Spatial and temporal variation of fish communities in secondary channels of the San Juan River, New Mexico and Utah

Spatial and temporal variation of fish communities in four secondary channels of the San Juan River between Shiprock, NM and Bluff, UT were investigated from July 1993 through November 1994. Fish abundance and habitat availability data were collected to determine if physical attributes of sites influenced spatial and temporal variation in their fish communities. Stability of habitat was shown to positively influence the stability of the fish community. Analysis of variance revealed greater spatial than temporal variation in the abundance of red shiner, Cyprinella lutrensis, fathead minnow, Pimephales promelas, and flannelmouth sucker Catostomus latipinnis, while speckled dace, Rhinichthys osculus showed greater temporal variation. Ordination, using detrended correspondence analysis, revealed variation in fish communities by site, date, and sample year. Spatial variation was greatest during low-flow periods when the greatest differences in habitat among the four sites occurred. Spring runoff had the greatest temporal effect on the fish communities in secondary channels and appeared to reset the communities by displacing those species that were less resistant to increased current velocities. This annual event may help maintain native fish species adapted to these conditions in the San Juan River while moderating the abundance of nonnative fish species.     

Contemporary Land Change Alters Fish Communities in a San Francisco Bay Watershed,California, U.S.A.

Urbanization is one of the leading threats to freshwater biodiversity, and urban regions continue to expand globally. Here we examined the relationship between recent urbanization and shifts in stream fish communities. We sampled fishes at 32 sites in the Alameda Creek Watershed, near San Francisco, California, in 1993–1994 and again in 2009, and we quantified univariate and multivariate changes in fish communities between the sampling periods. Sampling sites were classified into those downstream of a rapidly urbanizing area (“urbanized sites”), and those found in less impacted areas (“low-impacted sites”). We calculated the change from non-urban to urban land cover between 1993 and 2009 at two scales for each site (the total watershed and a 3km buffer zone immediately upstream of each site). Neither the mean relative abundance of native fish nor nonnative species richness changed significantly between the survey periods. However, we observed significant changes in fish community composition (as measured by Bray-Curtis dissimilarity) and a decrease in native species richness between the sampling periods at urbanized sites, but not at low-impacted sites. Moreover, the relative abundance of one native cyprinid (Lavinia symmetricus) decreased at the urbanized sites but not at low-impacted sites. Increased urbanization was associated with changes in the fish community, and this relationship was strongest at the smaller (3km buffer) scale. Our results suggest that ongoing land change alters fish communities and that contemporary resurveys are an important tool for examining how freshwater taxa are responding to recent environmental change.     

Spawning ecology of flannelmouth sucker, Catostomus lattipinnis (Catostomidae), in two small tributaries of the lower Colorado River

We report the first published accounts of spawning behavior and spawning site selection of the flannelmouth sucker in two small tributaries of the lower Colorado River in the Grand Canyon, Arizona. Spawning was observed on 20 March 1992 and from 28 March to 10 April 1993 in the Paria River, and from 16 to 19 March 1993 in Bright Angel Creek. Flannelmouth suckers exhibited promiscuous spawning behavior–individual females were typically paired with two or more males for a given event and sometimes changed partners between events. Multiple egg deposits by different females sometimes occurred at one spawning site. Flannelmouth sucker selected substrates from 16 to 32 mm diameter in both streams. Spawning occurred at depths of 10 to 25 cm in the Paria River and 19 to 41 cm in Bright Angel Creek. Mean column water velocities at spawning locations ranged from 0.15 to 1.0 m sec^-1 in the Paria River and from 0.23 to 0.89 m sec^-1 in Bright Angel Creek. Water temperatures recorded during spawning ranged from 9 to 18° C in the Paria River and 13 to 15° C in Bright Angel Creek. Spawning flannelmouth sucker ascended 9.8 km upstream in the Paria River and 1.25 km in Bright Angel Creek. Spawning females (410–580 mm) were significantly larger than spawning males (385–530 mm) in the Paria River. The mean size of spawning fish in the Paria River was significantly smaller than the entire stock, averaged throughout the study period (380–620 mm). However, fish spawning in 1992–1993 averaged 53 mm larger than fish spawning in the same reach of the Paria River in 1981, indicating a shift in the size structure of this stock.     

