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In regulated rivers, canalization and reduced water discharge may lead to loss or impairment of salmonid spawning areas, and thereby negative effects on stock recruitment. We discuss the possibility of mitigating such effects through establishing new spawning areas by addition of gravel. We monitored the occurrence of nests in seven gravel deposition sites in five different Norwegian rivers. The total area covered by each of the depositions spanned from 25 m2to 300 m2. The areas were examined for nests during a 2–5 year period following the addition of gravel, that is, a total of 27 spawning periods. The fish were found to spawn at all localities and years with the exception of two occasions when gravel was lost during floods. Spawning success measured as egg survival was generally high (>80%). These results show that the additions met the criteria for successful spawning. Nests of Atlantic salmon were found to be aggregated in the area with the highest water current. However, successful nests were also found in areas with added gravel where the water current was much reduced. This unusual selection of spawning site most likely reflects the lack of other, more favourable spawning areas. The results therefore suggest that gravel additions can be successful even if the only available areas are suboptimal with respect to water flow and water depth. Three of the seven localities were unsuitable as the gravel was partly or totally displaced downstream during floods. At two of these localities, nests were probably lost because the floods occurred after the spawning season. This shows that large floods can be a major drawback. Monitoring of the spawning success and displacement of gravel is therefore essential to evaluate the method. Subsequently, stable areas, favoured by the spawners, can be expanded by supplementary addition of gravel. In sum, the results show that the careful addition of gravel areas can be used to provide suitable spawning locations for salmonids in regulated rivers. Copyright © 2008 John Wiley & Sons, Ltd.

The global increase in the production of domestic farmed fish in open net pens has created concerns about the resilience of wild populations owing to shifts in host–parasite systems in coastal ecosystems. However, little is known about the effects of increased parasite abundance on life-history traits in wild fish populations. Here, we report the results of two separate studies in which 379 779 hatchery-reared Atlantic salmon smolts were treated (or not) against salmon lice, marked and released. Adults were later recaptured, and we specifically tested whether the age distribution of the returning spawners was affected by the treatment. The estimates of parasite-induced mortality were 31.9% and 0.6% in the River Vosso and River Dale stock experiments, respectively. Age of returning salmon was on average higher in treated versus untreated fish. The percentages of fish returning after one winter at sea were 37.5% and 29.9% for the treated and untreated groups, respectively. We conclude that salmon lice increase the age of returning salmon, either by affecting their age at maturity or by disproportionately increasing mortality in fish that mature early.