Edinburgh Napier University

  Understanding the distribution of spawning resident and migratory trout in a river system is important for fishery management. In most cases this information is lacking or only partial, and mostly derived from surveys of breeding adults. Stable isotope analysis of fry provides an alternative method by which to determine the relative importance of different areas for migratory and resident fish breeding. The ratios of carbon and nitrogen stable isotopes in fish tissue vary predictably between offspring from different forms; sea trout typically show higher abundance of the heavier stable isotopes of both elements. These are passed to their offspring, allowing distinction at this phase. Here we show the results from applying this technique to two different river systems; the Gala Water tributary of the River Tweed and tributaries of the River Deveron. Although it is generally possible to distinguish migratory and resident offspring, interpretation is complicated by considerable isotopic variation within river systems. This may be linked to the influence of freshwater feeding, catchment-scale land-use patterns and isotopic variation in adults. In order to maximise the utility of stable isotope-based techniques in fisheries management, these factors need to be more fully understood.