Resource partitioning among the fishes of rainforest streams in Sri Lanka

The fish assemblage typical of small rainforest streams of Sri Lanka was investigated to see if the fishes were characterized by a high degree of specialization, expected of equilibrium communities, or if they were relatively unspecialized, expected of more stochastic (non-equilibrium) communities. Morphological features, habitats, microhabitats and diets of the 20 most abundant fish species were measured. The fishes included 11 Cyprinidae (including seven Barbus spp. and two Rasbara spp.), two Gobiidae, two Channidae, and one species each in the Cobitidae, Cyprinodontidae, Belontidae, Belonidae, and Mastacembelidae. Microhabitats were determined by measuring (for 6760 fish) water column depth, distance of fish from bottom, mean water column velocity, water velocity at fish, and substrate. Morphologically, the species showed a high degree of specialization, especially in structures related to feeding. Although several species were habitat generalists, most species occurred in distinct habitats. Within habitats, microhabitat overlap among co-occuring species was low, particularly in relation to position in the water column. Principal component analysis of the microhabitat measurements produced three new variables. Most species not clearly segregated by the overlap analysis showed segregation on the new variables. Fishes not segregated by habitat or microhabitat tended to show low dietary overlaps. Specialization in feeding habits was more pronounced among the Sri Lankan fishes than noted for fishes in small rainforest streams elsewhere, in part because of the greater reliance of the Sri Lanka fishes on autochthonous foods. Overall, the fish assemblage had the characteristics expected of an equilibrium (deterministic) assemblage.     

Morphology and habitat use by fishes of the Rio das Velhas basin in southeastern Brazil

We analyzed the morphology and habitat use of 16 small-sized fish species that are abundant in the Rio das Velhas basin using 17 morphological attributes. Habitat use was characterized in terms of the species mean density considering three hydraulic factors: substrate, water depth and mean water velocity. The distribution of species within the morphological space demonstrated congruence between the morphological attributes of a species, and their occupation of vertical microhabitats. Based on this data, the species could be classified as benthic A, benthic B or nektonic zone-dwelling species. Benthic A and B species differed mainly in relation to their body shapes, which were fusiform or depressed, respectively. Generally, habitat selection by each species was related to certain hydraulic factors, water velocity in particular. Siluriformes generally had morphological or behavioral adaptations to live in fast flowing habitats, and they were found more frequently than Characiformes in microhabitats with greater water velocities. Benthic species were the most selective with respect to substrate type. Between congener species, the habitat use of Serrapinnus heterodon and Serrapinnus piaba greatly overlapped, but these species were separated in morphological space. In contrast, Characidium species were closely related in morphological space but had great habitat segregation. Fish requirements and habitat use are poorly understood but are important to species management and habitat restoration.     

Is sexual segregation in the guppy, <Emphasis Type="Italic">Poecilia reticulata</Emphasis>, consistentwith the predation risk hypothesis?

We examined the importance of sex differences in predation risk in generating sexual segregation in the guppy, Poecilia reticulata. We hypothesised that sex differences in predation risk will result in habitat segregation and ultimately social segregation of the sexes, with the more vulnerable sex (males in this case) using safer habitats. In accordance with the predation risk hypothesis we observed sexual segregation in a population associated with high but not low predation risk. Under high predation risk we observed a larger proportion of males in shallow marginal habitats resulting in habitat segregation and ultimately social segregation of the sexes. Furthermore, habitat segregation by sex was associated with habitat segregation by body length with shoals in deeper water having a larger mean body length. Shoaling fish species have been key models in investigating group living, and further research directed towards understanding sexual segregation in other fish species would be valuable.     

