The neotropical river otter (Lontralongicaudis annectens) is threatened in Mexico, and in order to design soundconservation strategies for its conservation, basic studies on their biologyand ecology are necessary. The diet and abundance were evaluated between July,1999 and August, 2000 in the rivers Ayuta, Copalita and Zimatán in thecoast of Oaxaca. We considered estimated of four otters in 147.5 km the riverAyuta. 86 otters in 330.75 km the rive Copalita and 177 otters in 228.85 kmthe river Zimatán. We found a relation in an indirect way between theabundance of the otter and dissolved O2. From 330 scats it was determinedthat they use four prey categories, which include crustaceans (53.0 ±3.6%), fish (33.1 ± 9.9%), insects(9.8 ± 7.6%) and amphibians (4.0 ± 3.3%). In relation to the biomassingested, the main prey were Macrobrachium americanum, M. acanthochirusand Gobiexos mexicanus. The largest number of fish and crustaceans usedwas obtained in the river Zimatán (n = 258 samples, 16 spp.) followedby the river Ayuta (n = 253, 14 spp) and the river Copalita (n = 197, 16 spp).We found a relation among the frequency of appearance of prey species in thescats, with their abundance (p < 0.01; r2 = 0.40), as well asa significant relation between the consumed biomass and the abundance of theprey. Abundance of prey is determined by O2 dissolved in the water(p = 0.04; r2 = 0.09) as well as turbidity (p = 0.04; r2= 0.22). There was no relation between the physiochemical variables of the waterwith the diversity of the potential prey.
