Fish vulnerability to capture by trapping is modulated by individual parasite density.

Commercial fishery harvest is a powerful evolutionary agent, but we know little about whether environmental stressors affect harvest-associated selection. We test how parasite infection relates to trapping vulnerability through selective processes underlying capture. We used fish naturally infected with parasites, including trematodes causing black spots under fish skin. We first assessed how individual parasite density related to standard metabolic rate (SMR), maximum metabolic rate (MMR) and absolute aerobic scope (AAS)-then used laboratory fishing simulations to test how capture vulnerability was related to parasite density. We further explored group-trapping dynamics using experimental shoals containing varying proportions of infected fish (groups of six with either 0, 2, 4 or 6 infected individuals). At the individual level, we found a positive relationship between parasite presence and SMR, but not MMR or AAS. While we saw no relationship between individual metabolic capacity and vulnerability to trapping, we found the length of time fish spent in traps increased with increasing parasite density, a predictor of trapping-related capture probability. At the group level, the number of infected individuals in a shoal did not affect overall group trapping vulnerability. Our results suggest that parasite infection has some capacity to shift individual vulnerability patterns in fisheries, and potentially influence the evolutionary outcomes of fisheries-induced evolution.