A common marking method influences fish-parasite interactions

Being able to identify the same individual organism through time is essential to robustly address many outstanding questions in ecology and evolution. For organisms that can move, and that have no reliable identifying features, researchers must often use some form of artificial marking system. In fish, a commonly used system is Visible Implant Elastomer (VIE) marking. Previous studies indicate that this marking system does not alter the survival, growth, or behaviour of marked fish. Here, using 26 VIE-marked and 46 control guppies, Poecilia reticulata, I found that VIE marking indeed did not impact fish growth. However, one month after marking, VIE-marked fish experienced an estimated ~3.7-fold higher Gyrodactylus turnbulli parasite burden during experimental infections. Two months post-marking, G. turnbulli individuals transmitted faster to VIE-marked than unmarked fish, although at this timepoint their subsequent growth was not different on fish from different treatment groups. Despite its lack of impact on common metrics of fish health, therefore, VIE marking appears to have important, long-lasting impacts on fish-parasite interactions. These findings suggest caution with assuming that marking animals is biologically neutral: that assumption may fail when the response variable involves host-parasite interactions.