Community structure of fishes attracted to shallow water fish aggregation devices off South Carolina,U.S.A.

Synopsis Twenty-two fish aggregation devices were deployed in 14 m of water off South Carolina. Species composition and abundance were determined by diver visual census on eight occasions from May through November, 1985. A total of 21 families and 36 species of fishes was observed at 121 stations. Pelagic fishes dominated the fauna with a 99.3% relative abundance, and Decapterus punctatus accounted for 97.6% of the individuals. Caranx crysos, Diplectrum formosum, Decapterus punctatus, Centropristis striata and Monacanthus hispidus were the most frequent species. Total fish abundance, number of species and abundance of four of the six most common species were significantly affected by season. Hurricane activity may have caused a significant drop in pelagic fish abundance at the FADs in July. No significant correlations among species abundances were found after removal of season and FAD type effects. Spatial zonation and seasonal occurrence patterns suggest some competition among pelagic fishes. Several factors that regulate FAD faunal abundance and composition are hypothesized, including: juvenile fish availability, availability of shelter, availability of adequate food resources, interspecific and intraspecific competition, severe sea conditions, and sporadic intrusions of large predatory fishes. It is hypothesized that the abundances of benthic and pelagic FAD fishes are correlated and that there is a direct or indirect energetic link between shallow water pelagic and benthic fish assemblages near FADs.     

Interspecific trophic relationships among pelagic fish species underneath FADs

Fortnightly experimental purse-seine hauls at fish aggregation devices (FADs) and open water control sites, over a 2-year period in oceanic waters o. the eastern coast of Majorca revealed that carangid, coryphaenid, serranid, balistid and centrolophid fishes caught there were mostly planktivores. Most of the species had a high food intake. The dominance of neustonic and holoplanktonic epipelagic prey could indicate a direct link between FADs, invertebrates (biofouling) and fish. Polyprion and Schedophilus were more generalist predators than the more specialized Naucrates and Trachurus spp. There was low variation in feeding intake and the types of prey categories important for each species. Naucrates , Coryphaena and Schedophilus characterized the autumn community under FADs, while Trachurus , Seriola and Balistes were present throughout the summer. There was little diet overlap among the species suggesting only limited competition for the food resources among Trachurus spp, Naucrates and Seriola , and among Seriola and Coryphaena.     

The influence of flood cycle and fish predation on invertebrate production on a restored California floodplain

Although floodplains are known to be tightly controlled by the flood cycle, we know comparatively little about how flooding influences predators and their consumption of secondary production, particularly in highly seasonal floodplains typical of Mediterranean climates. In this study, we investigate how the seasonal dynamics of a central California floodplain influence the timing and magnitude of fish predation and the abundance and composition of invertebrates. For 3 years (2000–2002), we compared changes in abundances and size distributions of invertebrates through the flood season (January–June) with seasonal changes in the abundance of larval and juvenile fishes. Using diet analysis of fishes and manipulative feeding experiments with fishes in field enclosures, we link specific changes in invertebrate populations directly to feeding preferences of seasonally abundant fish. Early in the flood season prior to March, we found little influence of fish predation, consistent with the near absence of larval and juvenile fishes during this period. Coinciding with the midseason increase in the abundance of larval and juvenile fishes in April, we found significant declines in zooplankton abundance as well as declines in the size of zooplankton consistent with fish feeding preferences. Our results were consistent with results from feeding enclosure experiments that showed that fish rapidly depressed populations of larger cladocerans with much less effect on smaller cladocerans and calanoid copepods. At the end of the flood season, zooplankton abundances rapidly increased, consistent with a switch in the feeding of juvenile fish to aquatic insects and subsequent fish mortality. We also found that zooplankton biomass on the floodplain reached a maximum 2–3 weeks after disconnection with the river. We suggest that floodplain restoration in this region should consider management strategies that would ensure repeated flooding every 2–3 weeks during periods that would best match the peaks in abundance of native fishes. Electronic supplementary material Electronic supplementary material is available for this article at and accessible for authorised users.     

