Exposing Northern shrimp (Pandalus borealis) to fish feed containing the antiparasitic drug diflubenzuron caused high mortality during molting

Use of the chitin synthesis inhibitor diflubenzuron (DFB) as an antiparasitic drug in salmon aquaculture raises concern over its impact on marine ecosystems. Further, global drivers, such as ocean warming and acidification (OAW), may increase the toxicity of hazardous substances including DFB. The aim of the present study was to examine the combined effects of DFB-medicated salmon feed on ovigerous Northern shrimp (Pandalus borealis) under Control (pH_NBS 8.0, 7.0ºC) and OAW conditions (pH_NBS 7.6, 9.5ºC). DFB-exposed shrimp consumed on average 0.1–0.3 g medicated feed during the 2-week exposure period, and high mortality (61–73%) was documented at both environmental conditions. There was no significant interaction between OAW and DFB. Only 2–7% of DFB-exposed shrimp molted successfully compared to 65% in Control and 63% in OAW. The shrimp molted earlier (shorter intermolt period) and exhibited higher feeding rate at OAW compared to Control conditions. An additional experiment, where female shrimp were exposed to DFB closer to molting, noted increased mortality after only 4 d exposure, and successful molting for some shrimp after 2 to 3 weeks of depuration. High mortality of shrimp exposed to DFB-medicated feed indicates that the use of this feed in aquaculture could affect local shrimp populations.