Patterns of availability and use of resources by otters (Lutra lutra L.) in southwest Portugal
The use of resources by otters (Lutra lutra L.) living along coasts and streams in Southwest Portugal was described, and related to their spatial and temporal patterns of availability. The use of freshwater along coasts was inferred from the spatial distribution of spraints, and from radio-tracking. Spraints were mostly concentrated at the mouth of streams that provided freshwater throughout the dry season, and where rest-sites were found within dense thickets of brambles. Rest-sites were considered a scarce resource, because they seemed to be associated with the few permanent sources of freshwater. Along the coast prey availability was highest in winter and lowest in later summer and autumn. The corkwing wrasse was the dominant prey, and the seasonal variation in diet was primarily related to fluctuations in its abundance: wrasse were consumed when they were most abundant, other prey being taken to compensate for lows in their availability. In streams there were large year-to-year fluctuations in fish abundance, and these were related to the severity of summer droughts. Spring was probably the period of maximum prey abundance. Eels and crayfish were the dominant prey, but cyprinids, toads and frogs were also important seasonally. Prey were consumed relative to availability, except eels, which were consumed less than expected during the peak abundance of crayfish. Crayfish and eels were more associated in spraints that expected by chance. The introduction of the American crayfish to Iberia contributed to increase food abundance from spring to early autumn. Breeding of otters occurred seasonally, but births were earlier on the coast (October-December) than inland (January-March), most probably as a consequence of the seasonal patterns of food availability. Litter sizes inland were larger than on the coast. Differences between otter habitats in Southwest Portugal and elsewhere were discussed. A conservation strategy for otters in this region was developed.