Journal of Fish Biology (2001) 59, 1218–1238 doi:10.1006/jfbi.2001.1734, available online at http://www.idealibrary.com on Dynamics of the shallow-water fish assemblage of the Patos Lagoon estuary (Brazil) during cold and warm ENSO episodes A. M. G*‡, J. P. V*  K. O. W† *Fundac ¸a ˜o Universidade Federal de Rio Grande (FURG), Pos-graduac ¸a ˜o em Oceanografia Biolo ´gica, Departamento de Oceanografia, Laborato ´rio de Ictiologia, C.P. 474—Rio Grande, RS, Brasil and †Department of Wildlife and Fisheries Sciences, Texas A&M University, College Station, TX 77843-2258, U.S.A. (Received 24 October 2000, Accepted 26 July 2001) In southern Brazil, cold (La Nin ˜a) and warm (El Nin ˜o) episodes of the El Nin ˜o Southern Oscillation (ENSO) phenomenon cause drought and high rainfall, respectively. The low precipitation and freshwater outflow associated with La Nin ˜a during 1995–1996 were associated with an increase in the abundance of marine species in the Patos Lagoon estuary. During the 1997–1998 El Nin ˜o, high precipitation and river discharge were associated with low abundance of marine species in the estuary. ANOVA results showed a higher abundance during La Nin ˜a than El Nin ˜o for estuarine resident (RES) and estuarine dependent (DEP) fishes. During La Nin ˜a catch per unit of effort (CPUE) of RES increased from the marine to estuarine area, but during El Nin ˜o CPUE increased at the marine area and diminished during summer and autumn in some estuarine sites. DEP fishes had an opposite abundance pattern. During La Nin ˜a, these fishes were abundant at the coastal marine area and along some estuarine sites, but during El Nin ˜o, CPUE remained almost the same at the marine area but dropped along some estuarine sites. These different abundance patterns for dominant fish groups yielded a positive inter- action between stations and climatic events. With higher river discharge and the consequent decline of dominant euryhaline fishes, such as Mugil platanus and Atherinella brasiliensis, freshwater species increased in abundance and richness in the shallow waters of the estuary. The ENSO phenomenon influences precipitation and estuarine salinity in southern Brazil and thereby seems to have a strong influence on recruitment, immigration, and emigration dynamics of fish species living within and adjacent to estuarine habitats.  2001 The Fisheries Society of the British Isles Key words: La Nin ˜a; El Nin ˜o; ENSO; fish assemblage; estuary; Patos Lagoon. INTRODUCTION The El Nin ˜o Southern Oscillation (ENSO) is associated with a warming of surface waters that occurs at irregular intervals in the eastern tropical Pacific. A severe ENSO affects not only the Pacific marine environment, but also continen- tal systems worldwide (Trillmich & Ono, 1991). Cold (La Nin ˜a) and warm (El Nin ˜o) episodes of ENSO in the Pacific cause drought and abundant rainfall, respectively, in southern Brazil, Uruguay and north-eastern Argentina (Philander, 1990). The intensity of the 1995–1996 La Nin ˜a was moderate, but the 1997–1998 El Nin ˜o was, by some measures, the strongest on record, with major climatic impacts felt around the world (McPhaden, 1999). In estuaries and riverine sytems, ENSO can affect salinity, temperature and water circulation ‡Author to whom correspondence should be addressed. email: amgarcia@mikrus.com.br 1218 0022–1112/01/111218+21 $35.00/0  2001 The Fisheries Society of the British Isles