third trial, both control pens were treated with hydrogen peroxide owing to heavy lice burdens. However, L. salmonis numbers rapidly increased again and at day 55, fish treated only with
emamectin benzoate still had 80 fewer lice than control fish. In the two summer trials, large numbers of C. elongatus were rapidly reduced by treatment with 82–84 efficacy by day 21.
Despite the potential for continuous re-infestation, oral treatment with emamectin benzoate presented an effective means of controlling all parasitic stages of L. salmonis and C. elongatus on
farmed salmon, and in one trial, numbers remained lower on treated fish for at least 55 days. q
2000 Elsevier Science B.V. All rights reserved.
Keywords: Sea lice; Lepeophtheirus salmonis; Caligus elongatus; Salmo salar; Emamectin benzoate
1. Introduction
Ž . The success of Atlantic salmon, Salmo salar
L. , farming operations may be seriously affected by parasitic copepods, commonly known as sea lice, Lepeophtheirus
Ž .
Ž .
salmonis Krøyer and Caligus elongatus Nordmann . Control of these parasites is largely dependent on the use of immersion bath treatments. These are carried out using
Ž
w w
. hydrogen peroxide Salartect , Brenntag, Paramove , Solvay-Interox or the synthetic
Ž
w
. Ž
w
. pyrethoids, cypermethrin Excis , Vericore and deltamethrin Alphamax , Alpharma .
Ž
w
. Ž
The organophosphates, dichlorvos Aquagard , Novartis and azamethiphos Salmo-
w
. Ž
san , Novartis are also used, depending on the regulations of individual countries Roth .
et al., 1993 . However, immersion treatments are impractical for use on exposed sites and during adverse weather conditions, and the procedures involved are very stressful to
Ž .
fish. With the exception of cypermethrin Jakobsen and Holm, 1990 , bath treatments are only effective against pre-adult and adult stages of sea lice, allowing chalimus stages
to survive and continue the cycle of infestation. Hydrogen peroxide may cause damage to the gills and its use is restricted in summer owing to its toxicity at higher water
Ž .
temperatures Thomassen, 1993; Bruno and Raynard, 1994 . Resistance to the
Ž organophosphate, dichlorvos has been identified in some populations of sea lice Jones
. et al., 1992 . As a result of these limitations, treatments are being developed that can be
Ž
w
. administered in feed. Two insect growth regulators, diflubenzuron Lepsidon , Ewos
Ž
w
. Ž .
and teflubenzuron Calicide , Nutreco Erdal, 1997 are available in Norway as in-feed
Ž treatments. Their mode of action is the inhibition of chitin synthesis Horst and Walker,
. 1996 and activity is therefore restricted to the molting stages of sea lice. Ivermectin has
Ž also been used as an in-feed treatment Johnson and Margolis, 1993; Smith et al., 1993;
. Palmer et al., 1997 but can only be prescribed in the UK under the cascade procedure
Ž .
where authorised products fail to provide effective control Anonymous, 1998 . Ž
Y Y
. The efficacy of emamectin benzoate 4 -deoxy-4 epimethylaminoavermectin B
as
1
an in-feed treatment for Atlantic salmon was initially demonstrated in a series of Ž
. replicated tank trials employing induced infestations of L. salmonis Stone et al., 1999 .
A dose of 50 mg kg
y1
day
y1
for 7 consecutive days was found to be effective against both adult and immature stages of L. salmonis.
The sea trials reported here evaluated the efficacy of emamectin benzoate under field conditions, where fish may be continuously exposed to re-infestation. Two trials carried
out during the summer allowed evaluation of both L. salmonis and C. elongatus, the species commonly infesting farmed fish in the UK. These trials examined treatment of
post-smolts in their first year at sea. The third trial evaluated efficacy in April when sea temperatures were lower and allowed assessment of larger fish in their second year at
sea.
2. Materials and methods