Temporal fluctuations of the phytoplankton community of the Baía River, in the upper Paraná River floodplain, Mato Grosso do Sul, Brazil

Subsurface samples were taken monthly (March/1993 through February/1994) in the Baía River, a tributary of the right bank of the Paraná River (22° 43 S and 53° 17 W). We analyzed temporal changes in the phytoplankton community in relation to density, biomass, richness and species diversity, equitability, size structure, and dominance. We related these to regional climate and hydrology, and to the physical and chemical variables of the water column. We determined 119 taxa, wich were numerically dominated by the class Chlorophyceae with 37 taxa. The classes Cyanophyceae and Bacillariophyceae contributed the most abundant biomass and were responsible for the two peaks that were observed. The high water period was generally characterized by lower phytoplankton biomass, higher richness and species diversity, and higher density of nannoplanktonic species such as Monoraphidium tortile in March, and Cryptomonas brasiliensis in January. During low water, however, the highest values of phytoplankton biomass were recorded, with heterocytous cyanophytes dominating during the phase of greater stability of the water column, and filamentous diatoms during periods of mixing turbulence. The fluctuations observed indicate that the hydrosedimentological regime of the Paraná River, together with the climatological factors, constitute the main forcing functions acting on the Baía River phytoplankton.     

Fish-habitat associations across multiple spatial scales

Important to the study of reef fish ecology is understanding the degree to which fish community structure varies across space, what factors can account for such variation, and whether these factors are scale dependent. This study examined the structure of reef fish communities across four spatial scales (1, 10 100, and 200 m²) visually censused from seven sites within Tague Bay, St. Croix, U.S. Virgin Islands. Relative differences in the number of individuals and species among sites indicated a pattern that was consistent across spatial scales. Spearmans rank correlation revealed significant positive correlation in site rankings, in terms of species richness, between 1 and 10 m², and 100 and 200 m²; and for the abundance of individuals between 100 and 200 m². In order to understand the degree to which quantified habitat variables account for patterns in the abundance of individuals and species, and whether these fish-habitat relationships were consistent regardless of spatial scale, separate canonical correlation analyses were conducted at each scale. Independent of scale, the total number of individuals and species were correlated with specific habitat variables, either negatively (with areas of pavement, sand, no algae, and low structural complexity) or positively (with areas of Amphiroa rigida, Halimeda incrassata, high structural complexity, and diverse algae/seagrass communities). These habitat variables explained 31–81% (at scales of 1–200 m²) of the variation in the number of individuals and species. Similar analyses were also performed on the abundances of the nine most common species, and whether their specific habitat associations were independent of scale. Results indicated that habitat variables explained 19–73% (at scales of 1–200 m²) of the variation in abundances of each species. Unique fish-habitat relationships were observed for each species, and most such relationships were consistent across spatial scales. The structure of reef fish communities of Tague Bay was explained in large part by the composition of coral and algae communities present. Both the spatial variation in community structure and the fish-habitat relationships, at the community and population level, appeared to be largely independent of the spatial scale examined. This suggests that generalizations across Tague Bay are possible. Similar habitat associations reported in the literature are discussed with regard to the possibility for generalizations across regions.     

DNA in Action: Rapid Application of DNA Variation to Sockeye Salmon Fisheries Management

Commitment to conservation-based management of exploited fish species imposes unprecedented requirements for adaptive, real-time management of biologically and socially complex mixed-stock fisheries such as those conducted for Pacific salmon. Stock identification is a key component of the management process, with population-specific timing and abundance information often incorporated into management decisions. By using both microsatellite and major histocompatibility complex genetic variation, we achieved highly accurate estimates of stock composition for Fraser River sockeye salmon. Over a 2-month period in 2002, we analyzed 9300 returning Fraser River sockeye salmon sampled in mixed-stock fisheries, and provided stock composition estimates to fishery managers within 9–30 h of sample delivery. Stock-specific exploitation targets governed by conservation concerns were achieved in this fishery.     

Changes in the Tagus nursery function for commercial fish species: some perspectives for management

The Tagus estuary has long been subjected to several human activities and uses. Changes in the estuarine fish community were evaluated based on beam trawl surveys in 1978–1980 and 1994–1996. There was a decrease of abundance of many species that use the Tagus as a nursery, namely bib (Trisopterus luscus), five bearded rockling (Ciliata mustela), tub gurnard (Trigla lucerna), and flounder (Platichthys flesus), possibly due to reduction of the water quality. In 1994–1996, only sea bass (Dicentrarchus labrax), sole (Solea solea) and Solea senegalensis Kaup, 1858, were found within the nursery grounds, showing an irregular pattern of abundance. Condition and vacuity index were highly variable. The major problems affecting the fish community, namely pollution, fishery regulation and waterflow regulation, are discussed in relation to the management and conservation of this estuarine system.     

