Enigmatic decline of a common fish parasite ( spp.) in the St. Lawrence River: Evidence for a dilution effect induced by the invasive round goby.

As they integrate into recipient food webs, invasive exotic species may influence the population dynamics of native parasites. Here we assess the potential impact of the Eurasian round goby () on the abundance of eyeflukes of the genus , which are common parasites in fishes of the St. Lawrence River (Canada). Analyses of data collected over nearly two decades revealed that the infection levels in three native fish [spottail shiner (), golden shiner () yellow perch ()] declined sharply throughout the St. Lawrence River after the introduction of the goby. At two sites where data were collected at regular time intervals, declines of spp. in spottail shiners occurred within two years of the goby's first recorded appearance, with prevalence dropping as much as 77-80% between pre-invasion and post-invasion periods. Furthermore, in localities where gobies remained scarce, infection in native species did not change significantly over time. Altogether, these observations suggest that gobies play a role in the eyefluke collapse. The decline in populations of the main definitive host (ring-billed gulls, ) and changes in hydrology during periods of parasite recruitment were not strongly supported as alternate explanations for this phenomenon. Since other snail-transmitted trematodes with similar life cycles to spp. did not show the same decreasing pattern, we conclude that eyeflukes did not decline as a result of snail depletion due to goby predation. Rather, we suggest that gobies acted as decoys, diluting the infection. As spp. occurred at lower abundance in gobies than in native fish hosts, the replacement of native fish with exotic gobies in the diet of gulls might have played a part in reducing parasite transmission. In contrast to the typically negative impact of invasions, the goby-induced decline of this pathogen may have beneficial effects for native fishes.