Influence of irrigation infrastructures and water quality on fish assemblages in Lake Tana tributaries,north-west Ethiopia

Lake Tana is one of East Africa’s largest freshwater bodies, yet many of its fishes are migratory and utilize in-flowing tributaries as critical spawning habitat. However, factors such as expanding water resources developments and sand mining along these rivers and streams may disrupt this ecosystem function. We monitored juvenile and adult fish abundance and water quality across five lake tributaries from August 2014 to April 2015 to examine how irrigation schemes and water quality affect assemblage and population structure. Adult assemblages were dominated by Labeobarbus cyprinids and varied between tributaries, albeit without separation by irrigation development or sand mining. Overall, adult abundances of the dominant migratory Labeobarbus species were four-fold higher below the Shini River irrigation weir than upstream. Contrastingly, juvenile abundances were often significantly higher above these structures. Juvenile abundances decreased on average by 46% along the first 1000 m of two irrigation canals, suggesting poor habitat suitability or high mortalities from water withdrawals. Water quality varied more between rivers than sampling times, but without any separation of tributaries by irrigation or sand mining. Conductivity and turbidity-related parameters had the highest correlation with adult assemblage structure and individual species abundances. These findings indicate that Lake Tana tributaries must be managed on a case by case basis, with more focus given to mechanisms allowing fish to bypass irrigation developments and the direct assessment of fish populations between sand mining and other sites.

     

The influence of gummy sharks, Mustelus antarcticus, on observed fish assemblage structure

Large predatory fish that alter the behaviour of smaller species may affect visual estimates of abundance by making organisms more or less difficult to observe and quantify. To evaluate the non-consumptive influence of large predators on rocky reef fishes observed with Baited Remote Underwater Video (BRUV), we tested the hypothesis that fish assemblages monitored when large (ca 1.5 m) piscivorous sharks, Mustelus antarcticus, were present differ from those observed when sharks were absent. We did this in two ways using recordings from rocky reefs in Batemans Marine Park, NSW, Australia. First, we examined 6 min of each 30 min tape; 3 min when sharks were present and 3 min when they were absent, in a paired sample design from 17 sites. Second, we compared fish assemblages for complete tapes (30 min) at sites with sharks present compared to sites where they were absent. The diversity and total abundance of fishes was consistently lower in the presence of sharks; we detected significant assemblage-wide change (PERMANOVA, P?<?0.05). Importantly, the diversity and total abundance of fishes for complete tapes (30 min) decreased in the presence of sharks by 18 % and 36 %, respectively, underscoring the likelihood that sharks influence observed fish assemblages. Individual species responses were variable and while the abundance of many fish species decreased in response to sharks, others increased 6-fold. Overall, these results highlight the need to consider systematic differences in the abundance of large predators to avoid biases when testing hypotheses about fish assemblages using non-destructive visual methodologies.     

Fish assemblages on artificial and natural reefs in the Flower Garden Banks National Marine Sanctuary, USA

 Visual censusing was used to characterize fish assemblages on artificial and natural reefs located within the boundaries of the Flower Garden Banks National Marine Sanctuary (FGBNMS) in the northwestern Gulf of Mexico. Emphasis was placed on determining spatial and temporal patterns in habitat utilization by fishes on an offshore artificial reef (Mobil Platform HI-A389A). Overall, 43 species were observed during diurnal surveys in the upper 24 m of the artificial reef. Midwater pelagic fishes (i.e., carangids and scombrids) accounted for over 50% of all taxa enumerated on the artificial reef; however, these taxa were transient members of the assemblage and were observed infrequently. Labrids, pomacentrids, and serranids were the dominant reef-dependent taxa. Distinct trends in vertical, diel, and seasonal abundances were observed for juvenile and adult fishes. Of the three designated depth zones (upper 1.5–9.0, middle 9.0–16.5; lower 16.5–24.0 m), abundance and species diversity were lowest in the upper zone. Nocturnal counts were characterized by a marked reduction or complete absence of most species, due in part to twilight cover-seeking and movement activities. Seasonal variation in community composition and species abundance (May versus September) was primarily due to recruitment of juveniles (0-age fishes) to the artificial reef in late summer. Increases in total fish abundance (all taxa combined) coincided with both increasing habitat rugosity and degree of fouling. Species richness on natural coral reefs in the FGBNMS was higher than on the artificial reef. Unlike the artificial reef, fish assemblages on the natural reefs were dominated by a single family (Pomacentridae) which accounted for over 50% of all individuals observed. Accepted: 1 August 1996     

Spatial and temporal patterns in the fish assemblages of individual pools in a midwestern stream (U.S.A.)