The zooplankton of Lake Mikri Prespa

A study from June 1990 to October 1992 revealed thatthe pelagic zooplankton community of Lake Mikri Prespaconsists of 45 invertebrate species (28 Rotifera, 11Cladocera, 6 Copepoda and 1 mollusc). It includes A. steindachneri, an endemic calanoid of the WesternBalkans. Half of the species are first records for thelake.The total abundance and biomass ranged from 61 to 905 indl^–1 and 58 to 646 µg l^–1,respectively. Seasonal fluctuations agreed with thePEG-model. Copepods prevailed throughout the year. Thespecies composition and seasonal variability wereaffected by temperature-dependent food conditions,competition and predation. These factors induced aseasonal succession which involved a replacement oflarger species or individuals by smaller ones duringsummer. The cladoceran population, as a whole, wascharacterised by small size classes of the speciespresent, probably due to fish predation. Anexamination of the horizontal distribution showedthat, although seasonal variability was the same atall stations sampled, the central station showedhigher abundance. Inshore-offshore diel migrations ofjuvenile fish is considered to account for this unevendistribution. Shannons diversity index ranged from0.98 to 2.95 and evenness from 0.3 to 0.9. Both wereinfluenced by temperature.     

Fish Communities and Their Associations with Environmental Variables, Lower San Joaquin River Drainage, California

Twenty sites in the lower San Joaquin River drainage, California, were sampled from 1993 to 1995 to characterize fish communities and their associations with measures of water quality and habitat quality. The feasibility of developing an Index of Biotic Integrity was assessed by evaluating four fish community metrics, including percentages of native fish, omnivorous fish, fish intolerant of environmental degradation, and fish with external anomalies. Of the thirty-one taxa of fish captured during the study, only 10 taxa were native to the drainage. Multivariate analyses of percentage data identified four site groups characterized by different groups of species. The distributions of fish species were related to specific conductance, gradient, and mean depth; however, specific conductance acted as a surrogate variable for a large group of correlated variables. Two of the fish community metrics – percentage of introduced fish and percentage of intolerant fish – appeared to be responsive to environmental quality but the responses of the other two metrics – percentage of omnivorous fish and percentage of fish with anomalies – were less direct. The conclusion of the study is that fish communities are responsive to environmental conditions, including conditions associated with human-caused disturbances, particularly agriculture and water development. The results suggest that changes in water management and water quality could result in changes in species distributions. Balancing the costs and benefits of such changes poses a considerable challenge to resource managers.     

The Impact of Artisanal Fishery on a Tropical Intertidal Benthic Fish Community

We examined the benthic fishes and artisanal fishery in the intertidal flats of Inhaca Island, Mozambique. Results of a questionnaire indicated that catches had decreased, and that piscivorous fish have disappeared. Results of a catch sampling study indicated that current catch rates are low, < 2 kg person^–1 fishing trip^–1. Use of fishing gear was significantly related to season, diel and lunar tidal phase, and habitat. Forty-eight fish species were observed in the catches with eight species comprising 80% of the catch of 1814 specimens. The annual catch was estimated at 26.2t for the whole bay. Highest fishing pressure was observed in the central section of the bay. A demersal fish survey was carried out with a 2-m beam trawl to sample the fish community. Two different areas were sampled, one area with a low, and one with a high fishing pressure. A total of 19889 fishes were caught comprising 93 species. Gobies dominated the catches and accounted for 56% of all specimens. Fishes were small with a mean standard length of 29mm. The Saco area exhibited the highest catch rates and biomass (maximum of 1040 fish 1000 m^–2 and 1490g 1000 m^–2), and the highest species richness and evenness values. Catch composition was different between the two sampling areas, and was strongly affected by season, but less by habitat. Total fish biomass was estimated at 5.6t for the whole area. Stomach content varied with habitat, and season, and was dominated by benthic invertebrates. The largest estimates of consumption were obtained in the tidal channel and the Zostera beds. Mean consumption of benthic organisms was 1.3g AFDW m^–2 yr^–1. The area seemed to be overfished. The heavily fished areas exhibited lower catch rates, lower proportion of piscivorous fish, increased proportion of small fish, and a decrease in species diversity.     

