surber sampler. Conditions under which samples were collected were variable, ranging from severely polluted to non-polluted over two seasons. Metrics evaluated were the Shannon-Weiner index and the random runs value plus four measures of richness; Ephemeroptera taxa richness, Ephemeroptera/Plecoptera/Trichoptera taxa richness, total taxa richness, and Plecoptera taxa richness. Regression analysis indicated that linear relations existed between average metric values estimated through use of the D-net and Surber sampler. These results indicated that average D-net values for these six metrics can be expressed as average Surber sampler values, and vice versa, in these two streams when using either the D-net or Surber sampler in the manner described. This study presented two ways (i.e., the F test or the mo...

Abundância e distribuição de Aeglidae (Crustacea: Anomura) em função do uso da terra na bacia hidrográfica do Rio Jacutinga, Rio Grande do Sul, Brasil

This study investigated the influence of land use and occupation on the density and distribution of aeglids in the hydrographic basin of the Jacutinga river (northern state of Rio Grande do Sul, Brazil). Sampling was carried out during the summer and winter of 2004, at 12 sites distributed in river spring, agriculture, grazing, and urban areas. Animals were collected using a Surber sampler. An analysis of variance (ANOVA) was conducted to evaluate differences in the density of species in each land use and sampling period. Biological data were classified using cluster analysis (Euclidian distance; UPGMA). A total of 226 aeglids were captured, including 167 (78.89%) specimens of Aegla platensis Schmitt, 1942 and 59 (26.11%) specimens of A. singularis Ringuelett, 1948. The aeglid distribution was associated with land use and occupation. In river spring and grazing areas, A. platensis was predominant, whereas in urban and agricultural areas A. singularis was the most prevalent species.

Macroinvertebrate Index of Biotic Integrity (M-IBI) for monitoring rivers in the upper catchment of Lake Victoria Basin, Kenya

The water quality of many streams in Lake Victoria Basin, Kenya is affected by agricultural intensification, municipal and industrial effluent, as well as water abstraction, livestock and deforestation. Management efforts have been hampered by lack of clear standards against which to judge the degree of environmental degradation. To achieve this goal, a macroinvertebrate-based Index of Biotic Integrity was developed to monitor ecological integrity of selected rivers occurring in the same ecological zone in the upper reaches of the basin. Macroinvertebrates were sampled with a Surber sampler at 22 stations. The stations were grouped into three condition categories (reference, intermediate and impaired stations) according to the level of degradation. Twenty metrics representing the structural and functional organization of macroinvertebrates were computed using Mann-Whitney U tests. The separation power of the metrics was evaluated using box plots. Final metrics were evaluated for responsiveness by Pearson’s correlations with physicochemical parameters. Of the 20 metrics, 9 met the test criteria, as they displayed variability across the stations, and were used to provide the final scores for Macroinvertebrate Index of Biotic Integrity (M-IBI). Metric values at both reference and impaired sites were used to establish the scoring criteria using inter-quartile ranges. This index provided preliminary evidence of response to changes in ecosystem integrity exhibited by resident macroinvertebtrate assemblages in rivers in the upper reaches of Lake Victoria Basin. It was recommended that the index developed be used to assess and monitor rivers in the upper catchments of Lake Victoria Basin in Kenya.

Artificial steps to stabilize mountain rivers: a post‐project ecological assessment

AbstractThis paper compares the ecological response of reaches treated with traditional (check dams, CDs) and morphologically based (artificial steps, ASs) grade‐control works installed in a mountain river, where an unmodified reach provided reference conditions. Coarse particulate organic matter (CPOM) retentiveness was measured at the reach scale using artificial leaf release experiments. Macroinvertebrate sampling was carried out by a Surber net at pool‐ and run‐type morphological units within the three reach types, and hydraulic and sediment characteristics were measured at each Surber sampling site. In addition to calculating a variety of parameters to describe hydrodynamic and macroinvertebrate characteristics within reaches and morphological units, biotic (Extended Biotic Index, IBE) and ecological (Fluvial Functionality Index, IFF) index scores were also calculated. Results averaged at the reach scale show that CPOM retention, macrobenthos taxa richness and diversity were higher in the presence of a natural bed morphology, whereas both types of grade‐control works alter CPOM dynamics and macroinvertebrate communities. However, ASs showed CPOM retention dynamics and macroinvertebrate richness and diversity that were closer to the unmodified reach than CDs. Analysis at the unit scale demonstrated strong links between hydraulic variables and macroinvertebrate parameters such as abundance, diversity and proportion of shredders (SHR). Overall, ASs seem to provide a good trade‐off between the need to limit channel incision whilst maintaining aquatic ecosystems. Copyright © 2009 John Wiley & Sons, Ltd.

