Foraging response of bluegill Lepomis macrochirus to microplastics and subsequent impacts on swimming performance

Microplastics are an emerging threat to aquatic and terrestrial species, and growing evidence suggests a suite of negative impacts, including reduced food consumption and direct physiological effects. Uptake of microplastics by animals can occur passively through accidental ingestion (i.e. fish swimming through contaminated water and taking microplastics into their buccal cavity/gills), or through active selection and consumption of particles. The rate of active plastic consumption for any animal depends on how well it can recognize and avoid eating the plastic. This likely has important impacts on the animal’s health and fitness, with subsequent impacts on reproductive fitness. In this study we explored the willingness of bluegill, Lepomis macrochirus, to directly consume microplastics (small, easily consumable pieces of high-density polyethylene bag films or polypropylene rope fibres) and whether this varied with continued exposure over time. Wild-caught bluegill were stocked in groups of three or six individuals in 37-litre aquaria and offered food (controls) or microplastics followed by food over a period of 4–6 days. Results showed that direct consumption of microplastics declined over time while foraging on food increased, indicating learned avoidance of microplastics. Foraging was impacted by plastic type, with bluegill more likely to forage on films than on fibres. Following group testing, each fish underwent an individual test for swimming endurance in a modified swim apparatus; however, no significant differences were found in swimming performance between treatments. Our results add to the growing literature revealing how animals, including fish, may actively consume microplastics and the potential effects of that exposure.