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Neotropical otter diet variation between a lentic and a lotic systems

Neotropical otters have been scarcely studied at the northern edge of their distribution in Mexico.  The species has nocturnal-twilight habits, and their principal food is fish and crustaceans.  The aim of this work was to know the foraging habits of Neotropical otters ( Lontra longicaudis ) in two monitoring sites one lentic and the other one lotic systems of the Rio San Lorenzo, Sinaloa Mexico.  From February 2009 to January 2010, we surveyed the area during the four seasons by walking 15 km of the river edge of Rio San Lorenzo, and 15 km at the El Comedero reservoir.  We collected 318 scats, 155 in the reservoir and 163 in the river.  Fish were the most abundant prey item in both places, with a frequency of occurrence of 98.1 % in the river and 100 % in the reservoir.  The percentage of occurrence varied largely in the other prey items in both places.  Seven taxonomic groups compose their foraging habits in the river (relative frequency in parentheses): fish (64.3 %), insects (9.2 %), mollusks (6.8 %), birds (6.0 %), mammals (5.6 %), crustaceans (5.2 %) and fruits (2.8 %).  Five taxonomic groups in the reservoir compose it: fish (65.1 %), birds (22.7 %), mammals (9.2 %), fruits (2.5 %) and insects (0.4 %). The most important fish in the diet belonged to the genus Oreochromis , which includes an introduced species.  We conclude that the Neotropical otter is an opportunistic carnivore that preys mainly on fish, probably due to their high availability, but they also forage in other taxa that are available at Rio San Lorenzo basin.