Microplastic fibers are frequent contaminants of aquatic ecosystems. Early life stages of aquatic organisms are predicted to be especially vulnerable to microplastic pollution. We hypothesized that microplastic fibers in the water column might interfere with fertilization and embryonic development of fish. We tested this with an in vitro fertilization system with three-spined sticklebacks. Six egg clutches were divided and one half was fertilized and bread out in water with polyester fibers (PET fibers; mean diameter 9.7 ± 2.3 μm; mean length 245.6 ± 163.1 μm) at a concentration of 1 × 104 fibers/L while the other half served as control without fibers.Observation with a dissection microscope revealed that some polyester fibers stuck to the outside of the eggs in the fiber treatments. Yet, overall 67.4 ± 12.9% eggs were fertilized from which 97.2 ± 4.2% larvae hatched without any significant difference between treatments. Mortality and abnormal development of larvae was low and was not changed by microplastic fibers, as was the heart rate of developing embryos five days post fertilization.The present study illustrates that polyester fibers, even at concentrations three to four orders above levels reported from the environment, do not impair fertilization success, embryonic and early larval development of sticklebacks. Accordingly, concentrations of microplastic fibers currently observed in aquatic habitats do not appear to be harmful to early live stages of fish.

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