Synopsis The composition and consistency of fish assemblages in 14 adjacent pools (6–120 m long) of a clear-water, limestone and gravel creek in midwestern U.S.A. were quantified in eight snorkeling surveys over 19 months, to establish a baseline of natural variation in the system at this scale. The fauna of the stream was dominated numerically by minnows (Cyprinidae), sunfish and black bass (Centrarchidae), and topminnows (Fundulidae). The pool fish fauna of the total 1 km reach (including all 14 pools) was highly consistent throughout the study, despite two major floods. Assemblages in individual pools generally were consistent, but there was more variation within pools than at the scale of the entire reach. Throughout the study, most individual pools remained within discrete subsets of the total occupied multivariate space in a principal components analysis based on fish species abundances. Sunfishes (Lepomis spp.) and bass (Micropterus spp.) were more consistent in their distribution among pools than were minnows (Cyprinidae) or a topminnow (Fundulus). There were 25 significant correlations in occurrence of species pairs among stream pools, out of 91 possible comparisons of the 14 most abundant taxa in the reach. Many pools contained assemblages either dominated by large centrarchids or by abundant cyprinids and juvenile centrarchids, but intermediate assemblages also were observed. The dynamics of distribution of fish species and fish assemblages among individual stream pools are likely influenced by a combination of species-specific behaviors and habitat selection, predator constraints on use of individual pools by small fishes, riffles as size-selective barriers to fish movements between pools, dispersal of young-of-the-year fishes, and abiotic phenomena like floods. Individual stream pools appear to be discrete habitat units for fishes, and do represent an appropriate scale for biologically meaningful studies of fish assemblages or their effects on streams.Department of Zoology, University of Oklahoma     

1980—2007年大亚湾鱼类物种多样性、区系特征和数量变化

根据2004—2005年大亚湾海域底拖网鱼类调查数据,并结合1980—2007年的历史资料,分析了该海域鱼类的种类组成、区系特征、多样性、优势种和数量变化趋势.结果表明： 2004—2005年,大亚湾海域共记录鱼类107种,分属13目50科,以中下层鱼类的种类最多,为48种,其次是中上层和底层种类,分别为37种和21种.大亚湾鱼类区系具热带和亚热带特性,以暖水性种类占绝对优势,为97种,暖温性种类为10种.多样性指数以夏季最高（3.82）,其次是冬季（3.37）和秋季（3.00）,春季最低（2.40）.Pielou均匀度指数的季节变化情况与多样性指数相似.1980—2007年大亚湾海域鱼类群落特征发生了明显的变化：鱼类种类数减少,优势种更替明显.鱼类种类数由1980年的157种减少至1990年的110种,2004—2005年继续减少至107种;鱼类优势种由1980年以带鱼和银鲳等优质鱼为主,更替为以小型和低值的小沙丁鱼、小公鱼和二长棘鲷幼鱼为主.用包含年际变化趋势和季节性周期变化的回归模型模拟1980—2007大亚湾鱼类资源密度的变化,鱼类资源密度在1980—1999年和1990—2007年两个时期均呈下降趋势,但1990—2007年间下降幅度比1980—1999年间大;1980—1999年鱼类资源密度的季节波动幅度较平缓（振幅为0.099）,而1990—2007年的季节波动较大（振幅为0.420）,说明1990—2007年阶段大亚湾鱼类数量的季节变化更为显著.     

Spatially variable response of native fish assemblages to discharge,predators and habitat characteristics in an arid‐land river

1. Fish assemblages and habitats were sampled annually at fixed sites in three tributaries of the Gila River catchment over a 21‐year span that included prolonged low‐ and high‐flow periods. Model selection was used to evaluate responses of seven native fishes with variable ecological traits (four small‐bodied cyprinids, one large‐bodied cyprinid, and two large‐bodied catostomids) to mean annual discharge and predacious non‐native fishes across the three sites. We also compared habitat use and overlap of native and non‐native fishes to identify potential for negative interactions among species. 2. Assemblage structure (species abundance and richness) and recruitment of native species was strongly and primarily affected by mean annual discharge and secondarily by location and densities of non‐native predators (mainly the centrarchid Micropterus dolomieui). 3. Densities of age‐0 catostomids and small‐bodied cyprinids were positively associated with discharge, and this pattern was strongest in the tributary with the lowest densities of non‐native predators. Absence or extreme low abundance of natives during low‐flow years was most pronounced at the sites where non‐native predators were comparatively common. Densities of adults of large‐bodied native species also varied by site, but often were positively associated with densities of non‐native predators. 4. Spatially variable responses of native fish assemblages indicated that the persistence of native fishes could be jeopardized if key habitats were lost or flow regimes unnaturally altered, particularly during low‐flow conditions when recruitment of native fishes is low and predation by non‐natives is high. Large‐bodied species may be less vulnerable to multiple years of poor conditions because adults are able to avoid predation by non‐natives and thus can rely on occasional high discharge years for successful recruitment. 5. As in other arid‐land streams, native fish assemblages of the Gila River Basin continue to decline. Our results indicate that conservation requires specific knowledge and consideration of physical influences as well as life‐history attributes of native and non‐native fishes.     

