Restricted movement in stream fish: The paradigm is incomplete, not lost

The 'restricted-movement paradigm' (RMP) states that adult fish in streams are sedentary and spend most of their lives in short (20û50 m) reaches of stream. In mark-recapture 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 ofstream salmonids in 27 populations. Empirical estimates of turnover rate were high (median:0.53), variable among populations (range: 0.15û0.78), but unrelated to displacement distance. Median displacement was <100 m for 24 populations and was typically <50 m. The proportion ofmobile individuals was low in most populations (median: 19%) and exceeded 50% in only five ofthe populations. Brook trout, a species central to studies critical of the RMP, appeared to beexceptionally mobile relative to other salmonids. The compatibility of high turnover rates withshort displacement distances and the finding that median displacement was usually limitedsupport the notion that restricted movement is the norm in populations of stream salmonidsduring nonmigratory periods. However, the finding of considerable intra- and interspecificheterogeneity in the extent of movement underscores the potential importance of the mobilecomponent to population processes. By providing an analytical framework that yields quantitativemeasures of different components of movement and allows for standardized comparisons, thesemodels can bring needed rigor to analysis and design in movement studies.[0001:RMISFT]2.0.CO;2
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