Oligochaetes in the southern basin of the Venetian Lagoon: community composition, species abundance and biomass

The oligochaete community of the Chioggia Lagoon (southern basin of the Venetian Lagoon) has been studied from May 1992 to May 1994, at ten fixed sampling stations, distributed across the lagoon. Numerous data regarding composition, abundance, biomass and the breeding periods of the species in the community have been obtained. Nine tubificid species, recently reported for the Mediterranean and Ponto-Caspian areas have been collected. Among them only three were abundant everywhere, except in the innermost part of the lagoon, and throughout the year: Tubificoides vestibulatus, T. swirencowi and Limnodriloides maslinicensis. Only one enchytraeid genus, Grania, has been found restricted to an area near the connection with the sea. The values for the Shannon Diversity Index for the community never exceeded 0.8 and were mainly between 0.4 and 0.7. The most interesting result was the decrease of abundance and biomass, proceeding from the sea mouth (about 6000 ind m^–2) towards the innermost part of the lagoon (less than 100 ind. m^–2). Correlations are suggested with the different grain size of the sediment.     

The Impact of the Extreme Floods in July 1997 on the Ichthyocenosis of the Oder Catchment Area (Czech Republic)

Changes in the ichthyocenosis of five smaller streams of 2nd to 4th order existing in the spring section of the River Odra (Oder) catchment area, in the territory of the Czech Republic, were observed before and after the catastrophic floods of July 1997. Quantitative catches using electro-fishing were performed in June, August, and September 1997, i.e. a month before and 2 months after the floods. The maximum flow of water in the examined sites usually reached or exceeded the limit of the so-called ȁ8once in a century high waterȁ9. Extreme floods did not influence statistically the proven average readings of species diversity of ichthyocenosis in the examined sites (p = 0.5625), species variety (p = 0.7316), abundance per hectare (p = 0.3125), and biomass per hectare (p = 0.4375). A distinctive decline in abundance and also in the biomass of the ichthyocenosis took place in geomorphic reaches. Significant incisions and alterations in the routes of the streams were also observed. Statistical data of catches of key reophilous fish species from the River Oder catchment basin, all of which were affected by the floods, in the periods 1995–1996, 1997, and 1998–1999, were evaluated. Catches of the species of Salmo trutta and Thymallus thymallus in 1997 considerably increased. Only Thymallus thymallus experienced a more pronounced decrease in the years after the floods, when compared with the average readings made in the period 1995–1996. The readings of the adult abundance of Barbus barbus and Chondrostoma nasus did not prove a negative impact of the floods on the adult population of both species in the assessed years. Our findings suggest that the original fish communities may be naturally well adapted to culmination flows.     

Temporal variability in abundance and biomass of ciliates and copepods in the eutrophicated part of Kaštela Bay (Middle Adriatic Sea)

Seasonal variations in abundance and carbon biomass of ciliated protozoa and micrometazoa were studied from May 1998 to November 1999 in the eutrophicated area of Katela Bay (Middle Adriatic Sea). Ciliates showed peaks in spring and autumn, primarily due to changes in the abundance and biomass of tintinnines, which participated in total ciliate abundance and biomass with 40.48 and 60.02%, respectively. The highest tintinnine density was 4,278 ind. l^–1, while their average biomass varied from 0.611 to 26.557 gC l^–1 . Maximal average density and biomass of non-loricates were 1,430 ind. l^–1 and 3.925 gC l^–1, respectively. The micrometazoa community was dominated by copepod nauplii, especially during the summer and autumn. The copepod biomass ranged between 3.47 and 26.75 gC l^–1 . High abundance and biomass values of the investigated zooplankton groups point to an important role of these organisms in the secondary production in the Bay, indicating that they may be (1) a crucial factor in controlling the populations of nano-/pico-phytoplankton and heterotrophic nanoflagellates, and (2) a significant prey for larger micrometazoans.     

