Eurasian otter (Lutra lutra) diet as an early indicator of recovery in defaunated river communities

•We investigated the diet of the Eurasian otter in a river suffering a toxic spill.•60 km of river length were defaunated, but otters occurred again six months later.•Otter diet was similar to that of undisturbed rivers 1–3 years after the spill.•Fish species appeared in the otter diet 6–12 months before than in human samplings.•Otter diet could be a good early indicator of recovery of fish species. In April 1998 the dam of the Aznalcóllar mine tailings pond in the surroundings of the Doñana National Park (Southwestern Spain) was broken, discharging into the Guadiamar River 6 hm3 of toxic mud and acidic water with high concentrations of heavy metals and arsenic. As consequence, 60 km of the river were defaunated. First signs of the Eurasian otter (Lutra lutra), a top predator of aquatic ecosystems, were detected at the affected river sectors six months after the spill. We studied temporal and spatial variations in the otter diet at the Guadiamar River throughout two years in order to investigate its putative role as indicator of the river recolonization process by otter prey. We collected 893 otter faeces (or spraints) (560 the first year and 333 the second one) beginning 13 months after the accident. Spraint content was identified using morphological keys. Results are expressed as frequency of occurrence of each type of prey, percentage of individual prey, and relative frequency of occurrence. Most of the diet was made by fish (occurring in 77% of the samples) and crayfish (39%), with minor representation of amphibians (15%), insects (7%) and birds (6%). The importance of fish in the affected sectors increased the second year of study, contrasting with a decrease in the role of crayfish and the group “other vertebrates”. Just over a year after the toxic spill the otter diet was rather similar to those detected in other Iberian Mediterranean rivers not affected by such a catastrophic event, suggesting the recovery of the riverine biocenose was almost completed. However, several studies on different animal groups showed that the spill impact was still very pronounced at this time. This fact questions the role of the otter diet as a suitable indicator of the complete river recovery. Alternatively, otter diet seems to be a powerful indicator of the early stages of fish recolonization, since most species occurred in the spraints between six months and one year before they were captured in human samplings.