Young-of-the-year fish assemblages as an alternative to adult fish monitoring for ecological quality evaluation of running waters

The EU's Water Framework Directive (WFD) in particular makes good use of riverine biota-including fish-as ecological indicators of river integrity. However, it is also important to examine when our indicators fail to serve the purpose for which they were originally intended: to consistently and reliably describe the quality of the environment. We argue that in the Czech Republic (and probably several other European countries), the use of adult fish as ecological indicators often fails to fulfill these purposes for several reasons, especially when fish are stocked in natural waters, for practical considerations during field sampling, and problems with public relations. The ecological quality of running waters sensu WFD is usually assessed on the basis of the adult fish assemblage. The aim of this study is to describe disadvantages of monitoring river integrity using adult fish in Czech Republic, which has regular and widespread stocking of more than one-third of fish species present in running waters and to suggest the alternative strategy of young-of-the-year (YOY) fish monitoring for assessing river ecological quality with the ability to provide a sensitive response to water quality and habitat structure regardless of the effect of stocking or river size. We conclude that even though YOY sampling is often a more logical and practical approach to water quality monitoring under specific conditions, no method is perfect and caution should be taken. For example, YOY can have high annual variability in density and higher sensitivity to summer flood events than adults. The suitable time for sampling YOY fishes in late summer is much shorter than for the adult fish community, and therefore must be considered when designing a successful monitoring strategy. Monitoring of YOY fishes does not provide direct information about age structure of fish populations, which is required by WFD. YOY sampling is a useful-albeit imperfect-method for providing a sensitive response to ecological status of rivers, especially under specific stocking conditions.