Composition and diversity of Ephemeroptera (Insecta) nymph communities in the middle section of the Jacuí River and some tributaries, southern Brazil

The taxonomic composition and diversity of assemblages of Ephemeroptera nymphs of four lotic environments in the central region of State of Rio Grande do Sul, a subtropical area in southern Brazil, were evaluated. Samplings were done monthly, with a Surber sampler, from June 2001 to May 2002, in the Jacuí River and three of its tributaries. The total number of nymphs collected in the four sampling sites was 11,007 in five families and 19 genera, of these, 11 are new records for the State. The highest diversity occurred in Point 4 (H'=2.41) and the lowest in Point 2 (H'=1.69). Point 4 had the highest environmental stability, conservation of the riparian vegetation and the lowest anthropic impact, while Point 2 presented a large environmental simplification due to a direct anthropic influence (e.g. domestic sewerage, trampling by cattle). The diversity of nymphs observed in the total area is high, compared to the estimated maximum theoretical diversity; a result of the high evenness and richness recorded. Rarefaction curves, calculated for a sample of 1,018 specimens, showed a similar expectation of richness for the four sampling sites. This result seems to be associated with the overall environmental homogeneity of the region caused by long-term alterations (land use and deforestation). In summary, higher diversity of Ephemeroptera nymph assemblages seems to be associated with habitat complexity, a good vegetation cover and a lower anthropic influence.

Changes in macroalgae and bottom fauna in the winter period in the regulated Alta River in Northern Norway

AbstractThe study was carried out in the upper part of the Alta River (69°58N; 23°24E), on a 3 km stretch that is not covered by ice due to increased winter temperatures after hydroelectric power development in 1987. The aim was to detect potential effects on the macroalgae and bottom fauna from the altered physical conditions. The presented data, based on Surber samples, describe the late winter situation (March to May) for the macroalgae (1996–2004) and the bottom fauna (1993–2004). The maximum biomasses of algae showed a highly significant decline from close to 200 g dry weight (ash free) m−2 in 1996 to less than 50 g dry weight m−2 in the last years of the investigation. Microspora amoena was the overall dominating species in the first phase of the study and Didymosphenia geminata in the last 5 years. Ulothrix zonata and Hydrurus foetidus were common throughout the entire study. A highly significant linear decline (r2 = 0.752, p = 0.001) was found in the total bottom fauna densities, showing means from 9500 to 16 000 ind. m−2 in 1993–1996 and from 2200 to 2700 ind. m−2 in 2001–2004. The reduction primarily concerned Chironomidae larvae. The mean biomass of the bottom fauna did not change to the same extent due to low individual weight of the Chironomidae larvae. A strong correlation between algal biomass and bottom fauna density was found (r2 = 0.796, p = 0.0029). The study demonstrates a eutrophication effect in the ice‐free river section following the regulation. The effect decreased through the investigated period and terminated 13–15 years after the regulation took place. Copyright © 2008 John Wiley & Sons, Ltd.

The relationships between benthic macroinvertebrates and biotic and abiotic environmental characteristics downstream of the Yahagi Dam, Central Japan, and the State Change Caused by inflow from a Tributary

AbstractAnthropogenic modifications have disrupted aquatic ecosystems by modification of the flow regime and transport of sediment. The impacts of dams are believed to be very large, but, despite the abundance of dams in monsoonal Asia, great scientific uncertainty still exists about the effect of dams on macroinvertebrates. Therefore, we studied macroinvertebrate assemblages at three reaches (downstream of a dam, downstream of a confluence and on a tributary) of the Yahagi River, central Japan, to confirm the impact of long‐term impoundment on the relationships between macroinvertebrate assemblages and biotic and abiotic environmental factors and also the role of a major tributary in community shift.Four Surber samples and associated physical measurements (depth, velocity and substrate composition) were taken from four study sites (riffles) at each study reach during the period from 10 to 21 February 2004. Drifting materials (zooplankton, POM and bedload sediment) and periphyton were also sampled. Significant differences were found in the macroinvertebrate fauna of these different reaches. Faunal distributions downstream of the dam were severely altered, with high macroinvertebrate abundance and low taxa richness in contrast to those in the tributary. The two‐way indicator species analysis (TWINSPAN) classification clearly distinguished these samples from the others. Based on nonmetric multidimensional scaling (NMS) ordination and correlation analysis (environmental variables vs. NMS axis score), these differences in community structure reflected changes in substrate composition and quantity and quality of suspended and benthic FPOMs. Our observations suggest that while dam‐derived zooplankton and reduced sediment transport had a great impact on the fauna, the tributary inflow, with a catchment size of less than 20% of that above the dam, acted as a major source of sediment input and facilitated macroinvertebrate community recovery. Copyright © 2008 John Wiley & Sons, Ltd.