Relationships among habitat,ecomorphology and diets of cichlids in the Bladen River,Belize

The Neotropical Cichlidae is among the most species-rich and ecologically diverse groups of freshwater fishes. This study investigated interspecific morphological and ecological relationships within an assemblage of six cichlids in the Upper Bladen River, Belize. This portion of the river drains a nearly pristine watershed within a nature reserve, and thus should provide a natural ecological context for interpretation of ecological patterns. Species distributions within morphological, habitat and dietary space yielded patterns consistent with a hypothesis of niche partitioning. Statistical analyses of the species assemblage revealed relationships between two principal morphological gradients from multivariate analysis with several diet and habitat variables, and these patterns were consistent with prior functional morphological interpretations. Given that this local cichlid assemblage contains no congeneric species, it is apparent that morphological divergence resulting in niche segregation reflects selective establishment of species from a more species-rich regional species pool rather than in situ adaptive evolution.     

Relationships between fishes and habitat in rainforest streams in Sabah, Malaysia

Distinct fish assemblages were found at the mesohabitat scale in 14 streams in eastern Sabah, Malaysia. Sites were designated a priori as pool, run or riffle on the basis of physical habitat structure and properties. Principal components analysis of physical habitat data confirmed the validity of the a priori designation with a major axis of three correlated variables: water velocity, depth and substratum type. Canonical discriminant analysis on fish abundance and biomass data confirmed the existence of a specialized assemblage of fishes from riffle areas of all streams. Overall, pool and run assemblages were highly variable, dependent on stream size, but also variable between streams of the same size. Multiple regression of species richness, diversity, abundance and biomass data on principal components revealed significant but low correlations with measured habitat variables. Riffle habitats showed lower species richness and diversity but high abundance. The fish assemblage in riffles was dominated by balitorid species, specialized for fast-water conditions. Pool assemblages had the highest species diversity and were dominated by cyprinid species of a number of morphological and ecological guilds. Run assemblages were intermediate in assemblage characteristics between riffle and pool assemblages. Between-stream variation in assemblage composition was less than within-stream variation. Of 38 species collected, seven could be designated as riffle specialists, 18 as pool specialists and 13 as ubiquitous, although most of the latter showed size-specific habitat use with larger size classes found in slower, deeper water.     

Static habitat partitioning and dynamic selection by sympatric young Atlantic salmon and brown trout in south-west England streams

Direct underwater observation of micro‐habitat use by 1838 young Atlantic salmon Salmo salar [mean L_T 7·9 ± 3.1(s.d.) cm, range 3·19] and 1227 brown trout Salmo trutta (L_T 10·9 ± 5·0 cm, range 3·56) showed both species were selective in habitat use, with differences between species and fish size. Atlantic salmon and brown trout selected relatively narrow ranges for the two micro‐habitat variables snout water velocity and height above bottom, but with differences between size‐classes. The smaller fishes <7 cm held positions in slower water closer to the bottom. On a larger scale, the Atlantic salmon more often used shallower stream areas, compared with brown trout. The larger parr preferred the deeper stream areas. Atlantic salmon used higher and slightly more variable mean water velocities than brown trout. Substrata used by the two species were similar. Finer substrata, although variable, were selected at the snout position, and differences were pronounced between size‐classes. On a meso‐habitat scale, brown trout were more frequently observed in slow pool‐glide habitats, while young Atlantic salmon favoured the faster high‐gradient meso‐habitats. Small juveniles <7 cm of both species were observed most frequently in riffle‐chute habitats. Atlantic salmon and brown trout segregated with respect to use of habitat, but considerable niche overlap between species indicated competitive interactions. In particular, for small fishes <7 cm of the two species, there was almost complete niche overlap for use of water depth, while they segregated with respect to water velocity. Habitat suitability indices developed for both species for mean water velocity and water depth, tended to have their optimum at lower values compared with previous studies in larger streams, with Atlantic salmon parr in the small streams occupying the same habitat as favoured by brown trout in larger streams. The data indicate both species may be flexible in their habitat selection depending on habitat availability. Species‐specific habitat overlap between streams may be complete. However, between‐species habitat partitioning remains similar.     

