Journal of Fish Biology (1996) 48, 437–446 Pike and red swamp crayfish: a new case on predator–prey relationship between aliens in central Spain B. E*, G. G. N*  A. A† *Department of Animal Biology I, Faculty of Biology, University of Madrid, E-28040 Madrid and †Department of Ecology, Agricultural Research Service, Community of Madrid, El Encín, P.O. Box 127, E-28800 Alcalá de Henares, Madrid, Spain (Received 25 March 1995, Accepted 25 April 1995) Pike in the Ruidera Lakes (central Spain) chiefly ate the recently introduced crayfish Procambarus clarkii. It was the dominant prey in occurrence, number and biomass for every size class and season. Likewise, number (up to 27) and total weight of ingested crayfish were directly related with pike size. The principal prey fish were all introduced species (except the endangered Blennius fluviatilis), most native species having disappeared since pike were introduced in 1953.  1996 The Fisheries Society of the British Isles Key words: pike; Esox lucius; Procambarus clarkii; predation; feeding; Spain. INTRODUCTION The pike Esox lucius L., as well as other true predaceous fish (sensu Popova, 1967, 1978), was originally absent from the Iberian Peninsula, where it was first stocked for angling purposes in 1949 (Gutiérrez-Calderón, 1955; Pena, 1986; Elvira, 1995a). Its present distribution in Spain includes most of the large river basins (Doadrio et al., 1991; Elvira, 1995b). Biological invasions cause a great number of disruptions in original ecosystems all over the world (Drake et al., 1989). In fact, introduction of fish has usually been accompanied by decline or extinction of many native species (Brown, 1989). The pike becomes increasingly piscivorous as it grows, reaching the top of the aquatic food chain in most communities throughout its circumpolar distribution in the northern hemisphere (Raat, 1988; Maitland & Campbell, 1992). It is considered to be a serious threat to native Spanish fishes (Elvira, 1990, 1995a–c, 1996; Elvira & Barrachina, 1996). However, accurate studies on pike diet in Spain are scarce (Pena et al., 1987; Sánchez-Isarria et al., 1989; Santamaría, 1993, 1995). Sostoa & Lobón-Cerviá (1989) described its feeding pattern and Rincón et al. (1990) examined its impact on native fish assemblages in small streams. The fish community of the Natural Park of the Ruidera Lakes consists of nine native species (eight of them Iberian endemics), and six well-acclimatized, introduced species (Almodóvar & Elvira, 1994). Pike were introduced in Ruidera in 1953 (Elvira & García-Utrilla, 1991) and presently are widespread and common (Almodóvar & Elvira, 1994). Our first aim was to study the predator–prey interactions of pike with native fishes. However, native fishes are restricted now mostly to the middle and upper Tel.: +34-1-3944946; fax: +34-1-3944947. 437 0022–1112/96/030437+10 $18.00/0  1996 The Fisheries Society of the British Isles