The ''restricted-movement paradigm'' (RMP) states that adult fish in streams are sedentary and spend most of their lives in short (20-50 m) reaches of stream. In markrecapture studies, however, many fish initially marked are often never recaptured. As well, turnover rates of individuals in the home section (where fish were originally marked) can be high when marked fish moving out are rapidly replaced by unmarked ones. Recent challenges to the RMP have been based on the inference that high turnover indicates high mobility. However, when the home section is small many individuals may leave (high turnover) but not move far away (low displacement). I present two models for the frequency distribution of displacement distances: one represents populations as homogeneous ensembles with a single mobility parameter; the second represents populations as a mixture of stationary and mobile individuals. Both readily distinguish the turnover and displacement components of movement and show that high turnover rate is compatible with low displacement. The models were then fit to dispersal curves for six species of stream salmonids in 27 populations. Empirical estimates of turnover rate were high (median: 0.53), variable among populations (range: 0.15-0.78), but unrelated to displacement distance. Median displacement was Ͻ100 m for 24 populations and was typically Ͻ50 m. The proportion of mobile individuals was low in most populations (median: 19%) and exceeded 50% in only five of the populations. Brook trout, a species central to studies critical of the RMP, appeared to be exceptionally mobile relative to other salmonids. The compatibility of high turnover rates with short displacement distances and the finding that median displacement was usually limited support the notion that restricted movement is the norm in populations of stream salmonids during nonmigratory periods. However, the finding of considerable intra-and interspecific heterogeneity in the extent of movement underscores the potential importance of the mobile component to population processes. By providing an analytical framework that yields quantitative measures of different components of movement and allows for standardized comparisons, these models can bring needed rigor to analysis and design in movement studies.

Fish assemblages of temperate lakes are structured primarily by an interaction between piscivory and a small number of environmental variables, but tropical floodplain assemblages have often been viewed as unpredictably structured. We tested the predictability of fish assemblage structure in floodplain lakes of the Orinoco River, Venezuela, in relation to 22 variables describing environmental variation at the microhabitat, habitat, and supra‐lake levels. Fish species abundances were estimated through electrofishing surveys of 20 lakes in three regions for the early and late dry seasons of each of two consecutive years. Canonical correspondence analyses indicated that assemblage structure was predictably related to only four descriptors of lakes: transparency, conductance, depth, and area. Discriminant function analyses revealed that transparency (“clear”: Secchi transparency >20 cm; or “turbid”: Secchi transparency ≤20 cm) was tightly associated with the numerical density of six major taxa (82% classification accuracy) and the numerical density of piscivorous species (89% accuracy). Depth and area probably derived their significance from causal relationships to transparency and availability of cover, whereas the influence of conductance arose incidentally through an association with biogeographical zonation. Mantel tests indicated that similarity in structure of assemblages was not strongly related to the distance between lakes. Transparency was a remarkably reliable predictor of species composition. Fish with sensory adaptations to low light were dominant in turbid lakes, whereas visually oriented fishes predominated in clear lakes; seasonal change involved decline in the proportion of visually oriented fishes concomitant with a decline in transparency. The effect of transparency on assemblage structure was probably mediated by the relationship of transparency to visibility of prey. The structuring of Orinoco fish assemblages by piscivory, under the influence of transparency as controlled by depth and area, contrasts with previous views emphasizing random assemblage variation in neotropical floodplain lakes and extends the applicability of a conceptual model originally developed for temperate lakes.

Fish assemblages of temperate lakes are structured primarily by an interaction between piscivory and a small number of environmental variables, but tropical floodplain assemblages have often been viewed as unpredictably structured. We tested the predictability of fish assemblage structure in floodplain lakes of the Orinoco River, Venezuela, in relation to 22 variables describing environmental variation at the microhabitat, habitat, and supra-lake levels. Fish species abundances were estimated through electrofishing surveys of 20 lakes in three regions for the early and late dry seasons of each of two consecutive years. Canonical correspondence analyses indicated that assemblage structure was predictably related to only four descriptors of lakes: transparency, conductance, depth, and area. Discriminant function analyses revealed that transparency (''clear'': Secchi transparency Ͼ20 cm; or ''turbid'': Secchi transparency Յ20 cm) was tightly associated with the numerical density of six major taxa (82% classification accuracy) and the numerical density of piscivorous species (89% accuracy). Depth and area probably derived their significance from causal relationships to transparency and availability of cover, whereas the influence of conductance arose incidentally through an association with biogeographical zonation. Mantel tests indicated that similarity in structure of assemblages was not strongly related to the distance between lakes. Transparency was a remarkably reliable predictor of species composition. Fish with sensory adaptations to low light were dominant in turbid lakes, whereas visually oriented fishes predominated in clear lakes; seasonal change involved decline in the proportion of visually oriented fishes concomitant with a decline in transparency. The effect of transparency on assemblage structure was probably mediated by the relationship of transparency to visibility of prey. The structuring of Orinoco fish assemblages by piscivory, under the influence of transparency as controlled by depth and area, contrasts with previous views emphasizing random assemblage variation in neotropical floodplain lakes and extends the applicability of a conceptual model originally developed for temperate lakes.

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