BIOTROPICA 40(3): 386–389 2008 10.1111/j.1744-7429.2007.00378.x Big Fish are the Best: Seed Dispersal of Bactris glaucescens by the Pacu Fish (Piaractus mesopotamicus) in the Pantanal, Brazil Mauro Galetti 1,4 , Camila I. Donatti 2 , Marco Aur´ elio Pizo 3 , and Henrique C. Giacomini 1 1 Laborat ´ orio de Biologia da Conservac ¸˜ ao, Departamento de Ecologia, Universidade Estadual Paulista (UNESP), Av. 24-A, 1515, 13506–900, Rio Claro, S˜ ao Paulo, Brazil 2 Department of Biological Sciences, Stanford University, Stanford, California 94305, U.S.A. 3 Universidade do Vale do Rio dos Sinos (UNISINOS), Ciˆ encias da Sa ´ ude, Av. Unisinos, 950, 93022–900, S˜ ao Leopoldo, RS, Brazil ABSTRACT We studied the potential role as seed disperser of the pacu fish (Piaractus mesopotamicus, Characidae) in the Pantanal of Brazil. The most important food item in the diet of the pacu in the wet season was fruits of the palm Bactris glaucescens found in the guts of 73 percent of all fishes collected (N = 70). We found a positive relationship between fish length, weight, and gape size and the number of intact seeds in their gut. Therefore, large pacus are especially important in dispersing B. glaucescens seeds within the studied system. Since the best seed dispersers are the largest fishes, which are preferred by commercial fisheries, we predict that the ongoing over fishing in freshwater ecosystems will have major impacts on the dispersal system of fish-dependent plants. We suggest that it is paramount to change the attitudes in fisheries management of fruit-eating fishes and urgent to evaluate the impact of fishing on forest regeneration. Key words: Arecaceae; ichthyochory; mutualism disruption; seed predation. IN TROPICAL HABITATS, BIRDS, PRIMATES, AND RODENTS are the most studied vertebrates in respect to fruit consumption and seed dis- persal (see Estrada & Fleming 1986, Fleming et al. 1993, Levey et al. 2002). Nevertheless, the presence of intact seeds in the di- gestive tract of bony fishes has been reported in various tropical regions, mainly in the Amazon region, and can be a positive in- dication of seed dispersal by these vertebrates (Gottsberger 1978; Goulding 1980, 1983; Waldhoff 1996; Horn 1997; Banack et al. 2002; Mannheimer & Bevilacqua 2003). In riparian and/or flood plains forests, fish may play an important role in ecosystem dy- namics, acting as upstream carriers of seeds (Kubitzki & Ziburski 1994, Waldhoff 1996). However, this potential effect of fishes in certain tropical habitats subject to periodic large-scale floods, such as varzea forests and riparian and riverine gallery forests, has been under-investigated. Due to the deleterious human impact on aquatic ecosystems worldwide, the poorly known interactions between fruits and fresh- water fishes are probably among the world’s most threatened mu- tualisms. Interactions between fishes and fruits may be particularly susceptible to disruption due to deforestation, dam construction, pollution, ornamental fish trade, and the introduction of exotic species into aquatic ecosystems, but the major threat for large- bodied fishes is overfishing. Although overfishing is well docu- mented in the ocean, several fishes are known to have been over- exploited (Catella 2001). For instance, 125 tones of the highly frugivorous fish, the pacu (Piaractus mesopotamicus) were harvested in 2002 in the Pantanal of Brazil and fishing stocks are declining sharply (Catella et al. 2005). Received 30 October 2006; revision accepted 6 July 2007. 4 Corresponding author; e-mail: mgaletti@rc.unesp.br The removal of a large biomass of terrestrial frugivores in trop- ical forests due to commercial and subsistence hunting has led some scientists to coin the term ‘empty forests’ (Redford 1992), and may have serious consequences for the entire community. The cascading effects of mutualism disruption, especially seed disper- sal, have been documented recently for lizards, rodents, primates, birds, and ants, and may have far-reaching consequences for plant populations (Rold´ an & Simonetti 2001, Cordeiro & Howe 2003, Traveset & Riera 2005, Galetti et al. 2006). Thus, it is vital to understand the role of frugivorous fishes as seed dispersers to pre- dict the impact of their decline on the plant populations whose seeds they disperse and, ultimately, on the forest ecosystems they inhabit. We studied the potential role as seed disperser of the pacu fish (P. mesopotamicus, Characidae) in the Brazilian Pantanal. The Pan- tanal is a large, seasonally flooded area located in central-western Brazil, eastern Bolivia, and northeastern Paraguay, which comprises ca 3 percent of the world’s wetlands (Swarts 2000). In the upper Paraguay River, a major drainage of the Pantanal, various frugivo- rous fish species are found, including the piraputanga (Brycon hilarii, Characidae) and the pacu, but little is known about their role as seed dispersers (Pott & Pott 1994, Sabino & Sazima 1999, Reys et al. in press). This study was conducted at Fazenda Rio Negro (19 ◦ 34 ′ 29 ′′ S, 56 ◦ 14 ′ 37 ′′ W), a 7500 ha private ranch in the Nhecolˆ andia region in Brazil (Donatti et al. 2007). Several habitat types are represented at Fazenda Rio Negro, including cerrado, semi-deciduous forests, and riverine forests, where our study was conducted. These habitats are representative of the main vegetation types of Pantanal (Prance & Schaller 1982). A brief description of the riverine forest can be found below, following Prance and Schaller (1982) and Pott and Pott (1994). 386 C  2008 The Author(s) Journal compilation C  2008 by The Association for Tropical Biology and Conservation THE฀JOURNAL฀OF฀TROPICAL฀BIOLOGY฀AND฀CONSERVATION