Revisiting the restricted movement paradigm: the dispersal of Atlantic salmon fry from artificial redds

The Restricted Movement Paradigm (RMP) asserts that stream fishes are sedentary, living much of their lives within a single reach. To test the RMP, we implanted eyed Atlantic salmon eggs (Salmo salar L.) into a total of 19 artificial redds, in seven salmon-free streams, in six years, and estimated summer fry dispersal through electrofishing surveys. As expected, most fry dispersed downstream, but an average of 35% moved upstream. Surprisingly, fry moved just as far upstream as downstream (medians = 403 and 404 m, respectively). Fry were larger at lower densities and farther from redd sites, consistent with density-dependent growth, and larger upstream than downstream, possibly indicating that larger fry were better able to move upstream against the current. Dispersal distances were normally distributed around all but two of 19 redds, and half of the downstream dispersal curves were best described by unimodal distributions, consistent with a homogeneous movement strategy. Our data suggest that salmon fry were more mobile and move upstream more than previously thought, which should facilitate their stocking or reintroduction to new habitats.

Measuring and modelling Pacific herring spawning-site fidelity and dispersal using tag-recovery dispersal curves

Flostrand, L. A., Schweigert, J. F., Daniel, K. S., and Cleary, J. S. 2009. Measuring and modelling Pacific herring spawning-site fidelity and dispersal using tag-recovery dispersal curves. – ICES Journal of Marine Science, 66: 1754–1761. An approach of relating Pacific herring (Clupea pallasi) tag-recovery data to dispersal distances is presented. Observations from coded wire tag sampling (1999–2006) were used to represent adult herring interannual spawning patterns on the British Columbia coast. Six datasets were applied, differing by number of years-at-liberty (1, 2, or 3) and gear type of recapture (purse-seine or gillnet). In total, 227 tag-recovery samples, consisting of 5687 tag recoveries, were used. Distances were approximated to the shortest paths through water between the release and recapture sites. Recovery rate and distance relationships suggest that exponential models fit the data reasonably well, with average rates of change in recovery rates (slopes) varying from approximately −0.009 to −0.005. A combined slope estimate of −0.007 is similar to four of the six estimates. Using these models, the intensity of movement among five stock-assessment regions was estimated by applying distances relative to their centres. Fidelity estimates range from 53 to 90% across all models and regions, which is consistent with previous findings and premises that influence resource management. Interpretation and application of the modelling exercise are discussed in terms of previous and future work.

Modeling seed dispersal of black cherry, an invasive forest tree: how microsatellites may help?

We used empirical models and three dispersal functions (Weibull, lognormal and 2Dt) to model seed distributions derived from the black cherry (Prunus serotina Ehrh.) understorey of a pine-dominated stand. Two different approaches were used to disentangle the overlapping seed shadows: the traditional inverse modeling approach and the genetic approach that uses microsatellite markers to assign a dispersed seed to its maternal parent. The distinction was made between the seeds passively dispersed by gravity (fruits with mesocarp) and those dispersed by birds (fruits without mesocarp). Our main objectives were to compare the three dispersal functions and assess the differences between the two approaches used. The functions performed equally well, but the lognormal function often showed a better data correlation. The best dispersal curves obtained by both traditional and genetic approaches were quite similar for the seeds dispersed by gravity, and 95% of these seeds were predicted to fall 5 and 3 m away from the parent tree for the traditional and genetic approaches, respectively. Differences were more important for the seeds dispersed by birds. The traditional approach predicted a lower number of seeds near the parent plant and a higher dispersal distance. Microsatellites provided accurate information on individual dispersal events and led to a better insight into the dispersal process.