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

transcript

2007) 123–129www.elsevier.com/locate/aqua-online

Aquaculture 269 (

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

S.L. Waddy ⁎, S.M. Mercer, M.N. Hamilton-Gibson, D.E. Aiken, L.E. Burridge

Department of Fisheries and Oceans, Biological Station, 531 Brandy Cove Road, St. Andrews, NB Canada E5B 2L9

Received 27 April 2006; received in revised form 20 April 2007; accepted 24 April 2007

Abstract

SLICE® (0.2% emamectin benzoate) is a widely-prescribed in-feed treatment for the control of sea lice (Lepeophtheirussalmonis and Caligus spp.) on farmed salmon (primarily Salmo salar). Large doses of emamectin benzoate disrupt the moltcycle 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 orunmedicated salmon pellets were assessed in laboratory feeding trials. Most lobsters were initially attracted to and ate salmonfeed and 87.5% of the 48 lobsters offered SLICE®-medicated feed acquired quantifiable levels of emamectin B1a in theirdigestive 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 ofnatural foods in the feeding trials was similar each day, but in five of the six trials consumption of salmon feed declinedsignificantly after day 1. The mean dose of emamectin benzoate ingested in 2 weeks ranged from 0.02 μg g− 1 for the intermoltovigerous 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 tosalmon feed medicated with SLICE® at a concentration used by the aquaculture industry. The most significant findings are thatovigerous intermolt lobsters ate relatively little medicated feed and both postmolt and intermolt lobsters preferred their naturalfoods and became conditioned to reject medicated feed.Crown Copyright © 2007 Published by Elsevier B.V. All rights reserved.

Keywords: American lobster; SLICE™; Emamectin benzoate; Molt

1. Introduction

A widely-prescribed treatment for the control of sealice (Lepeophtheirus salmonis and Caligus spp.) onfarmed salmon (primarily Salmo salar) is the drug

⁎ Corresponding author. Tel.: +1 506 529 5890; fax: +1 506 5294609.

E-mail address: waddys@mar.dfo-mpo.gc.ca (S.L. Waddy).

SLICE® (0.2% emamectin benzoate), which is admin-istered as an in-feed additive. Emamectin benzoate is anavermectin derivative (4″-deoxy-4″-epimethylamino-avermectin B1) that binds to the chloride channels ofthe nerve cells of sea lice, disrupting nerve impulses andcausing paralysis and death (SPAH, 2002a). The acutetoxicity of the drug to non-target crustaceans is low(Willis and Ling, 2003; Burridge et al., 2004), butsublethal doses can disrupt the molt cycle of ovigerousAmerican lobsters (Homarus americanus), causing

124 S.L. Waddy et al. / Aquaculture 269 (2007) 123–129

them to molt prematurely and lose their attached eggs(LOEL=0.22 μg g−1;Waddy et al., 2007).

Emamectin benzoate is released to the environmentthrough uneaten medicated feed, fish faeces, and urine(Telfer et al., 2006). The risk of non-target crustaceansingesting the drug is greatest when salmon farmers areusing SLICE® as the compound is unlikely to bio-concentrate in animals or biomagnify in the food chain(Chukwudebe et al., 1996) and has not been detected inthe water column (Telfer et al., 2006).

The presence of emamectin benzoate residues incrabs (Pagurus spp. and Carcinus maenas) collectednear a farm in Scotland following the use of SLICE®

(Telfer et al., 2006) demonstrates that scavengingcrustaceans will eat medicated feed. American lobstersand other invertebrates aggregate near salmon farms(Iwama, 1991; Findlay et al., 1995), and it has beensuggested that waste fish pellets provide a food supplythat attracts lobsters to the sites (Hargrave, 2003).Although there is no evidence that the use of emamectinbenzoate is having toxic impacts on non-target crusta-ceans (Willis et al., 2005; Telfer et al., 2006), questionsremain about the potential for the drug to disruptmolting and egg production in crustaceans foraging nearsalmon farms (Bright and Dionne, 2005).

In this paper we report on laboratory feeding trials inwhich adult female American lobsters were offered achoice between their natural foods and SLICE®-medicat-ed salmon feed. The results provide the first informationon the feeding response and ingestion rates of a non-targetcrustacean to salmon feed medicated with SLICE® at aconcentration used by the aquaculture industry.

