BioInvasions Records (2016) Volume 5 Article in press © 2016 The Author(s). Journal compilation © 2016 REABIC Open Access Rapid Communication CORRECTED PROOF The transatlantic introduction of weakfish Cynoscion regalis (Bloch & Schneider, 1801) (Sciaenidae, Pisces) into Europe Pedro Morais 1,2, * and Maria Alexandra Teodósio 1 1 CCMAR – Centre of Marine Sciences, Campus de Gambelas, University of Algarve, 8005-139 Faro, Portugal 2 CIIMAR – Interdisciplinary Centre of Marine and Environmental Research, University of Porto, Terminal de Cruzeiros do Porto de Leixões, Avenida Norton de Matos, 4450-208 Matosinhos, Portugal E-mail addresses: pmorais@ualg.pt (PM), mchichar@ualg.pt (MAT) *Corresponding author Received: 28 June 2016 / Accepted: 28 September 2016 / Published online: 7 October 2016 Handling editor: Charles W. Martin Abstract Weakfish Cynoscion regalis (Bloch and Schneider, 1801) is a sciaenid fish native to the east coast of North America and has been recently collected in three areas of the Iberian Peninsula (Europe). We aimed to i) provide the first report of the presence of weakfish in Europe, ii) hypothesize the most likely introduction vector, iii) discuss the potential for ecological overlap between weakfish and meagre Argyrosomus regius (Asso, 1801), the native Sciaenidae species, and iv) highlight the importance of citizen science in the detection of non- native species. Weakfish were captured in the Sado estuary (July 2014), Gulf of Cadiz (November 2015) and the adjacent Guadiana estuary (June 2016), and in two Galician Rías (June 2016). Anglers reported that weakfish was present in the Sado estuary for “some” years, while their presence was only noticed recently in the other two areas. We hypothesize that ballast water was the introduction vector, that weakfish established a reproducing population in the non-native range, and that it dispersed from the Sado estuary, a central region of its current distribution range. The Sado estuary might have been the introduction area for weakfish via ballast water because there is a busy transoceanic commercial port in the estuary. Moreover, they are not used in European aquaculture facilities, nor in the aquarium trade. The collected specimens were ≤50 cm, with average lengths of 30 cm, which corresponds to a median age of 2 years and to individuals capable of reproducing. As a result, the year of introduction must be before 2012. Weakfish have a similar ecological niche to meagre, but the outcome of potential interactions is uncertain. Citizen science, especially the initiative of local fishermen, was critical to detect this non-native species. Key words: non-native, fish, ballast water, citizen science, Portugal, Spain, Iberian Peninsula Introduction Reports on transoceanic marine invasions are becoming more common (Callaway et al. 2006), and they consist primarily of introductions of holoplanktonic (Berg et al. 2002) and meroplanktonic invertebrate species via ballast water (Roman 2006). In the case of vertebrates (i.e. fish), intentional introductions (e.g., for recreational fishing), escapees from aquaculture facilities (Bartley 2011 (Copp et al. 2007), and aquarium release (Brice et al. 2004) are the most common vectors of introduction rather than via ballast water (Wonham et al. 2000; Grigorovich et al. 2003; Copp et al. 2007). The presence of weakfish Cynoscion regalis (Bloch and Schneider, 1801) (Sciaenidae, Pisces) in the Iberian Peninsula (western Europe) is a recent example of a transoceanic marine fish introduction. This species is native to the east coast of North America (Froese and Pauly 2016), was detected by fishermen in several regions of the Iberian Peninsula and subsequently announced in specialized magazines (Mundo da Pesca 2016), personal blogs (OsPescas 2015), and regional online media (e.g., A Voz do Algarve 2016; Europapress 2016; Vigoe 2016). The native distribution of weakfish is from Florida’s Atlantic coast (USA) to Nova Scotia (Canada), where it is exploited in commercial and sport fisheries (Froese and Pauly 2016; NMFS 2016). In Europe, the meagre Argyrosomus regius (Asso, 1801) is the most abundant Sciaenidae species. The meagre is a valuable species for local and artisanal fisheries and sport fishing, and it is also reared in aquaculture (Mañanós et al. 2009; Duncan et al. 2013).