Reproductive traits of the European catfish (Silurus glanis) during the early stages of invasion in the Lower Tagus River

Freshwater ecosystems face severe challenges from biological invasions, leading to biodiversity loss, disruption of ecosystem services, and economic impacts. Human-mediated activities, such as aquarium trade and sport angling, contribute to species introductions, with potential negative consequences for native ecosystems. The European catfish (Silurus glanis) is one of the worlds largest freshwater fish and has been intentionally introduced into diverse regions, impacting native ecosystems. However, limited research exists on its reproductive traits outside its native range. This study addresses this gap by examining the reproductive characteristics of non-native European catfish populations in the Lower Tagus River in Portugal, focusing on size at maturity, spawning period, and fecundity. The observed balanced sex ratio aligns with studies of native populations. Variations in size at first maturity (TL50) among populations highlight the influence of habitat conditions, temperature, food availability, growth rate, and geographical location on this trait. The extended spawning season (March to June) in the Tagus River is consistent with native populations, but variations may occur based on environmental conditions and water temperature. Absolute fecundity ranged from 8,961 to 335,500 oocytes, showing positive relationships with body size and emphasizing the reproductive potential of European catfish in Portugal. Egg size variations, along with asynchronous egg development, contribute to the species reproductive strategy, favoring its invasive success. Management efforts should include monitoring, regulations on introductions, removal programs, and public awareness to mitigate their impact. Future research should focus on understanding how non-native European catfish populations adapt in various regions and continue to impact ecosystems.