2. Materials and methods

2.1. General methods

2.1.1. Preliminary studyA test was done to determine if emamectin benzoate

can be detected in the digestive glands of lobsters thatingest relatively small quantities of the drug. Eightlobsters were given a controlled dose of 0.06 μg g−1 andsacrificed 12, 24, 48, or 72 h after feeding (see Waddyet al., 2002 for the method of forced oral administrationof the drug). The digestive glands were removed andshipped frozen to Enviro-Test Laboratories/XenosDivision (Nepean, ON) for determination of the quantityof emamectin B1a (the major homolog of emamectinbenzoate) and its major metabolite, desmethyl emamec-tin B1a, in the tissue using the high performance liquidchromatography (HPLC) fluorometric method of Bur-ridge et al. (2004).

2.1.2. FeedSalmon feed was medicated with emamectin benzo-

ate by Enviro-Test Laboratories using SLICE® premix(Schering-Plough Animal Health, Pointe Claire, QC),7.5-mm commercial salmon pellets (Skretting, St.Andrews, NB), and anchovy oil (Skretting). Theconcentration of emamectin benzoate (9.2 μg g−1) onthe pellets was determined using the HPLC method ofBurridge et al. (2004).

Mussels (Mytilus edulis), sea urchins (Strongylocen-trotus droebachiensus), soft-shell clams (Mya arenaria),and crabs (C. maenas) were collected from local beachesand maintained alive until offered to the lobsters. Thelobsters in the communal feeding trials were also givensmall pieces of fresh-frozen shrimp (Pandalus montaguiand P. borealis), squid (Loligo pealei), hake (Merlucciusbilinearis), and herring (Clupea harengus harengus).Seaweed (Ascophyllum nodosum and Fucus sp.) attachedto small rocks was available to the lobsters at all times.

2.1.3. LobstersAdult female American lobsters were purchased from

the commercial fishery in the southern Gulf of St.Lawrence (Miminegash, PE). Prior to the feeding trials,they were fed to satiation on a diet of invertebrates andfish, and held communally with shelter under naturalday length (latitude 45°N) and temperature (varyingfrom 0° to 15 °C seasonally) in flow-through seawater.The lobsters were of a size that molt on a 2-year cycleand spawn in the intervening years (Waddy and Aiken,2005). When the studies began, they were eitherovigerous and in intermolt (mean weight 500 g; moltstage C4) or recently molted and non-ovigerous (meanweight 550 g; molt stages C1 and C2). Molt stages weredetermined by the methods of Aiken (1980). Theovigerous group spawned in July before the feedingtrials began in the late summer and autumn. The post-molt lobsters molted in August prior to being used infeeding trials in September.

2.1.4. Statistical analysisData collected in the study were analyzed using

Sigma-Stat statistical software (version 3.0, Systat Soft-ware Inc., San Jose, CA). Comparisons between groupsof the quantity of food consumed, concentration of drugresidues in the digestive gland, and mean time to moltwere made using the Mann–Whitney rank sum test,Kruskal–Wallis one-way analysis of variance on ranks,or t-test. The relationship between time since ingestionof emamectin benzoate and concentration of residues inthe digestive glands was analyzed using least-squaresanalysis. Comparisons of the proportion of the groups

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eating salmon feed, displaying neurological problems,or having quantifiable levels of the metabolite in theirdigestive gland were done using a z-test or chi-squareanalysis. Differences were considered to be significant ifαb0.05.

2.2. Individual feeding study

Ovigerous lobsters held individually in large tanks(3×1.3×0.5 m deep) were given a choice betweensalmon pellets and invertebrates in a study conductedbetween 4 August and 18 December. Each tank wasdivided longitudinally at the upstream end by an opaqueplastic barrier that kept the two food types separate, butallowed the lobsters to move freely throughout the tank.Incoming water entered behind a baffle that forced thewater to flow uniformly downstream along the bottomof the tank, past the food to the opposite end where theoutflow and the lobster's shelter were located. Duringthe 3-day acclimation period, the lobsters were fed tosatiation on invertebrates.

On each day of the trials, an excess of salmon feed andinvertebrates was placed on opposite sides of the “Y”barrier late in the afternoon. The location of the two typesof food was alternated each day. Crabs and sea urchinswere cut open to prevent them from eating the salmon feedor crawling under the baffle. This also avoided the prob-lem of comparing preference for formulated pellets withthat of mobile prey. Trials were conducted using eitherSLICE®-medicated feed or unmedicated feed. Uneatenfood was removed from the tanks each morning and thequantity recorded. Most of the feeding trials lasted 4 days,but six lobsters were offered a choice of foods for 4 daysone week and 5 days the following week, with a 3-daybreak between when they were offered only invertebrates.