An offshore buoy as a small artificial island and a fish-aggregating device (FAD) in the Mediterranean

The ODAS Italia 1 oceanographic buoy is moored in the Ligurian Sea, 37 nm from Genoa, along the Genoa-Cape Corse transect (43° 48.90 N–09° 06.80 E), over a 1270 m deep sea bottom. The underwater portion of the buoy is 37 m long and 0.60 m in diameter, acting as a small island for colonization of fouling organisms and as a fish-aggregating device (FAD). The role of the buoy in attracting and maintaining fish assemblages was investigated by visual censuses in different seasons at depths of 0–40 m. Fish from seven families, comprising 12 species, of which three are benthic, were recorded with maximum abundance in summer. Fouling was studied from samples collected on the buoy and on immersed panels. The fouling community of the buoy consisted of 34 algae and 100 animal species, including three fish. The settlement processes of the fouling community on the panels, in particular on those exposed for over 70 months at 12 m and 33 m depth, are described based on counts of settled organisms, the covering index of each taxa and biomass assessments. On the panels, 63 species were identified. The fouling biomass, on the panel submerged for 70 months, assessed as wet weight, reached 2.8 kg/m² at 12 m depth and 4.8 kg/m² at 33 m depth.Observations of benthic organisms settled directly on the buoy were made between 1988 and 1989 and when the buoy was retrieved and brought back to shore on April 15, 1991 after 52 months at sea. At this time, the fouling community along the full 37 m length of the buoy was sampled, and 91 taxa, including 83 species, were identified. Several of the species present on the buoy are shallow, coastal species, some with a very short larval period. Possible ways of colonization by such species are discussed. Despite seasonal changes, the pelagic fish community was more stable over the period of 11 years of study than the benthic community settled on the buoy (that is still developing).     

Environmental Biology of an Endemic Cyprinid,Varicorhinus alticorpus,in a Subtropical Mountain Stream of Taiwan

An endemic freshwater fish, Varicorhinus alticorpus, was studied from 1990 to 1993 in a fast flowing mountain stream in southern Taiwan (22°30'N, 120°30'E). The analysis of environmental conditions suggested that the fish could tolerate water temperatures between 19–24°C, a pH of 8.0–8.8, and flow velocity from 20–100cmsec^–1. The fish occupied riffles and pools. They foraged and schooled during daytime and aggregated in crevices between rocks at night. Most juveniles stayed at the shallow sandy-pebble flat by the pool where flow was slow. The species feed primarily on the periphyton growing on the rocks, and leave unique scars after grazing. One peak of juvenile recruitment was observed right after the rainy season, suggesting that reproduction of the fish has adapted to the climate. The change of abundance of adults along the habitats (at 150 to 800 m altitude) also indicated that the fish might migrate to low altitudes for spawning and disperse back to higher altitudes for exploiting new resources. The environmental biology of this fish shows an example of a cyprinid adapted to habitats in a subtropical mountain stream.     

Macrozoobenthos of Lake Peipsi-Pihkva: taxonomical composition, abundance, biomass, and their relations to some ecological parameters

The large but shallow (3,558 km², up to 15.3 m deep) lake is eutrophic, with Chironomus plumosus and Potamothrix hammoniensis as dominating macroinvertebrates in the profundal. The extensive well-aerated sublittoral with sandy bottom sediments has a mesotrophic appearance and supports a diverse fauna with several oxyphilous species, including a very abundant population of Dreissena polymorpha. The phytophilous fauna is limited to small sheltered areas only. The average abundance of the small animals of macrozoobenthos (without big molluscs) was 2,617 ind. m^–2, their biomass 12.34 g m^–2 (corresponding to 52.2 kJ m^–2) in 1964–1994. The same figures for big molluscs (mostly Dreissena) were 304 ind. m^–2 and 238 g m^–2 in 1964–1994, and even 864 ind. m^–2 and 687 g m^–2 in 1985–1988, at the time of their special mapping. The sublittoral zone revealed the lowest biomass of small animals but the highest biomass of big molluscs. The southern, shallower lake regions, more enriched with nutrients and better protected from wind, revealed a significantly higher biomass of small macrozoobenthos in the near-shore zone than the cleaner and open northern part, while no positive effect of enrichment was observed neither in the biomass of profundal zoobenthos nor in that of big molluscs. The production of the small macrozoobenthos was calculated as 111 and 53 kJ m^–2 during two annual cycles in Lake Peipsi s. s., the most productive period being the autumn overturn. Lake Peipsi-Pihkva has the highest abundance and biomass of macrozoobenthos among the large lakes of North Europe.     