Ecomorphology of Anolis lizards of the Choco' region in Colombia and comparisons with Greater Antillean ecomorphs

We analysed the ecomorphological relationships in four species of Anolis lizard that occur in the Choco' region in Colombia. The region is one of the most diverse of the Neotropical lowlands. The species were assigned to traditionally recognized Greater Antillean ecomorph categories based on habitat use data. Principal component analyses were carried out to examine correlations between the morphological traits, body size, and habitat use. We found that species are separated in morphological space principally by body size and lamella number. Upon removal of the effect of body size, correlations between morphology and habitat use became apparent. However, when compared with Greater Antillean ecomorphs, we found little evidence of morphological convergence in species occupying similar habitats. The species of the Choco' region are, however, clearly separated in the multidimensional morphological space from the Antillean taxa, and appear to form a separate cluster differentiating principally in body size and the number of lamellae. Mainland species clearly constitute an ecomorphological radiation but apparently this is independent of that of the West Indian fauna. More studies are needed to understand the causes for the independence of evolutionary trajectories on the mainland and the Greater Antilles, and to obtain a better understanding of the ecological and evolutionary processes underlying the radiation of these faunas.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 92 , 29–39.     

Habitat selection as a mechanism of resource partitioning in two cryptic bat species Pipistrellus pipistrellus and Pipistrellus pygmaeus

Ecomorphological studies of bat communities often reveal the spatial and temporal coexistence of morphologically similar species, leading to suggestions that these communities are structured by non-deterministic processes. However, the diversification of echolocation call structure in bats allows for considerable morphological similarity while still permitting niche differentiation based on specialisation for prey type and habitat structure. The recent separation of a common Palaearctic bat, the pipistrelle, into Pipistrellus pipistrellus and P. pygmaeus, which are sympatrically distributed throughout their range, raises the question as to whether these two morphologically similar species partition resources in time and space.
To test the hypothesis that the coexistence of these cryptic species is facilitated by differential habitat use, 14 P . pipistrellus , and 12 P. pygmaeus were radio-tracked from adjacent maternity roosts, in northeast Scotland, from May to September 2002/2003. The two species showed distinct habitat partitioning with P. pygmaeus foraging predominantly in riparian woodland and over water, and P. pipistrellus foraging along woodland edges and short isolated tree lines. Inter-specific overlap in habitat use was low and consequently foraging ranges were segregated spatially.
The degree of habitat partitioning revealed in these species, which show considerable overlap in echolocation call parameters and functional morphology, suggests that morphological features, whilst useful in separating chiropteran species into coarse-grained foraging guilds, may not predict fine-grained ecological segregation.     

The morphological plastic response to water current velocity varies with age and sexual state in juvenile Atlantic salmon, Salmo salar

1. Salmonids, like many other fish species, exhibit morphological plasticity to variations in water current velocity. However, little is known about how this response varies with age and alternative sexual tactics that usually coexist in the same area. We therefore sampled immature 1- and 2-year-old and sexually mature Salmo salar parr to determine how the morphological response to slow and rapid water currents varies across these groups.
2. Both 1- and 2-year-old immature parr in rapid habitats can be distinguished from individuals in slow habitats using a combination of fin measurements. In contrast, body shape measurements were useful only to distinguish 2-year-old individuals in the different habitat types. We also showed that mature parr are notably robust, irrespective of habitat type. For these individuals, only their body length differed between slow and rapid water currents, being bigger in slow water currents.
3. Our results imply that fins are the first structures to respond to water current velocity, followed by changes in body shape as individuals grow bigger. The robust phenotype observed for mature parr is likely to pose extra limitations on movement due to an increase in drag forces, thus contributing to their smaller size in rapid water currents.     

