Density dependence of herbivorous central stoneroller Campostoma anomalum in stream mesocosms

Abstract

Herbivorous fish can have strong effects on stream ecosystem function by consuming primary producers and excreting limiting nutrients, but it is unclear whether they are resource limited. Thus, understanding factors regulating abundance of these fish might help predict ecosystem function. We used stream mesocosms to test whether populations of central stoneroller Campostoma anomalum exhibit density dependence across a range of typical densities and resource abundance found in Great Plains streams. We predicted that incrementally increasing stocking biomass from 3·7 to 24·9 g·m−2 would reduce standing stocks of resources resulting in lower growth of stocked fish. Fish growth (over 41 days) was compared to initial stocking biomass and primary production as well as standing stocks of algae and invertebrates using regression analysis. Mean growth of individuals was negatively associated with stocking biomass (Radj2 = 0·55; P = 0·02), as predicted. Contrary to our prediction, increases in fish biomass led to increased primary productivity (Radj2 = 0·31, P = 0·07), but resulted in no relationship among algal filament lengths (Radj2 = 0·00; P = 0·34), algal biomass (Radj2 = 0·12; P = 0·19) or invertebrate biomass (Radj2 = 0·03; P = 0·30). Thus, density dependence occurred without an apparent reduction in food resources. We hypothesised that stoneroller growth was possibly limited by competition for high-quality algae or invertebrates, or behavioural interactions causing interference competition.