Low-amplitude noise elicits the Lombard effect in plainfin midshipman mating vocalizations in the wild

Anthropogenic noise pollution is an emerging global threat to fish populations. Among a suite of deleterious effects, noise can potentially impede reproductive success in some fishes by masking their mate advertisement vocalizations. Using the plainfin midshipman fish, Porichthys notatus, a marine toadfish that produces a distinctive ‘hum’ during courtship, we investigated how noise affects male vocalizations and spawning success in the wild. We recorded nesting males for 3 days and measured the frequency (i.e. pitch), amplitude and duration of their vocalizations before, during and after exposure to artificial noise (∼118Hz tone). We also counted eggs in nests exposed to 10 days of artificial noise versus control nests that were not exposed to artificial noise. Males exposed to noise produced fewer vocalizations, reduced the frequency of vocalizations and increased the amplitude of their mating hum (Lombard effect). However, chronic artificial noise exposure did not significantly affect spawning success, suggesting that the Lombard effect allowed males to sustain clear advertisement signals when competing with a relatively weak artificial noise source. Future studies are needed to determine whether such vocal adjustments incur costs for males, and how common anthropogenic noises, such as boat engines, affect spawning and reproductive success.