Habitat specialization in tropical continental shelf demersal fish assemblages

The implications of shallow water impacts such as fishing and climate change on fish assemblages are generally considered in isolation from the distribution and abundance of these fish assemblages in adjacent deeper waters. We investigate the abundance and length of demersal fish assemblages across a section of tropical continental shelf at Ningaloo Reef, Western Australia, to identify fish and fish habitat relationships across steep gradients in depth and in different benthic habitat types. The assemblage composition of demersal fish were assessed from baited remote underwater stereo-video samples (n = 304) collected from 16 depth and habitat combinations. Samples were collected across a depth range poorly represented in the literature from the fringing reef lagoon (1-10 m depth), down the fore reef slope to the reef base (10-30 m depth) then across the adjacent continental shelf (30-110 m depth). Multivariate analyses showed that there were distinctive fish assemblages and different sized fish were associated with each habitat/depth category. Species richness, MaxN and diversity declined with depth, while average length and trophic level increased. The assemblage structure, diversity, size and trophic structure of demersal fishes changes from shallow inshore habitats to deeper water habitats. More habitat specialists (unique species per habitat/depth category) were associated with the reef slope and reef base than other habitats, but offshore sponge-dominated habitats and inshore coral-dominated reef also supported unique species. This suggests that marine protected areas in shallow coral-dominated reef habitats may not adequately protect those species whose depth distribution extends beyond shallow habitats, or other significant elements of demersal fish biodiversity. The ontogenetic habitat partitioning which is characteristic of many species, suggests that to maintain entire species life histories it is necessary to protect corridors of connected habitats through which fish can migrate.     

Sexual segregation in ungulates: a comparative test of three hypotheses

In most social ungulate species, males are larger than females and the sexes live in separate groups outside the breeding season. It is important for our understanding of the evolution of sociality to find out why sexual segregation is so widespread not only in ungulates but also in other mammals. Sexual body size dimorphism was proposed as a central factor in the evolution of sexual segregation in ungulates. We tested three hypotheses put forward to explain sexual segregation: the predation-risk, the forage-selection, and the activity budget hypothesis. We included in our analyses ungulate species ranging from non-dimorphic to extremely dimorphic in body size. We observed oryx, zebra, bighorn sheep and ibex in the field and relied on literature data for 31 additional species. The predation-risk hypothesis predicts that females will use relatively predator-safe habitats, while males are predicted to use habitats with higher predation risk but better food quality. Out of 24 studies on different species of ungulates, females and their offspring chose poorer quality but safer habitat in only eight cases. The forage-selection hypothesis predicts that females would select habitat based on food quality, while males should prefer high forage biomass. In fact, females selected higher quality food in only six out of 18 studies where males and females segregated, in eight studies there was no difference in forage quality and in four studies males were in better quality habitat. The activity budget hypothesis predicts that with increasing dimorphism in body size males and females will increasingly differ in the time spent in different activities. Differences in activity budgets would make it difficult for males and females to stay in mixed-sex groups due to increased costs of synchrony to maintain group cohesion. The predictions of the activity budget hypothesis were confirmed in most cases (22 out of 23 studies). The heavier males were compared to females, the more time females spent foraging compared to males. The bigger the dimorphism in body mass, the more males spent time walking compared to females. Lactating females spent more time foraging than did non-lactating females or males. Whether species were mainly bulk or intermediate feeders did not affect sexual differences in time spent foraging. We conclude that sexual differences in activity budgets are most likely driving sexual segregation and that sexual differences in predation risk or forage selection are additive factors.     