Simulated natural day length was providedwith 40-Wincandescent bulbs controlled by a SunMatch™ system(Aquabiotech Inc., Coaticook, QC). The lobsters weremonitored using low-light video cameras and time-lapserecorders to confirm that food was eaten. The tanks wereilluminated at night with four 25-W incandescent bulbsfiltered through Kodak No. 1a safelight filters. This lowlevel of illumination in the near infrared has nodetectable effect on the diel activity pattern of lobsters(Lawton, 1987; Waddy and Aiken, 1991). Seawatertemperature ranged from 13.5 °C in September to 5.4 °Cin December.

The lobsters were retained for a year after the feedingtrials to determine whether those that ate SLICE®-me-dicated pellets molted prematurely or developed neuro-logical problems such as paralysis of the appendages orproblems maintaining proper orientation.

2.3. Communal feeding study

Two groups of ovigerous intermolt lobsters and twogroups of postmolt lobsters (12 per group, total of 48)were used in 7- and 14-day feeding trials. Each groupwas housed with an excess of shelter at a density of 5.5lobsters m−2 in tanks 0.9×2.4×0.5 m deep. Prior to thetrials, lobsters were acclimated to the holding conditionsfor 8 or 9 days and fed to satiation on invertebrates andfish. Beginning on 17 September, an excess of SLICE®-medicated pellets and other foods was distributedthroughout the tanks late each afternoon. The followingmorning (∼18 h later), uneaten food was removed andthe quantity and weight of the pellets, as well as thenumber of other food items, were recorded. The weightof the retrieved salmon feed was adjusted to account forthe 60% increase in weight that occurs due to absorptionof water, and the 6% loss in mass when pellets are inseawater for that length of time. Seawater temperatureduring the trial ranged from 12.5 to 13.0 °C.

At the end of the feeding trials, the digestive glands ofthe lobsters were removed and shipped frozen to Enviro-Test Laboratories for quantification of drug residues.

3. Results

3.1. Preliminary study

The 8 lobsters given a controlled dose (0.06 μg g−1)of emamectin benzoate had drug residues in theirdigestive glands 12 to 72 h later (mean (SD)=104 (74.9)ng g−1; range: 19 to 248 ng g−1). There was no cor-relation (r2 =0.22, F=0.572) between the concentrationof emamectin B1a in the tissue and time since ingestionof the drug. Residues of the desmethyl emamectin B1ametabolitewere below the level of quantification (0.5 ngg−1)in the 12-h samples, but were present at 24, 48 and 72 h afterexposure (mean (SD)=2.0 (0.5) ng g−1).

3.2. Individual feeding trial—ovigerous intermolt lobsters

3.2.1. Unmedicated feedFifteen (94%) of the 16 lobsters ate salmon feed

during the trial. A significantly greater percentage of thegroup (81%) ate salmon feed on the first day of the trialthan on subsequent days (25% to 38%) (v2 =11.80, 3 df,P=0.008). Mean consumption of salmon pellets wasgreater on day 1 than on subsequent days (mean (SD)=1.7 (1.4) and 0.3 (0.1) pellets, respectively; H=17.02,3 df, Pb0.001). The lobsters consumed similar numbersof invertebrates each day (mean (SD)=1.1 (0.4) items;H=0.24, 3 df, P=0.971).

Fig. 2. Daily consumption of SLICE®-medicated feed by a group ofintermolt ovigerous (molt stage C4) and a group of postmolt (stage C2–3)non-ovigerous lobsters (12 lobsters per group) offered a choice betweentheir natural foods and medicated feed. The postmolt group atesignificantly more medicated feed than the intermolt lobsters (T=142.0,n=14, 14, P=0.005) and both groups ate significantly more medicatedfeed in the first week than the second week (ovigerous: t=2.93, 12 df,P=0.013; postmolt: T=75.00, n=7, 7, P=0.002). Consumption ofmedicated feed by the postmolt group in week 2 was similar to that in the

The six lobsters offered a choice of foods for 9 daysover a 12-day period (days 1 to 4 and 8 to 12) atesignificantly more salmon feed on the first day than onany other day (Fig. 1).

