Fishing Out Marine Parasites? Impacts of Fishing on the Abundance and Diversity of Fish Parasites

Half of all species on the planet are parasites. Despite this ubiquity, we have only a rudimentary understanding of anthropogenic influences on parasite biodiversity. Fishing – one of the most disruptive human impacts on the ocean, and the impact that extends deepest into the past – may strongly influence parasites, because it directly removes parasite habitat and food resources (i.e., fish hosts). Because fishing reduces the density of fish (reducing transmission efficiency of directly transmitted parasites), selectively removes large fish (which tend to carry more parasites than small fish), and reduces food web complexity (reducing transmission efficiency of trophically transmitted parasites), the removal of fish from the world’s oceans over the course of hundreds of years may be driving a long-term, global decline in fish parasites. Here, we review evidence from two “natural experiments” to assess this hypothesis. The first study was conducted in a series of no-take marine reserves and matched fished areas along the central Chilean coast. The second study takes advantage of a natural gradient of fishing pressure across the northern Line Islands archipelago in the central equatorial Pacific, including pristine coral atolls that have never been intensively fished. The results shed light on the ways in which fishing can impact marine food webs through complex indirect effects.