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

Abstract

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 (Neogobius melanostomus) on the abundance of eyeflukes of the genus Diplostomum, 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 (Notropis hudsonius), golden shiner (Notemigonus crysoleucas) yellow perch (Perca flavescens)] 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 Diplostomum 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, Larus delawarensis) 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 Diplostomum 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 Diplostomum 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.

Limited parasite acquisition by non-native Lepomis gibbosus (Actinopterygii: Centrarchidae) at two ponds in the Upper Rhine basin, Germany

“…The sporadic occurrence of P. centrarchi at Knielingen may result from the low density of mollusc intermediate hosts as a result of high predation pressure and/or the encounter-dilution effect, which predicts that the number of parasites per host will be negatively correlated with host density, as the total number of transmission stages is divided between all hosts in the area, reducing the probability of host–parasite contact (Buck & Lutterschmidt, 2017). A related alternative hypothesis, encounter-dilution via non-compatible hosts, predicts that frequent encounters with non-competent fish species can decrease a parasite's capability of infecting competent hosts through energy depletion or damage (Gendron & Marcogliese, 2017). At Knielingen, the fish community comprised a wide range of local species, presumed to be non-compatible for North American Posthodiplostomum cercariae.…”

Biological Invasion Theories: Merging Perspectives from Population, Community and Ecosystem Scales

“…For instance, after the introduction of the non-native round goby, Neogobius melanostromus, the population of a local parasite, Dilostomum sp., declined. Invasive populations acted as a decoy for parasites and thus positively affected native fish which were less infected (Gendron & Marcogliese, 2017). These phenomena can be difficult to observe, because either they occur too quickly (Dunn & Hatcher, 2015) or the origin of the parasite is unclear.…”

Invasion ecology meets parasitology: Advances and challenges

“…(2017) , who studied native and invasive amphipods of the genus Gammarus , living in sympatry and sharing acanthocephalan parasites in French rivers. Finally, Gendron and Marcogliese (2017) investigate how the invasive round goby, Neogobius melanostomus , may act to dilute the risk of infection by eyeflukes, Diplostomum spp., in native fishes of the St. Lawrence River, in Canada. These studies capture the various ways in which native parasites are influenced by invasive hosts, and in turn modulate the relative success of those invaders.…”