The Impact of Didymosphenia geminata On Brown Trout: Biological Villain or Ecological Scapegoat?

Didymosphenia geminata nuisance blooms were first observed in Rapid Creek, South Dakota in 2002. Shortly after its appearance, large declines (>50%) of its naturalized brown trout population were observed. Drought conditions were found to be largely responsible for biomass declines, but size structure differences were observed within D. geminata impacted areas. Here, we evaluated the influence of D. geminata on diet and growth of brown trout. Brown trout in Rapid Creek consumed fewer ephemoropterans and a large amount of dipterans compared with their counterparts in non-impacted streams. However, diet analysis showed that brown trout in Rapid Creek consumed as much or more prey than trout from two other streams unaffected by D. geminata. Moreover, relative weight of brown trout from Rapid Creek was high (>100), implying that food availability was not limiting. Growth characteristics also supported the notion that Rapid Creek brown trout are not food limited, as mean length at age of Rapid Creek brown trout was greater than Spearfish Creek and global means. Recent research suggests that poor habitat and predation could be impacting brown trout size structure rather than D. geminata.