Round goby predation on Hemimysis anomala

Abstract

The invasive Ponto-Caspian mysid Hemimysis anomala (hereafter Hemimysis) was first observed in south-eastern Lake Ontario in May 2006. In Lake Ontario, Hemimysis is most abundant at nearshore habitats characterised by cobble–rubble substrate, which it shares with the round goby Neogobius melanostomus (hereafter goby). To better understand the interaction if any between gobies and Hemimysis, gobies were collected throughout the summer and fall periods at three shallow (<3m) nearshore locations representing eastern, central, and western Lake Ontario where Hemimysis are relatively abundant. Goby stomach contents were examined for evidence of Hemimysis and other prey. In addition, gobies were collected at two locations in southwestern Lake Ontario in shallow- (3m) and deep- (11m) water nearshore habitat where stomach contents were also used to characterise diet. Despite higher Hemimysis abundance nearshore, Hemimysis were not consumed by any gobies collected from shallow nearshore locations, and only found in three of 23 gobies collected at the deep site. Goby abundance and consumption of Hemimysis by gobies at the deep site were unrelated to Hemimysis abundance in traps. At all locations, gobies fed on soft-bodied invertebrates and dreissenids although dreissenids were the dominant prey among large (>74mm) gobies. We suspect that the ability of Hemimysis to avoid predation by gobies is the result of a combination of anti-predator behaviours such as an evasive startle response, reduced activity and depth dependent segregation in interstitial spaces during the day, and a feeding mode used by gobies that is poorly suited for an evasive species like Hemimysis.