Feeding response of female American lobsters, Homarus americanus, to SLICE®-medicated salmon feed

SLICE® (0.2% emamectin benzoate) is a widely-prescribed in-feed treatment for the control of sea lice (Lepeophtheirus salmonis and Caligus spp.) on farmed salmon (primarily Salmo salar). Large doses of emamectin benzoate disrupt the molt cycle of ovigerous American lobsters (Homarus americanus), causing them to molt prematurely and lose their attached eggs. The feeding responses of adult female lobsters offered a choice between their natural foods and either SLICE®-medicated or unmedicated salmon pellets were assessed in laboratory feeding trials. Most lobsters were initially attracted to and ate salmon feed and 87.5% of the 48 lobsters offered SLICE®-medicated feed acquired quantifiable levels of emamectin B1a in their digestive glands. Postmolt (stage C2–3) lobsters ate three to six times more medicated feed than did intermolt (stage C4) ovigerous lobsters, and 8% of the postmolt lobsters developed problems maintaining proper orientation. Consumption of natural foods in the feeding trials was similar each day, but in five of the six trials consumption of salmon feed declined significantly after day 1. The mean dose of emamectin benzoate ingested in 2 weeks ranged from 0.02 μg g− 1 for the intermolt ovigerous group (0.015 and 0.006 μg g− 1 in the first and second week, respectively) to 0.06 μg g− 1 for the postmolt group (0.043 and 0.012 μg g− 1 in the two weeks, respectively). These doses are lower than the LOEL (lowest observed effect level) on the molt cycle. The results of this study provide the first information on the feeding response of a non-target crustacean to salmon feed medicated with SLICE® at a concentration used by the aquaculture industry. The most significant findings are that ovigerous intermolt lobsters ate relatively little medicated feed and both postmolt and intermolt lobsters preferred their natural foods and became conditioned to reject medicated feed.