Distribution and abundance of the Patagonian lamprey ( Geotria macrostoma ) in the largest unfragmented river of southern Patagonia, Argentina

The distribution, relative abundance, and rearing habitat of larvae of the Patagonian lamprey ( Geotria macrostoma ) were investigated in the Santa Cruz River basin, Patagonia, Argentina, using electrofishing and bile acid (petromyzonol sulfate) concentration. Both techniques revealed that larvae are widely distributed along the basin, aggregated at habitats of fine substrates and slow waters. Both, density and pheromone concentrations were consistent in detecting the presence of larvae and for gauging their abundance. Throughout the basin, length distribution of larvae ranged from 10 to 122 mm; larvae of the smaller length classes were present at both, upper sections and at the lower course of the main stem river, suggesting that these sites contained 0 + recruits from the previous spawning season. Currently construction of two hydroelectric dams in the main course of the Santa Cruz River will interrupt the migratory corridor of adult lamprey in search of their spawning grounds and will flood critical habitats for larvae rearing. The information generated in this study may be used as guidance for developing sampling surveys for the purpose of monitoring changes in populations trends for Patagonian lamprey in this river and other Patagonian drainages altered by human activities.