枸杞岛潮下带沙地生境鱼类群落结构和季节变化

为了解岛礁水域沙地生境的鱼类群落结构特征,评估该生境对鱼类资源养护的潜在作用,于2009年采用多网目组合刺网对枸杞岛潮下带沙质区域进行了逐月采样,同步设置岩礁为对照生境.应用α和β多样性指数结合相对重要性指数、相对渔获率、ABC曲线和聚类排序方法对两种生境中的鱼类组成、多样性变化和群落格局与变化进行了全面探讨.全年在沙地和岩礁生境共采集鱼类63种,隶属11目38科56属,2种生境各自出现的鱼类皆为46种.受暖水种频繁出现在沙地生境的影响,潮下带沙质区域鱼类区系比岩礁生境略显丰富,且春夏季的渔获量普遍高于岩礁生境;由于种类组成均匀度较低,沙地生境各季节的α多样性普遍较低,夏季显著低于岩礁生境.日本须鳎是沙地生境的指示种,为早春、夏末和秋冬季沙地底层优势鱼种.5-7月鳀、多数月份鲻和10月份鳗鲇等种类对沙地生境的阶段性利用,使其形成了区别于以趋礁性鱼类为优势类群的岩礁生境的群落格局和季节动态.沙地是多种鱼类幼鱼阶段的庇护所和饵料场,是鲆鲽类的良好栖息地.沙地生境在维持鱼类区系和养护鱼类资源方面具有重要作用.     

Spatial segregation between the water opossum Chironectes minimus and the water rat Nectomys squamipes: Just competition avoidance or a conservation problem as well?

Riparian zones, as spatially constrained environments, can favor intense competition among similar species and thus the occurrence of spatial segregation. We investigated the spatial pattern of water opossum Chironectes minimus and water rat Nectomys squamipes captures in Atlantic Forest rivers in southeast Brazil. Our goals were to test if the water opossum and water rat are spatially segregated, to describe the habitat selection by these two species and to verify if habitat degradation favors the water rat, at the expense of water opossum. The abundances of water opossum and water rat were negatively correlated. Water opossum abundance was higher in wide rivers with high tree density in the riverbank, while water rat abundance was higher in narrow streams within non-forested areas. The ratio between the water opossum and water rat relative abundances was positively related to the degree of conservation of the riverine habitat. The segregation showed by the water opossum and the water rat was produced by their morphological and ecological differences, rather than by interespecific competition. Based on ours results we predict that deforestation, especially the removal of riparian vegetation, will result in a reduction of the suitable habitat for the water opossum while the water rat will expand its range.     

The future distribution of river fish: The complex interplay of climate and land use changes,species dispersal and movement barriers

The future distribution of river fishes will be jointly affected by climate and land use changes forcing species to move in space. However, little is known whether fish species will be able to keep pace with predicted climate and land use‐driven habitat shifts, in particular in fragmented river networks. In this study, we coupled species distribution models (stepwise boosted regression trees) of 17 fish species with species‐specific models of their dispersal (fish dispersal model FIDIMO) in the European River Elbe catchment. We quantified (i) the extent and direction (up‐ vs. downstream) of predicted habitat shifts under coupled “moderate” and “severe” climate and land use change scenarios for 2050, and (ii) the dispersal abilities of fishes to track predicted habitat shifts while explicitly considering movement barriers (e.g., weirs, dams). Our results revealed median net losses of suitable habitats of 24 and 94 river kilometers per species for the moderate and severe future scenarios, respectively. Predicted habitat gains and losses and the direction of habitat shifts were highly variable among species. Habitat gains were negatively related to fish body size, i.e., suitable habitats were projected to expand for smaller‐bodied fishes and to contract for larger‐bodied fishes. Moreover, habitats of lowland fish species were predicted to shift downstream, whereas those of headwater species showed upstream shifts. The dispersal model indicated that suitable habitats are likely to shift faster than species might disperse. In particular, smaller‐bodied fish (<200 mm) seem most vulnerable and least able to track future environmental change as their habitat shifted most and they are typically weaker dispersers. Furthermore, fishes and particularly larger‐bodied species might substantially be restricted by movement barriers to respond to predicted climate and land use changes, while smaller‐bodied species are rather restricted by their specific dispersal ability.     