Effect of Environmental Variables on the Distribution of Two Freshwater Crabs (Anomura: Aeglidae)

Abstract We evaluate whether the abundance of the freshwater crabs Aegla itacolomiensis and A. platensis is related to any or all of 11 environmental variables. We sampled 205 Surber samples (33 × 33 cm) in a stream five meters wide in southern Brazil. For each Surber sample we obtained measures of flow velocity, depth, type of substrate, and availability of coarse particulate organic matter. The relationships between abundances of the two species and the environmental variables were assessed by regression trees. Only 2 of the 11 environmental variables were important in describing the abundances of the two species, both of them related to availability of coarse particulate organic matter. The abundance of Aegla itacolomiensis was related positively to quantity of fragmented leaves and, to a lesser degree, to quantity of twigs. For the abundance of A. platensis, quantity of twigs, followed by fragmented leaves, were the most important environmental factors. The quantity of recently fallen, unfragmented le...

The effect of the Grand Ditch on the abundance of benthic invertebrates in the Colorado River, Rocky Mountain National Park

AbstractWe investigate herein the hypothesis that there is a significant relationship between bed particle mobility and benthic invertebrate abundance in the gravel‐bed channel of the upper Colorado River in Rocky Mountain National Park. A large diversion channel called the Grand Ditch normally diverts a significant portion (∼50%) of the annual snowmelt runoff from the watershed northward out of the basin. In May 2003, a ∼30‐m section of the ditch was breached, contributing substantially to the magnitude and duration of discharge in the Colorado River until the ditch breach was repaired in July of that year. As a result, all grain sizes in the river channel were mobilized, which contrasted sharply with the minimal gravel transport experienced during the exceptional drought of the previous year. Benthic macroinvertebrates were collected in the field using a Surber sampler at the same six locations for both years, and the number of individuals of the orders ephemeroptera (mayflies), plecoptera (stoneflies), trichoptera (caddisflies) and diptera (e.g. chironomids) was counted in the laboratory. The total number of individuals was 240% higher in 2003, and the proportion of mayflies in the samples increased from 25% in 2002 to 40% in 2003. In 2003, samples were also taken immediately upstream and downstream of a large flow obstruction in the channel in order to further isolate the relative importance of sediment transport against other variables affecting the stream habitat. Numbers of individuals for all taxa collected (particularly ephemeroptera and plecoptera) were nearly an order of magnitude higher at the upstream site than at the downstream, protected location. These results have important implications for the ecosystem management of streams within Rocky Mountain National Park and elsewhere. Copyright © 2008 John Wiley & Sons, Ltd.

Bootstrapping to investigate the effect of number of macroinvertebrate samples on confidence limits of the mean

A resampling technique (bootstrapping) was used to evaluate the effects of increasing sample number on the confidence limits of mean density and the mean number of taxa (Families) in addition to the performance of four different sampling devices used to collect macroinvertebrates in three streams of contrasting anthropogenic impact. The four sampling devices were a Hess sampler, a modified Hess sampler equipped with an internal water pump, a Surber sampler, and a two-pole kick-net. In general, confidence limits decreased predictably as the number of samples increased, most notably with the addition of a third sample. The density of organisms captured with the Surber sampler was significantly lower than that of the Hess sampler but greater than that of the kick-net sampler. No significant difference in the number of Families was found among sampling devices. Based upon a minimal 10% reduction in confidence limits compared to the previous sample, a minimum of four replicates was suggested when using the Surber sampler. In general, other methods required a greater number of replicates that the Surber to adequately represent density, but this did not hold when applied to the number of taxa. Despite the inconvenience of having to bend down to use the Surber sampler, this study found no evidence to reject its apparent popularity as a sampling method for macroinvertebrates.

Effect of Environmental Variables on the Distribution of Two Freshwater Crabs (Anomura: Aeglidae)

We evaluate whether the abundance of the freshwater crabs Aegla itacolomiensis and A. platensis is related to any or all of 11 environmental variables. We sampled 205 Surber samples (33 × 33 cm) in a stream five meters wide in southern Brazil. For each Surber sample we obtained measures of flow velocity, depth, type of substrate, and availability of coarse particulate organic matter. The relationships between abundances of the two species and the environmental variables were assessed by regression trees. Only 2 of the 11 environmental variables were important in describing the abundances of the two species, both of them related to availability of coarse particulate organic matter. The abundance of Aegla itacolomiensis was related positively to quantity of fragmented leaves and, to a lesser degree, to quantity of twigs. For the abundance of A. platensis, quantity of twigs, followed by fragmented leaves, were the most important environmental factors. The quantity of recently fallen, unfragmented leaves was unimportant. We conclude that the two species of Aegla select locations with abundant old plant fragments that are usually colonized by fungi and bacteria. This conclusion is corroborated by previous studies indicating that Aegla spp. feed mostly on plant fragments.