Colonization,diversity, and seasonality of fishes at pelagic fish aggregating devices

The pelagic zone of the ocean can be a challenging environment in which to conduct research and as a result we lack the robust baseline abundance and diversity data, compared to what is available in more accessible coastal habitats, to be able to track changes or stressors to the biota in this environment. Many large-scale fisheries target pelagic fish, and much of the information available on these species is based on fisheries-dependent data that may be biased towards hotspots and commercially valuable fishes. Here, a long-term video and visual fish survey was conducted on two subsurface moored fish aggregating devices (FADs) in the pelagic waters of the central Bahamas to determine the feasibility of using moored pelagic FADs as tools for collecting fish abundance and diversity data. A wide range of species was documented, including large migratory fish that are the focus of commercial and recreational fisheries, and smaller often overlooked species on which little abundance or seasonality information exists. We found that FADs colonize quickly and reach a peak stable (albeit seasonally cyclical) abundance and diversity within the first several months after deployment. Species richness was higher in video surveys, but abundance was higher in visual surveys, except for sharks. Our results highlight the need to tailor survey methods to fit the context and study objective, and provide further evidence for the importance of fisheries-independent data in monitoring pelagic species.     

Rocky intertidal fish communities of California: temporal and spatial variation

Synopsis We examined data from our own and published collections of intertidal and shallow littoral fishes of the North American Pacific Coast with respect to temporal and spatial trends in species composition and dominance. We compared (1) recent and past intertidal collections made five and seven years apart, respectively, for two California localities, (2) intertidal collections from twelve localities in California, Oregon, and British Columbia, and (3) intertidal versus subtidal collections at one California locality. Temporal comparisons indicated substantially lower abundance of the cottid Oligocottus snyderi at two California localities during 1984, at one locality due to depressed abundance of young. Interannual differences in abundances suggested that other tidepool fish assemblages undergo significant changes as well. Geographical comparisons indicated general similarity in species composition, with cottids predominating in tidepools although several other families also were well represented (e.g., Stichaeidae, Scorpaenidae). Eel-shaped stichaeids and pholids occurred at high densities in exposed boulder fields. Both the tidepool and boulder field assemblages showed north-south changes in species abundances. Comparison of collections from the intertidal and subtidal zones at one California locality demonstrated that fishes of these habitats form two essentially distinct assemblages, with most species restricted to or concentrated in one or the other habitat.     

Impacts of past glaciation events on contemporary fish assemblages of the Ohio River basin

Aim We evaluated variation in fish assemblages on the basis of taxonomic composition and functional groups based on Pleistocene glacial boundaries in the Ohio River basin. We tested for the influence of habitat and hydrology on fish assemblage variation. Location Ohio River basin of North America, including the states of Ohio, Indiana and Illinois. Methods Fish collection sites were identified as Wisconsinan, pre‐Wisconsinan or unglaciated regions. Multivariate analyses, multi‐response permutation procedures, discriminant analysis and indicator species analyses were used to test whether taxonomic and functional assemblages were distinct among regions with varying glacial histories. Principal components analysis was used to identify habitat and water quality, as well as hydrological gradients that could be discerned by glacial region. Results We identified significant differences in both taxonomic and functional fish assemblage structure and habitat variation among regions that had different glaciation histories. The largest differences in taxonomic and functionally based fish communities were for unglaciated and pre‐Wisconsinan regions, while unglaciated and Wisconsinan regions were most similar. We correctly classified study regions in 71% and 60% of sites using taxonomy and functional analyses, respectively. Wisconsinan sites were characterized by Cyprinidae and Catostomidae assemblages with high abundances of tolerant fishes that tended to occur in habitats with reduced current velocity. Pre‐Wisconsinan sites were characterized by Cyprinidae, Catostomidae, Centrarchidae and Percidae families with increased abundances of intolerant fishes that tended to occur in habitats with coarser substrates and increased water velocity in streams of varying size. Unglaciated sites were characterized by Cyprinidae and Percidae families and were not closely associated with any habitat‐based functional group. Habitat in the unglaciated and pre‐Wisconsinan sites was significantly different from that in the Wisconsinan sites, which were characterized by increased channel structure and reduced stream size. Main conclusions Pleistocene glaciation events formed a lasting template of predictable regional differences in stream habitat and in the corresponding taxonomic and functional fish assemblage structures. While many factors impact the distribution of fishes, these results suggest that historical influences such as glaciation may be used to further explain the underlying mechanisms of spatial variation in fish assemblages.     