The importance of colony structure versus shoot morphology for the water balance of 22 subarctic bryophyte species

Questions: What are the water economy strategies of the dominant subarctic bryophytes in terms of colony and shoot traits? Can colony water retention capacity be predicted from morphological traits of both colonies and separate shoots? Are suites of water retention traits consistently related to bryophyte habitat and phylogenetic position? Location: Abisko Research Station, North Sweden. Methods: We screened 22 abundant subarctic bryophyte species from diverse habitats for water economy traits of shoots and colonies, including desiccation rates, water content at field capacity, volume and density (mg cm^?3) of water‐saturated and oven‐dried patches, evaporation rate (g·m^?2·s^?1) and cell wall thickness. The relationships between these traits and shoot and colony desiccation rates were analysed with Spearman rank correlations. Subsequent multivariate (cluster followed by PCA) analyses were based on turf density, turf and shoot desiccation rate, cell wall thickness and amount of external and internal water. Results: Individual shoot properties, i.e. leaf cell wall properties, water retention capacity and desiccation rate, did not correspond with colony water retention capacity. Colony desiccation rate depended on density of water‐saturated colonies, and was marginally significantly negatively correlated with species individual shoot desiccation rate but not related to any other shoot or colony trait. Multivariate analyses based on traits assumed to determine colony desiccation rate revealed six distinct species groups reflecting habitat choice and phylogenetic relationships. Conclusions: General relationships between shoot and colony traits as determinants of water economy will help to predict and upscale changes in hydrological function of bryophyte‐dominated peatlands undergoing climate‐induced shifts in species abundance, and feedbacks of such species shifts on permafrost insulation and carbon sequestration functions.     

Habitat segregation among mimetic ithomiine butterflies (Nymphalidae)

The extensive wing pattern diversity observed among sympatric unpalatable mimetic butterflies is difficult to explain. Diversity is a paradox because selection by predators is expected to drive local species to use the same aposematic patterns. Habitat segregation among mimicry complexes has been suggested as a hypothesis to explain how diversity could be maintained. However, very few studies have tested this hypothesis. To test whether mimicry complexes are associated with particular habitats, I sampled a diverse assemblage of ithomiine butterflies from eastern Ecuador comprising nine discrete mimicry complexes. Butterflies were sampled in four habitats varying along a gradient of succession. A total of 43 species and 902 individuals were sampled. Ithomiine species richness and abundance were lowest in open habitats, and habitat preferences were documented for many species. Mimicry complexes exhibited significant habitat differences supporting the role of habitat segregation in maintaining mimetic diversity. However, there was obvious overlap among mimicry complexes, particularly involving the two numerically dominant patterns at the site. The pattern of segregation appears to be driven by common species, with relatively little evidence that the distribution of rarer species matches that of the more abundant species. Thus, habitat segregation is likely to play a role in the evolution of mimetic diversity as a result of segregated abundant model species, but the effect is probably weak and other factors are also important.     

Habitat Restoration as a Means of Controlling Non-Native Fish in a Mojave Desert Oasis

Non‐native fish generally cause native fish decline, and once non‐natives are established, control or elimination is usually problematic. Because non‐native fish colonization has been greatest in anthropogenically altered habitats, restoring habitat similar to predisturbance conditions may offer a viable means of non‐native fish control. In this investigation we identified habitats favoring native over non‐native fish in a Mojave Desert oasis (Ash Meadows) and used this information to restore one of its major warm water spring systems (Kings Pool Spring). Prior to restoration, native fishes predominated in warm water (25–32°C) stream and spring‐pool habitat, whereas non‐natives predominated in cool water (≤23°C) spring‐pool and marsh/slack water habitat. Native Amargosa pupfish (Cyprinodon nevadensis) and Ash Meadows speckled dace (Rhinichthys osculus nevadensis) inhabited significantly faster mean water column velocities (MWCV) and greater total depth (TD) than non‐native Sailfin molly (Poecilia latipinna) and Mosquitofish (Gambusia affinis) in warm water stream habitat, and Ash Meadows speckled dace inhabited significantly faster water than non‐natives in cool water stream habitat. Modification of the outflow of Kings Pool Spring from marsh to warm water stream, with MWCV, TD, and temperature favoring native fish, changed the fish composition from predominantly non‐native Sailfin molly and Mosquitofish to predominantly Ash Meadows pupfish. This result supports the hypothesis that restoring spring systems to a semblance of predisturbance conditions would promote recolonization of native fishes and deter non‐native fish invasion and proliferation.     