3.2.2. Medicated feedFourteen (78%) of the 18 lobsters ate SLICE®-

medicated feed, and all 14 lobsters ate medicated feedon the first day. There was a significant decrease(v2 =47.41, 3 df, Pb0.001) in the percentage of thelobsters that ate medicated feed after day 1. One lobster(6%) ate some feed on day 3, but none (0%) of thelobsters ate medicated feed on days 2 and 4. The meannumber of pellets consumed on days 2 to 4 was signi-ficantly lower than on day 1 (mean (SD)=0.01 (0.05)pellets vs 1.3 (1.2) pellets, respectively; H=46.63, 3 df,Pb0.001). The estimated dose of emamectin benzoateingested by the lobsters ranged from 0 to 0.04 μg g−1

(mean (SD)=0.01 (0.01) μg g−1).The lobsters consumed similar numbers of inverte-

brates each day (mean (SD)=0.4 (0.2) items; H=1.33, 3df, P=0.722). All the lobsters survived for the year they

Fig. 1. Mean daily consumption of salmon pellets by six ovigerousintermolt lobsters offered a choice of foods that included unmedicatedsalmon feed and a variety of invertebrates. Lobsters were given thischoice of food on days 1 to 4 and 8 to 12, but on days 5 to 7 wereoffered only invertebrates. This trial was conducted to determine if thetypical feeding pattern of consuming significantly more salmon feedon the first day it was encountered would be repeated in the secondweek. Mean consumption of salmon feed on day 1 was significantlygreater than on any other day (mean (SD)=1.8 (1.7) vs 0.3 (0.5)pellets, respectively; T=260.5, n=6.6, P=0.009).

intermolt group in week 1 (t=0.36, 12 df, P=0.725).

were retained for observation, and the mean time of moltwas similar in the groups that ate medicated and un-medicated feed (calendar day 251 (SD 13) and 245 (SD9), respectively; t=1.38, 32 df, P=0.176).

3.3. Communal feeding trial—postmolt and intermoltovigerous lobsters

3.3.1. 7-day groupsThe postmolt group ate significantly more medicated

feed per day than did the intermolt ovigerous group(mean (SD)=6.2 (5.1) g vs 1.0 (0.4) g, respectively;T=28.00, n=7, 7, Pb0.001). The mean dose ingestedby the lobsters was estimated to be 0.01 μg g−1 for theintermolt ovigerous lobsters and 0.06 μg g−1 for thepostmolt lobsters.

The postmolt lobsters consumed twice as manynatural food items as the intermolt group (mean (SD)=26 (6) items and 13 (4) items per day, respectively; t=−4.42, 12 df, Pb0.001). All (100%) the lobsters sur-vived the feeding trial, but two (17%) of the postmoltlobsters became disoriented, lying ventral-side-up intheir shelters (z=0.741, P=0.46). The two disorientedlobsters had less emamectin B1a in their digestive glands(0.9 and 3.6 ng g−1) than did several of the animals (upto 20.3 ng g−1) that did not display any neurologicalproblems.

3.3.2. 14-day groupsThe postmolt group ate significantly more medicated

feed per day than did the intermolt ovigerous group(mean (SD)=2.9 (3.5) g vs 0.96 (0.6) g, respectively)(Fig. 2). Both groups consumed significantly moremedicated feed in the first week of the trial than in thesecond week (Fig. 2). Consumption of medicated feedby the postmolt group in week 2 was similar to that inthe intermolt group in week 1 (Fig. 2). The mean dose ofemamectin benzoate ingested by the postmolt lobsters was0.06 μg g−1 (0.043 μg g−1 in week 1 and 0.012 μg g−1 inweek 2). The intermolt lobsters ingested 0.015 μg g−1 inweek 1 and 0.006 μg g−1 in week 2.

The postmolt group consumed twice as many of thenatural food items as did the intermolt ovigerous group(t=−7.46, 26 df, Pb0.001). Within the groups, con-sumption of these items was similar in the two weeks(postmolt lobsters: 169 vs 182 items per week, t=0.41,12 df, P=0.679; intermolt ovigerous lobsters: 93 vs 84items per week, t=0.49, 12 df, P=0.637). All the lob-sters survived the feeding trial and none developed anyneurological problems.