An ecological and economic review of meiofauna as food for fish*

After briefly reviewing the evidence that meiofauna are an integral part of the complex food web of invertebrate infaunal and epifaunal organisms, it is shown that, at least in intertidal and shallow subtidal habitats, meiofauna are an important food for the small (30–60 mm) juveniles of some commercially important fishes (flatfish and salmonids) or for fish which are themselves food for commercial species. The evidence that fish feed preferentially on a few species of harpacticoid copepod rather than on the numerically dominant meiofaunal taxa is presented and discussed. It is argued that, in addition to benthic copepods being energetically efficient as food for fishes, it is differences in the activity and availability between major taxa of meiofauna as they relate to the feeding strategies of the fish which go a long way to explaining this apparent general preference of fish for harpacticoid copepods rather than nematodes. From literature estimates of consumption rates, it is concluded that the supply of meiofaunal food to juvenile fish is unlikely to be limiting although this may not be the case in some shallow subtidal muddy sand habitats. Whilst it is recognized that fish predation may influence the magnitude of the seasonal peaks in numbers, predation is not the principal cause of seasonal fluctuations in harpacticoid copepod species abundances. The effects of predation on other aspects of community structure is unknown. Finally, the possible role of meiofauna in mariculture systems is briefly discussed.     

Changes in demersal wild fish aggregations beneath a sea-cage fish farm after the cessation of farming

Demersal, non-cryptic, wild fishes were counted in replicate 100 m² transects beneath a floating sea-cage fish farm and two nearby sandy locations at Gran Canaria (Canary Islands, eastern Atlantic) four times before and after the cessation of farming. Cessation of farming involved the removal of farmed fishes and ceasing of the daily feeding, although farm structures (cages and moorings) remained. A 'beyond-BACI' sampling design provided the framework to detect the effect of the cessation of farming, which produced qualitative and quantitative changes in the composition and structure of the fishes assemblages beneath the sea-cage fish farm compared with two nearby controls. The aggregative effect on wild fishes due to the existence of the farm decreased from c. 50 times compared to nearby controls when the farm was in full operation to less than two times when only the farm structures remained. Abundances of particulate organic matter feeders (large-sized mugilids), large benthic chondrichthyid rays and Pagellus spp. declined markedly at the fish farm after the cessation of farming, suggesting that the removal of daily feeding was responsible for their disappearance. In contrast, abundances of herbivores, benthic macro- and meso-carnivores were similar beneath the fish farm both before and after the cessation of farming. Benthic macro-carnivores, however, were more abundant beneath the sea-cages compared to control locations, supporting the hypothesis that the increase in the physical structure beneath farms plays a role in aggregating these species. Sparids occurred beneath the sea-cages only after the cessation of farming, while the two natural control locations did not show differences from before to after the cessation of farming. Overall, the results show that the wild fish assemblage beneath the farm partially changed after the cessation of farming to a more natural state, approaching the assemblages observed at the control sites.     

Patterns of abundance variation in reef fishes near an artificial reef at Guam

Synopsis Development of the fish community on a submerged 16 m barge and variation in fish abundance on nearby transects were surveyed twice monthly for twenty months. A steady increase in abundance was observed for certain fishes on the barge, whereas a few species exhibited distinct seasonal variation on both the barge and transects. Most of the seasonal species settled between March and May.Some seasonal species appeared to be site selective in their settlement and consequently settled juveniles were clumped in their distribution. An abundance of preferred topographical features may be why settlement was relatively high at the study site and indirectly why predators became significantly (r₃ = 7.67^***, N = 37) more abundant at the study area during the months of maximum prey settlement. Concurrent settlement of several species during the same few months may be important because juveniles become an abundant food source to predators during those few months only. Periodic swamping of predators by abundant juvenile prey may improve the chances for individuals of rarer prey species to be overlooked and therefore be succesfully recruited.     

