The neotropical otter Lontra longicaudis (Olfers, 1818) has been considered as generalist due to the plasticity of its diet (^{Quadros & Monteiro-Filho, 2001}; ^{Rheingantz et al., 2017}). Fish are the main prey item throughout its distribution range (^{Rheingantz et al., 2017}); but otters consume alternative prey frequently, such as amphibians, reptiles, birds, mammals, insects, crustaceans, mollusks, and fruits (see ^{Macías-Sánchez & Aranda, 1999}; ^{Casariego-Madorell et al., 2008}; ^{Monroy-Vilchis & Mundo, 2009}; ^{Gallo & Casariego, 2014}). Food habits of L. longicaudis in Mexico have been determined at several locations (see ^{Macías-Sánchez & Aranda, 1999}; ^{Casariego-Madorell et al., 2008}; ^{Monroy-Vilchis & Mundo, 2009}; ^{Briones-Salas et al., 2013}; ^{Duque-Dávila et al., 2013}; ^{Monterrubio-Rico & Charre-Medellín, 2014}; ^{Cruz-García et al., 2017}); however, considering its wide range of distribution, knowledge on the diet of the species in the country is far from complete. The results presented here were obtained from analyses of 81 fecal samples (spraints) collected during January 2005-December 2008 at three sites in the Santiago River, Nayarit/Jalisco, Mexico (elevation 245-480 m, 22°C mean temperature, 800-1000 mm annual rainfall): confluence of the Bolaños river (Number of spraints = 13; reference point: 21°.192174, -104°.067054), El Cajón reservoir (N = 52; 21°.119621, -103°.977932), and Santo Domingo (N = 16; 21°.427587, -104°.437750). Spraints were individually processed, washed, and air-dried; diet items were identified to the lowest possible taxonomic level. To identify the taxonomic category of the items registered in the spraints we used scales and other bony structures of fish, reptiles, and amphibians; seeds for plant matter, and appendices, elytrae and wings of insects. We compared those items with specimens voucher of vertebrates, entomology and botanic collections of the Departamento de Botánica y Zoología of the Universidad de Guadalajara. The results were subjected to the following formulae (^{Maehr & Brady, 1986}; ^{Jacobsen & Hansen, 1996}):

Frequency occurrence FO=nN,

where n = number of spraints containing a specific diet item, and N = total number of spraints

Percentage occurrence PO=FiFt ,

where F _i = total frequency of a diet item, and F _t = the sum of all frequencies

The predominant diet items were: fish (PO 43.86%), insects (PO 22.81%), reptiles (PO 9.36%), and amphibians (PO 8.77%). Plant matter, mammals, crustaceans, mollusks, and birds, showed much lower PO and were not analyzed to further detail (Table 1). Six species of fish were identified as prey; the introduced Oreochromis aureus and Cyprinus carpio were the main prey, but native species as Moxostoma austrinum and the three endemics Ictalurus dugesii, Cichlasoma beani, and Chirostoma arge were also found in spraints. The main insect prey items were Hemiptera and Coleoptera, followed by Orthoptera and Hymenoptera; reptile prey, in order of importance, were Ctenosaura pectinata and horned lizards (Phrynosomatidae). Fish consumption was similar in all sites (χ² = 2.56, p-value = 0.278), but was highest at the Bolaños river for insects (χ² = 9.35, p-value = 0.009) and at El Cajón reservoir and Bolaños river for reptiles (χ² = 14.48, p-value = 0.001). The PO of the main fish items were different between sites; the highest values of O. aureus (PO 23.91%) and C. carpio (PO 21.74%) were at Santo Domingo, but were similar at El Cajón reservoir (O. aureus 15.38%; C. carpio 7.69%) and Bolaños River (O. aureus 16.25%; C. carpio 12.5%).

The highest frequency of introduced fish species in the diet, and the lowest frequency of native species may reflect fish population abundance at the Santiago River, as fishery records suggest that O. aureus and C. carpio are the most abundant species at the study site (pers. obs.). Studies of food habits of L. longicaudis at other sites also identify fish as the main prey item, but differ from the present study, as reptiles and insects were less frequently consumed therein (^{Parera, 1993}; ^{Macías-Sánchez & Aranda, 1999}; ^{Quadros & Monteiro-Filho, 2001}; ^{Kasper et al., 2004}; ^{Casariego et al., 2006}; ^{Briones-Salas et al., 2013}; ^{Duque-Dávila et al., 2013}; ^{Rangel-Aguilar & Gallo-Reynoso, 2013}; ^{Monterrubio-Rico & Charre-Medellín, 2014}; ^{Cruz-García et al., 2017}). During this study, water levels were rising rapidly due to the filling-up process of the El Cajón reservoir, and individuals of Ctenosaura pectinata, horned lizards, small mammals, and insects were frequently observed on tree-tops surrounded by water and in open waters of the reservoir, which could make them a prey of opportunity to aquatic predators, including L. longicaudis. The differences in the PO between sampling sites of groups as fish, reptiles, and insects could be a reflection of the differential effects of the reservoir filling, between those sites. Results from this study show that fish are the main prey on the diet of L. longicaudis, but the presence of reptiles and insects could be influenced by the conditions of the sites during the sampling period. Our results support the assumption that L. longicaudis is a facultative predator according to ^{Rheingantz et al. (2017)}. On the other hand, the fact that L. longicaudis consumes mainly introduced fish prey might be a useful information for the management of these species, and the conservation of native diversity.