Regulated small rivers as ‘nursery’ areas for invasive largemouth bass Micropterus salmoides in Iberian waters



  1. Few studies have assessed the ecological role of regulated small rivers in the environmental biology of invasive largemouth bass Micropterus salmoides in Iberian waters.
  2. Size structure, diet, physical condition, and developmental stress of largemouth bass populations were compared between two contrasting habitats in the Iberian Peninsula, the Encinarejo Reservoir and the small River Jándula below the dam (Guadalquivir River Basin, southern Spain).
  3. Size structure differed between river and reservoir populations of small (<180 mm FL) largemouth bass, with a higher proportion of individuals between 30 and 60 mm in the river and a higher proportion of 20–40 mm individuals in the reservoir. In the river, the most important prey for small largemouth bass were Ephemeroptera, Plecoptera and Odonata nymphs. In the reservoir, Gerridae insects and pumpkinseed Lepomis gibbosus were the most important prey in terms of ingested biomass for small largemouth bass, whereas large individuals consumed high percentages of common carp Cyprinus carpio and pumpkinseed. Prey richness, trophic niche breadth, and trophic diversity were significantly higher in small largemouth bass from the river, as well as eviscerated mass and glucose level. Fluctuating asymmetry was lower in the river for branched rays of pectoral fins, eye diameter, and length of pectoral fins.
  4. Overall results indicate that largemouth bass are taking advantage of both habitat differentiation and flow regulation in Mediterranean streams. Therefore, regulated small rivers may play an ecological role as ‘nursery’ areas, where developmental stress is low and thus, juvenile fish can display better body condition and nutritional status than their reservoir counterparts.
  5. The present study provides information relevant to the management of largemouth bass and subsequent conservation of the valuable and endemic Iberian fish fauna. For example, size-specific culls in the reservoirs and below the dams are suggested strategies to control the reinforcement of downstream populations.

Copyright © 2013 John Wiley & Sons, Ltd.