3.4. Analysis of digestive glands

Emamectin B1a residues were found in the digestiveglands of 87.5% of both the ovigerous and postmoltlobsters. The postmolt 7-day group, which ate moremedicated feed than the other groups, had significantlyhigher levels of emamectin benzoate residues than theother groups (Table 1). However, the residue levels inthe 7-day group were lower than in the group of lobstersgiven a controlled dose of 0.06 μg g−1 (mean (SD)=5.5(5.1) ng g−1 vs 104.0 (74.9) ng g−1, respectively; T=131.0, n=8, 12, Pb0.001).

Table 1Quantity (mean (SD)) of emamectin B1a and its major metabolite,desmethyl emamectin B1a, in the digestive glands of groups ofovigerous and postmolt adult female lobsters (n=12 per group) offereda choice of foods that included SLICE®-medicated pellets and theirnatural foods for 7 or 14 consecutive days

Durationoffeedingtrial

SLICE® residues in lobster digestive glands

Intermolt ovigerous Postmolt

EmamectinB1a (ng g−1)

DMAEMB1a (ng g−1)

EmamectinB1a (ng g−1)

DMAEMB1a (ng g−1)

7 days 1.88 (1.39) 0.30 (0.12) 5.53 (5.12) 0.58 (0.40)14 days 1.76 (1.66) 0.27 (0.08) 2.62 (3.19) 0.35 (0.23)

Emamectin B1a residues in the 7-day postmolt group weresignificantly greater than in the other 3 groups (H=13.33, 3 df,P=0.004; ANOVA). There was no significant difference in thequantity of metabolite residues in the four groups (H=8.33, 3 df,P=0.04; ANOVA).

Similar proportions of the postmolt and intermoltlobsters had quantifiable levels of the desmethyl ema-mectin B1a metabolite in their digestive glands (33% vs13%, respectively, z=1.37, P=0.170) and there was nosignificant difference in the quantity of metabolites in thefour groups (Table 1).

4. Discussion

This study has shown that adult female Americanlobsters will eat SLICE®-medicated feed, but prefertheir natural foods. Regardless of the molt stage of thelobsters, the holding conditions, or whether or not thefeed was medicated with SLICE®, the response to salm-on feed was similar. Most (78 to 94%) lobsters wereinitially attracted to and ate salmon feed, but rapidlybegan feeding selectively on their natural foods. Lob-sters ate significantly more salmon feed on the first daythey encountered it than on any subsequent day. The oneexception was a group of intermolt ovigerous lobstersthat ate very little medicated feed. In the first week of thefeeding trials, postmolt lobsters ate three to six timesmore medicated feed than did intermolt ovigerouslobsters. But in the second week, consumption of medi-cated feed by the postmolt lobsters declined to a levelsimilar to that in the intermolt lobsters in the first week(Fig. 2).

Additional support for the hypothesis that femaleAmerican lobsters quickly lose interest in eating SLICE®-medicated pellets comes from the results of an acutetoxicity test (Burridge et al., 2004). Lobsters offeredsalmon feed containing extremely high concentrations ofSLICE® (72 to 1167 μg g−1) consumed more medicatedfeed on the first day than on any other day and were sooneating little or no medicated feed. van Aggelen et al.(2003) similarly reported that Dungeness crabs (Cancermagister) and spot prawns (Pandalus platyceros) offeredSLICE®-medicated feed in an acute toxicity test eitherignored the feed or, after some initial interest, stoppedeating the feed.

High doses of emamectin benzoate have a negativeeffect on appetence in salmon (Roy et al., 2000). Thedecline in consumption of SLICE®-medicated feed byAmerican lobsters, however, was clearly due to a pre-ference for other foods. In each trial, the consumption ofnatural foods was unaffected by the amount of emamectinbenzoate the lobsters ingested. Even lobsters offeredunmedicated salmon feed preferred natural foods.

Several other studies have found that homarid andspiny lobsters prefer their natural diet to pelleted crus-tacean feeds (Williams, 2007, for review). Althoughlobsters are omnivorous and eat a wide variety of foods,

they are well known for being discriminating in theirfood choices (Evans and Mann, 1977; Childress andJury, 2006; Nelson et al., 2006). Feeding experience(‘conditioning’) influences food preferences (Derby andAtema, 1981) and is involved in the rejection of food(Lee and Myers, 1996). Kurmaly et al. (1990) found thatlarval European lobsters (Homarus gammarus) becameconditioned to reject some formulated feeds. Condition-ing involves both preingestional (e.g., taste, texture, orshape) and postingestional factors (e.g., digestibility ornutritional value), and it appears crustaceans can be-come conditioned by the chemicals in pellets to rejectprepared feed (Lee and Myers, 1996). The results of ourwork suggest that although American lobsters are ini-tially attracted to and eat salmon feed, they rapidly becameconditioned not to select salmon pellets, whether med-icated with SLICE® or not.