Structure and diversity of fish assemblages on rocky and soft bottom shores on the Swedish west coast

Density and biomass of fishes, from shallow rocky and soft bottom habitats on the Swedish west coast, showed large seasonal variation with low values in winter and spring and with peaks in June. Season was also the most important factor determining the fish assemblage structure. Within season, however, there was a clear separation in assemblage structure between rocky-and soft-bottom habitats. There were significantly higher total fish abundances and biomasses during night compared with day catches. On soft bottoms density and biomass of fishes decreased with increasing depth, but no such pattern was seen in rocky habitats indicating that the distribution of fishes was related to vegetation cover. Altogether, 53 fish species were recorded of which 30 were common to both habitats. Species richness was similar on rocky and soft bottoms. Of the 10 most abundant species found in rocky habitats four belonged to the Labridae and three to the Gadidae. The fish assemblage on soft bottoms were of a more mixed nature with representatives among the 10 dominants from six families (Clupeidae, Cottidae, Gadidae, Gobiidae, Labridae and Pleuronectidae). When ranking the 10 dominant fish species on rocky bottoms according to biomass c . 50% of the mass was Labridae, 19% Gadidae and 13% Cottidae. In soft bottom habitats, fish biomass was mainly distributed between six families. Pleuronectidae and Gadidae were dominant and each made up 25% of the biomass, whereas Labridae only contributed 4% of the fish mass. It is concluded that the fish assemblage in rocky habitats is dominated by permanent non-commercial species (63% of biomass), whereas soft bottoms mainly function as nurseries for juvenile fishes and as feeding grounds for seasonal migrants of commercial species (80% of biomass).     

Eight decades of sampling reveal a contemporary novel fish assemblage in coastal nursery habitats

In order to adequately monitor biodiversity trends through time and their responses to natural or anthropogenic impacts, researchers require long time series that are often unavailable. This general lack of datasets that are several decades or longer makes establishing a background or baseline of diversity metrics difficult – especially when attempting to understand species composition changes against a backdrop of climate and ecological variability. Here, we present an analysis of a community of juvenile nearshore fishes based on nearly 8 decades of highly standardized Norwegian survey records. Using multivariate statistical techniques, we: (i) characterize the change in taxonomic community composition through time, (ii) determine whether there has been an increase in warm‐water affinity species relative to their cold water affinity counterparts, and (iii) characterize the temporal change in the species’ functional trait assemblage. Our results strongly indicate a shift toward a novel fish assemblage between the late 1990s and 2000s. The context of changes within the most recent two decades is in stark contrast to those during the 1960s and 1970s, but similar to those during the previous warm period during the 1930s and 1940s. This novel assemblage is tightly linked to the warming temperatures in the region portrayed by the increased presence of warm‐water species and a higher incidence of pelagic, planktivorous species. The results indicate a clear influence of ocean temperature on the region's juvenile fish community that points to climate‐mediated effects on the species assemblages of an important fish nursery area.     

Seasonal variation in the composition of fish assemblages in small Amazonian forest streams: evidence for predictable changes

1. The effects of seasonal inundation on the biology of fishes on floodplains of large Amazonian rivers are well studied. However, the small seasonal changes in headwater streams are generally considered to have little effect on fish assemblages. 2. In this study, we analysed seasonal changes in the species composition and abundance of fish in small Amazonian forest streams. We sampled fish with hand and seine nets in headwater streams in a 10 000 ha terra‐firme forest reserve near Manaus, Brazil. Each stream was surveyed at the end of the 2005 dry season, at the beginning of the 2006 rainy season and at the beginning of the 2006 dry season, by means of a standardized sampling effort. 3. The numbers of individuals and species caught were higher in the dry season, but rarefaction analyses indicated that greater species numbers could have been due simply to the larger number of individuals caught. 4. Between the dry and rainy season, the direction of changes in species composition in multivariate space varied among sites, especially for quantitative (abundance) data. However, the observed variation among sites was the less than expected if the directions of change were random. 5. Fish assemblages in the second dry season were more similar to those in the previous dry season than expected if changes in species composition among seasons were random. This indicates that a general seasonal pattern in fish assemblages can be detected, despite the existence of some erratic site‐specific changes. 6. Most of the species that showed large seasonal variations in density occupy temporary ponds during the rainy season, when much of the valley is inundated and pond networks form adjacent to streams. Short‐duration lateral migrations to these ponds may play an important role in the seasonal fish‐assemblage dynamics in Amazonian headwater streams. 7. Our results contrast with previous studies on small Amazonian streams, which have found little seasonal change in fish assemblages, and highlight the importance of the abundance of common species as an indicator of general fish assemblage structure in biological monitoring programmes.