A Large-scale Comparative Analysis of Riffle and Pool Fish Communities in an Upland Stream System

Fishes were sampled in riffle and pool habitats at 74 upland localities in the Little River system, southeastern Oklahoma and southwestern Arkansas, U.S.A. I asked how these two habitat-defined communities differed with regard to species abundance and incidence patterns, and how these differences varied along othree environmental gradients: elevation, stream gradient, and stream size. Riffle and pool communities showed distinct and significant differences when ordinated in multivariate space defined by species abundance patterns. Sites with similar pool communities did not have similar riffle communities, and riffle and pool communities responded to environmental gradients in different ways. Elevation was the best predictor of pool community structure, whereas stream size was the best predictor of riffle communities. Overall, riffle habitats had fewer species than pool habitats and formed significant subsets of pool communities at 12 of 74 sites. I predicted that at small stream localities where riffles were unstable, riffle species would form subsets of the pool species communities, and both community types should show high similarities. The presence of faunal subsets was not associated with stream size, but faunal similarities were significantly higher at small stream localities. At the species level, 14 species were significantly associated with pool habitats, while only two were associated with riffle habitats. Riffle and pool communities, although linked by a continuous habitat gradient at the local scale, responded differently to large-scale environmental gradients. Local differences between these communities were predictable based on stream size.     

Fish Assemblages in Different Shallow Water Habitats of the Venice Lagoon

The small-sized fish assemblages of the Venice Lagoon were investigated and compared among five shallow subtidal habitats (seagrass beds, sparsely vegetated habitats, unvegetated sand bottoms, mudflats and saltmarsh creeks) in the Northern lagoon basin. Sampling was carried out seasonally (Spring, Summer and Autumn of 2002) in 4–7 stations for each habitat type, by means of a fine-mesh, small beach seine. Two-way analysis of variance was applied to assess the differences in species richness, fish diversity, density and standing stock amongst habitats, whereas fish assemblage composition was investigated by using multivariate analyses (MDS, ANOSIM, SIMPER). The analyses indicated that seagrass beds and saltmarsh creeks are relevant shallow habitats in structuring the small-sized fish assemblages of the Venice Lagoon, supporting specialized and recognizable fish assemblages. Those in seagrass beds, in particular, were characterized by higher species richness and standing stock with respect to all the others. The structuring role of these habitats was discussed in terms of both habitat complexity and degree of confinement. In contrast, sandy bottoms, mudflats and sparsely vegetated habitats were identified as “transition” habitats, with highly variable fish assemblages, influenced by the contribution of the adjacent habitats, and acting probably as both ‘buffer zones’ between the other habitats and migration routes for many fish species in the lagoon.     

Seasonal and spatial microhabitat selection and segregation in young Atlantic salmon, Salmo salar L., and brown trout, Salmo trutta L., in a Norwegian river

Seasonal microhabitat selection by sympatric young Atlantic salmon and brown trout was studied by diving. Both species, especially Atlantic salmon, showed seasonal variation with respect to surface and mean water velocities and depth. This variation is partly attributed to varying water flows and water temperatures. In winter the fish sought shelter in the substratum. A spatial variation in habitat use along the river due to different habitat availabilities was observed. Both species occupied habitats within the ranges of the microhabitat variables, rather than selecting narrow optima. It is hypothesized that the genetic basis allows a certain range to the behavioural response. Microhabitat segregation between the two species was pronounced, with brown trout inhabiting the more slow-flowing and partly more shallow stream areas. Atlantic salmon tolerated a wider range of water velocities and depths. Habitat suitability curves were produced from both species. It is suggested that habitat suitability curves that are based on observations of fish occupancy of habitat at median or base flow may not be suitable in habitat simulation models, where available habitat is projected at substantially greater water flows.