Food preferences in the American lobster can varywith the availability of food, molt stage, seawatertemperature, and other factors (Lawton and Lavalli,1995). Our feeding trials were conducted at times of theyear (late summer and autumn) and seawater tempera-tures when farmers in the Bay of Fundy are most likelyto use SLICE®. We included lobsters in both intermoltand postmolt, and lobsters held both communally andindividually in a Y-maze. The response of lobsters tosalmon feed was similar in each trial, suggesting thatSLICE®-medicated feed will not be a preferred food forlobsters under any conditions.

All the lobsters that ate SLICE®-medicated feedsurvived the trials and the ovigerous lobsters molted atthe same time as the control group. Two (8%) of the 24postmolt lobsters developed an orientation problem,which is a common response of arthropods to neuro-toxins (Agee, 1985; Burridge et al., 2004). There was nocorrelation however between the quantity of emamectinB1a in the digestive glands and whether the lobstersbecame disoriented. The lobsters with the highest res-idue levels displayed no obvious neurological problems.

The actual doses of emamectin benzoate ingested bythe lobsters in this study are probably lower than ourestimates, which are based on the amount of salmonfeed recovered from the tanks each morning. Unlikefish, lobsters are slow, intermittent feeders that masticatetheir food externally (D'Abramo and Conklin, 1985).This behaviour causes a significant amount of pelletedfeed to be reduced to a size too small to be consumed(Nelson et al., 2006) and makes it difficult to retrieve allthe uneaten feed (D'Abramo and Conklin, 1985; Houserand Akiyama, 1997). Sheppard et al. (2002), for ex-ample, estimated that small spiny lobsters waste 10% to50% of pelleted feed during the feeding process.

The lobsters in our study had access to SLICE®-medicated feed for ∼18 h per day for up to 2 weeks, andthe drug residues in their digestive glands were similarto those found in hermit crabs collected beneath salmoncages after administration of SLICE®-medicated feed(Telfer et al., 2006). It is difficult to speculate, however,how often lobsters in the Bay of Fundy might encounterfeed or faeces containing emamectin benzoate. Within amanagement area, SLICE®-medicated feed is used 20 to50 days of the year at most because the farms treat theirsalmon at about the same time (Dr. Michael Beattie,New Brunswick Department of Agriculture and Aqua-culture, personal communication). Smolts are treatedwith emamectin benzoate in the hatchery before transferto seawater (SPAH, 2002b), so the first use of medicatedfeed at cage sites does not usually occur until late sum-mer. Feed that passes through a fish cage degrades rapid-ly (b5 days) as it quickly absorbs water and becomesfragile (Stewart and Grant, 2002). Faecal material is lessdense than fish feed, so is subject to more rapid break-up(Stewart and Grant, 2002). Recent improvements tosalmon husbandry practices and innovations such asunderwater sensing technology to monitor and controlfeeding have reduced the amount of waste feed per unitof salmon produced (Cromey et al., 2002; Parsonageand Petrell, 2003). Adult female American lobsters inthis area are very mobile, travelling in excess of 30 kmper year as they move between nearshore and deeperwater with the changing seasons (Campbell, 1986).When SLICE® treatments occur late in the year, mostadult female lobsters will be in deeper water, wherethere are no salmon farms.

The results presented here provide the first informa-tion on the feeding response and ingestion rates of anon-target crustacean to salmon feed medicated withSLICE® at a concentration used by the aquacultureindustry. The most significant finding is that ovigerousintermolt lobsters ate relatively little medicated feed.Postmolt lobsters ate significantly more medicated feedthan the ovigerous lobsters and appear to be more at riskof ingesting enough emamectin benzoate to causeneurological problems, than to affect the molt cycle.However, even the postmolt lobsters did not eat muchmedicated feed after the first week.

Acknowledgments

We thank Dr. Rejean Berman of Schering-PloughAnimal Health for providing the SLICE® premix used toprepare the medicated feed. Krista Brooksbank andVictoria Merritt assisted with the feeding trials. We alsothank Dr. Michael Beattie, NB Department of Agriculture

and Aquaculture, for providing information on the use ofSLICE® by salmon farmers in the Bay of Fundy.

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

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