The ecological impact of many invasive species is usually unknown because of the absence of data before their introduction. We exemplify the usefulness of comparing early and late invasion stages to understand the ecological changes caused by invasive species, particularly concerning large species and large ecosystems, such as reservoirs, where experimental work is not easily carried out. We also report the first data on growth and diet for the European catfish (Silurus glanis) in the Iberian Peninsula and compare three populations in the Ebro River system, corresponding to the earliest introductions (> 30 years ago) with two recently introduced populations in reservoirs of the Ter River basin. The total length and age of the catfish varied significantly among populations and recent introductions consisted of smaller and younger catfish with significantly higher condition and size-specific growth rates compared to older (as well as native) populations. For example, 7+ old catfish were around 100 cm in the older populations and 150 cm in the more recent ones. Diet also depended on site and catfish size. Catfish measuring less than 30 cm consumed mostly invertebrates, thereafter shifting to red swamp crayfish Procambarus clarkii (old introductions) or fish (recent introductions). A number of fish species were present in stomachs but common carp (Cyprinus carpio) and birds were only present in very large fish (> 120 cm). The median length of common carp was significantly larger in reservoirs with late-stage catfish invasion. The abundance of waterbirds, particularly anatids, was significantly lower in reservoirs where catfish were present, suggesting a direct ecological impact, or else avoidance learning by the birds. Our results suggest that in the early stages of invasion, catfish display higher condition and growth rates by profiting from relatively unexploited resources (large fish and, secondarily, waterbirds), shifting to other prey (and thus reducing mean growth) in the late invasion stages
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