Processes affecting fish populations range in scale from local to global. Fish response is also scale-dependent, with some activities varying locally while others depend on large-scale connectivity within or between watersheds. These issues are still only partly recognized, with large-scale research often affected by non-independent sampling, weak inference, poor model testing or model over-extrapolation. Available multi-scale studies can reach different conclusions about factors affecting fishes from local studies, but results vary between examples. Potential explanations are (i) effects on fishes are context-dependent; (ii)different species or life-stages are limited in different ways; (iii) multi-scale studies are too few for generalization.We advocate improved use of geostatistical tools to guide sampling or interpret the spatial extent of management problems, and we illustrate this using brown trout in Welsh streams. Our strongest recommendation is that fish ecologists recognize the importance of interactions across scales in quantifying effects on fishes so that management decisions can be better based on evidence rather than judgement or extrapolation.