Trophic ecology of the black drum, Pogonias cromis (Sciaenidae), in Mar Chiquita coastal lagoon (Argentina) By G. E. Blasina 1,2 , S. A. Barbini 1,3 and J. M. Dı´az de Astarloa 1,2 1 Departamento de Ciencias Marinas, Laboratorio de Ictiologı´a, Universidad Nacional de Mar del Plata (UNMdP), Mar del Plata; 2 Consejo Nacional de Investigaciones Cientı´ficas y Te ´cnicas (CONICET), Ciudad Auto ´noma de Buenos Aires; 3 Comisio ´n de Investigaciones Cientı´ficas de la Provincia de Buenos Aires (CIC), La Plata, Argentina Summary This is the first documentation on the feeding habits of the black drum, Pogonias cromis, in waters of South America. The aim of the investigation was to determine the trophic ecology of P. cromis in the Mar Chiquita coastal lagoon (37°32¢–37°45¢S; 57°19¢–57°26¢W) from stomach content analyses of 109 P. cromis specimens. Samples in various sizes were collected monthly using various types of fishing gear. The results showed that this species is a benthic predator focusing on bivalves and crabs. PERMANOVA PERMANOVA results indicated significant seasonal differences in the diet. This pattern reflected seasonal changes in prey abundance and availability, which produced changes in both foraging strategy and trophic niche breadth. This indicates that P. cromis is a versatile predator, adapting its feeding strategy in response to a seasonal shift of prey abundance, and indicating an opportunist behavior. Introduction Coastal lagoons are considered to be very productive estuarine environments (Day and Ya´n˜ez Arancibia, 1985). The func- tional roles of these fish habitats have been extensively investigated as offering physiologically suitable physicochem- ical conditions, abundant prey resources, and low predation risk (Joseph, 1973; Blaber, 2000). Coastal lagoons have also long been recognized as important feeding areas and repro- ductive grounds for fish, or for migration stopovers (Gibson, 1994). They are physically unstable environments and subject to rapid and extreme salinity variations, among other factors (Gray et al., 1996). These conditions have profound effects on behavior as well as on ecological interactions. Fishes respond to these variations by moving to preferred zones or by adapting to local salinity conditions, a common adaptation in many estuarine species (Ley et al., 1994). Likewise, euryhaline fishes may display other behavioral adaptations, such as changes in the optimal feeding strategies for maximize net energy intake (Crowder and Cooper, 1982). Continuous variations in environmental conditions obligate predators to evolve flexible feeding habits, lower tendencies towards specialization, and to maintain their flexibility to exploit temporary peaks in prey abundance (Ley et al., 1994; Giberto et al., 2007; Mendoza Carranza and Vieira, 2008). Thus, food habits of estuarine fishes likely reflect the types and variability of resources available in their environment (Livingston, 1984; Ley et al., 1994). To survive, organisms must cope with these fluctuating environments, and ecological generalization or tolerance may evolve when environments vary rapidly (Ruehl and DeWitt, 2007). The black drum, Pogonias cromis (Linnaeus, 1766), is a commercially important demersal coastal fish distributed along the western Atlantic Ocean from Massachusetts, USA, to south of Buenos Aires Province in Argentina (Macchi et al., 2002). It is an estuarine-dependent species and the largest sciaenid noted in estuarine environments of Argentina (Cous- seau and Perrotta, 2004). Like many other sciaenids, P. cromis juveniles live in estuarine areas where they can tolerate a wide range of salinities and water temperatures (Simmons and Breuer, 1962), and move into offshore marine waters when they reach the adult stage (Cervigo´ n, 1993). Adults are usually common in shallow coastal and estuarine waters, and occa- sionally occur further from the coast (Silverman, 1979; Urteaga and Perrotta, 2001). Studies of the P. cromis diet in the Gulf of Mexico revealed that they feed mainly on crustaceans and bivalves (Silverman, 1979; Peters and McMichael, 1990). Feeding habits of this species in waters of South America have not been documented. Our objective was to provide information on the trophic ecology of P. cromis in the Mar Chiquita coastal lagoon. We studied the relationship between diet composition (from stomach content analysis) and fish size, as well as the seasonal variation in the diet. We also analyzed the feeding strategy, estimating the niche breadth and relationship between pred- ator and prey sizes. Materials and methods Study area The Mar Chiquita coastal lagoon (37°32¢–37°45¢ S; 57°19¢– 57°26¢ W) is regarded as a World Biosphere Reserve by the UNESCO since 1996 (Iribarne, 2001). A temperate shallow estuary with a mean depth of approximately 1.2 m, it covers an area of 46 km 2 and is 25 km long. This irregularly shaped brackish water lagoon is separated from the sea by a littoral line of dunes and connected through an inlet channel of about 6 km length, 200 m width and 0.5–3.0 m depth (Reta et al., 2001). Freshwater inflows are from several streams, artificial channels and subterranean water, which contribute abundant quantities of water during the rainy periods. Salinity in the lagoon is lower than in the adjacent sea and a horizontal gradient fluctuates between 0 and 36 psu, depending on the tide and wind. Water temperature varies seasonally, with highest values in summer (approx. 25°C) and lowest values in winter (approx. 3°C) (Cousseau et al., 2001; Reta et al., 2001). J. Appl. Ichthyol. 26 (2010), 528–534 Ó 2010 Blackwell Verlag, Berlin ISSN 0175–8659 Received: May 15, 2009 Accepted: November 10, 2009 doi: 10.1111/j.1439-0426.2010.01459.x U.S. Copyright Clearance Centre Code Statement: 0175–8659/2010/2604–0528$15.00/0 Applied Ichthyology Journal of