Effects of different fish species and biomass on plankton interactions in a shallow lake

Thirty-six enclosures, surface area 4 m², were placed in Little Mere, a shallow fertile lake in Cheshire, U.K. The effects of different fish species (common carp, common bream, tench and roach) of zooplanktivorous size, and their biomass (0, 200 and 700 kg ha^–1) on water chemistry, zooplankton and phytoplankton communities were investigated. Fish biomass had a strong effect on mean zooplankton size and abundance. When fish biomass rose, larger zooplankters were replaced by more numerous smaller zooplankters. Consequently phytoplankton abundance rose in the presence of higher densities of zooplanktivorous fish, as zooplankton grazing was reduced. Fish species were also significant in determining zooplankton community size structure. In enclosures with bream there were significantly greater densities of small zooplankters than in enclosures stocked with either carp, tench and, in part, roach. When carp or roach were present, the phytoplankton had a greater abundance of Cyanophyta than when bream or tench were present. Whilst top-down effects of fish predation controlled the size partitioning of the zooplankton community, this, in turn apparently controlled the bottom-up regeneration of nutrients for the phytoplankton community. At the zooplankton–phytoplankton interface, both top-down and bottom-up processes were entwined in a reciprocal feedback mechanism with the extent and direction of that relationship altered by changes in fish species. This has consequences for the way that top-down and bottom-up processes are generalised.     

Correlates of Environmental Variables with Patterns in the Distribution and Abundance of Two Anemonefishes (Pomacentridae: Amphiprion) on an Eastern Australian Sub-tropical Reef System

The relationship between characteristics of the reef environment and variations in the distribution and abundance of the anemonefishes Amphiprion akindynos and A. latezonatus was investigated at North Solitary Island, a sub-tropical rocky reef system on the east coast of Australia. During the summers of 1994 and 1995, fish densities and host sea-anemone cover were assessed on replicate 25m transects at sites where host sea-anemones form semi-contiguous mats throughout the 6–21m depth range. Multiple regression analyses indicated that environmental variables accounted for 65–71% and 61–80% of the variations in the number of A. akindynos and A. latezonatus among-sites, respectively. Among-habitat comparisons indicated that A. latezonatus densities were positively correlated with depth (r=0.45–0.90), whereas A. akindynos showed no consistent depth-related abundance patterns. Poor correlations (p>0.05) between the densities of each species on transect lines suggested that present-day competition was unlikely to determine the preference of A. latezonatus for deeper depths. Correlations between host sea-anemone cover and fish densities at the within-habitat (depth) scale were comparatively stronger than correlations at among-habitats in both species. These results suggest that among-habitat comparisons can confound finer scale fish-habitat associations within habitat (depth) zones. Evidence suggests that while sea-anemone cover does, to an extent, regulate the local ecology of anemonefishes, other factors are also likely to interact to limit their densities.     

Effects of paper mill effluents on the fish fauna of stony shores of Lake Päijänne

The fish fauna of the stony littoral in the central parts of L. Päijänne was studied by electric fishing on four occasions during 1988–1989. Ten fish species and 1681 individuals (14.5 kg) were caught in the 15 fishing sites (4096 m²) which gives a mean density of 0.41 ind. m^–2 and biomass of 3.5 gm^–2. Effluent from two large paper mills causes a clear zonation of the fish fauna in the area. In the most polluted shores, only burbot and perch occurred regularly and the reproduction of other species was inhibited. In the semipolluted area (5–15 km from the Kaipola paper mill), burbot and stone loach occurred regularly, but owing to low numbers of perch and bullhead the total densities were usually low.The clean shores were characterized by an abundance of stone loach and bullhead and locally also of minnow, which seemed to be very sensitive to pollution. The strong floods during summer 1988 had positive effects on the reproduction of perch and pike, but the densities of other species were highest during the normal water level in autumn 1989.     

Short-term effects of a catastrophic beaver dam collapse on a stream fish community

Synopsis We examined the short-term effects of the natural catastrophic collapse of a beaver dam on downstream benthic insect density and fish community structure in a headwater tributary of the Mississippi River. The catastrophic collapse of the dam and ensuing flash flood resulted in a dramatic (< 90%) decrease in benthic insect density in riffle and pool habitats. Sixty days after collapse of the dam, insect densities in riffles were 62% of pre-collapse densities. Insect recolonization of pools was slower than for riffles; 60 days after collapse of the dam insect densities in pools were 8% of pre-collapse levels. Collapse of the beaver dam altered the structure of the downstream fish community by causing a short-term (2–4 days) influx of pond species, resulting in a brief increase in species richness and abundance. Fish species richness and abundance then decreased for 4–60 days to levels below those prior to the collapse.