Mechanisms of density dependence in stream fish: exploitation competition for food reduces growth of adult European bullheads (Cottus gobio)

摘要

In field experiments using cage enclosures, exploitation competition for invertebrate prey reduced individual growth of adult European bullheads (Cottus gobio) despite a dietary shift that maintained total prey biomass consumption. Growth of bullheadswas negatively density dependent over a range of densities found in the field and total bio-mass production per enclosure was unrelated to stocking density. Individuals grew faster when invertebrate densities were experimentally elevated above ambient levels, indicating that bullheads were food limited. Parallel dietary shifts in response to manipulation of both bullhead density and resource availability suggested that exploitation competition for preferred Gammarus pulex prey was the primary cause ofdensity-dependent growth. Switching to smaller prey species did not prevent growth of bullheads declining with increasing per capita food competition, indicating that the alternative prey were less profitable. Evidence for interference competition was limited. These results caution against the assumption that demographic rates of stream fish are density independent after the juvenile phase and demonstrate instead that density-dependent growth may potentially regulate populations at the adult stage via effects on fecundity and survival.