Population density, growth and reproduction of arapaima in an Amazonian river-floodplain

Compensatory density effects are key features of fish population dynamics that remain poorly understood in tropical river-floodplains. We investigated possible compensatory growth and reproductive processes for a river-floodplain population of Arapaima sp., an extinction-prone fish species of South America. Body growth was studied through analysis of ring patterns on the scales, and size and age at sexual maturity was studied through analysis of female gonads. Growth and maturity were compared for unmanaged conditions with relatively low population density (in 1990s) versus managed conditions with markedly higher density (in 2005-2006); between 1999 and 2005-2006, abundance increased 7.3 fold. Results contradict theoretical expectations for slower growth and delayed reproduction at higher population density. Total lengths of arapaima at low population density were significantly shorter for age classes 1-5 compared with lengths of those age classes at high population density (ANCOVA, P < 0.0001 for both slopes and intercepts). Total length at 50% maturity (L-50) only declined about 4% with increasing density (e.g., 164 cm at low density vs. 157 cm at high density). Apparent faster growth at high density and only a slight change in size at maturity resulted in fishes spawning at an earlier age with high density conditions (age 3 vs. age 4-5). We hypothesise that these patterns reflect compliance with minimum size limits of catch during the high density (managed) situation, where there was no harvest of immature fishes. Compliance with minimum size limits, thus, may have led to faster average body growth rate and earlier reproduction, which has greatly promoted population recovery.