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
2.1. Fish and holding conditions Three trials were carried out on commercial salmon farms on the northwest coast of
Scotland. The trials were conducted on pontoons of pens stocked with Atlantic salmon, S. salar. Fish were selected from the main ongrowing pens at each site and acclimatised
to small net pens of 12, 27 or 102 m
3
as described in Experimental design. Fish were naturally infested with L. salmonis and C. elongatus and were exposed potentially to
re-infestation by larval sea lice throughout the duration of the trial. In each trial, lice numbers were similar on fish in the surrounding commercial pens to those of the control
fish, prior to the start of treatment. All control and treated fish sampled for sea lice
Ž .
evaluation were weighed at each time point N s 20 or 30 fish . 2.2. Medicated feed
Ž .
The basal ration used was Trouw UK Wincham, Cheshire, UK 4.0 or 8.5 mm Ž
Y Y
. salmon feed pellets. Emamectin benzoate 4 -deoxy-4 epimethylaminoavermectin B
1
was dissolved in propylene glycol and mixed with fish oil prior to coating the feed pellets. Control feeds were prepared with propylene glycol and fish oil only. Treatment
was administered at a nominal dose of 50 mg kg
y1
fish biomass per day. All fish were given an initial unmedicated feed at a rate of 0.1–0.25 biomass at the start of the day
and medicated or control feeds were given at a rate of 0.4 or 0.5 biomass, 3 to 4 h Ž
. later. Medicated feed was administered for a period of 7 consecutive days days 0–6 .
Feeding rates and methods were identical for all control and treated groups in each trial. 2.3. EÕaluation of sea lice
At day y1 or y2, 20 or 30 fish were randomly selected from each pen to determine pre-treatment numbers of sea lice. Each fish was examined using a hand lens or low
power microscope and all attached lice removed and fixed in 5 formalin. Lice were identified as L. salmonis or C. elongatus and the number of chalimus I, II, III, IV,
pre-adult I, pre-adult II and adult stages recorded. Pre-adult and adult lice were further identified as males or females. Evaluation of sea lice numbers was carried out again at 7,
14 and 21 days from the start of treatment. In the third trial, additional counts of sea lice on five fish per pen in control pen II and treated pen I were conducted by site staff on
days 27, 35, 42, 48 and 55. For these counts, chalimus were counted only up to a
maximum of 100 per fish. The occurrence of sea lice damage was determined on sampled fish by visual assessment of typical epidermal lesions.
2.4. Experimental design The three trials were conducted blind with the identity of each treatment and control
ration unknown to personnel responsible for feed administration and collection of data. Treatments were randomly allocated to each trial pen.
2.4.1. First trial Two pens, one control and one treated, were used, each holding 180 fish at the start
of the trial. Each pen had a volume of 12 m
3
and they were located on a pontoon midway between 28 commercial pens, each measuring approximately 1150 m
3
. All pens were stocked with fish of the same year class in February at a density of 6.0–6.6 kg
m
y3
and the trial was started in August. The sea temperature over the trial period was 13.0–15.58C and salinity 35 ppt. None of the fish on the site had been treated previously
for sea lice. On day 8, fish in the surrounding commercial pens were treated for sea lice Ž
w
. with dichlorvos Aquagard , Novartis . The trial pens were surrounded by a fully
enclosed tarpaulin for the duration of this treatment, and for 30 min post-treatment, to protect fish from exposure to the treatment compound, which was rapidly flushed from
Ž
y1
. the site by high current speeds average 8–10 cm s
at 2 m depth . Pre-treatment fish Ž
. weights were 268 to 625 g mean 438 72 g S.D. . The sample size for parasite
enumeration was 30 fish per pen. 2.4.2. Second trial
There were two control and two treated pens with 149 fish per pen at the start of the
3
Ž
3
. trial. Each pen had a volume of 27 m
stocking density 2.8 kg m and was located on
the same pontoon described above with each commercial pen stocked at 6.7 kg m
3
. The pens were stocked in February and the trial started in September. The sea temperature
over the trial period was 13.8–14.28C and salinity 33–35 ppt. Trial fish had been treated Ž
w
. previously for sea lice with the organophosphate, dichlorvos Aquagard , Novartis 27
and 47 days before the start of the trial. Pre-treatment fish weights were 251 to 878 g Ž
. mean 513 136 g S.D. . The sample size for parasite enumeration was 20 fish per pen.
2.4.3. Third trial The trial consisted of two control and two treated pens with 360 fish per pen at a
density of 9.4 kg m
3
. This trial used larger pens each with a volume of 102 m
3
, located on a pontoon of 32 pens of similar size and stocking density. All pens were stocked with
fish of the same year class in April of the previous year and fish were 1026–4464 g Ž
. mean 2662 597 g S.D. in weight. There were also three large commercial rearing
units located within 1 km of this site. The trial started in April when the sea temperature was 7.2–8.58C and salinity 31–33 ppt. The fish had been treated for sea lice with
Ž
w
. dichlorvos Aquagard
in the previous summer and with hydrogen peroxide 20 days before the start of the trial. The entire site, including the two control pens, but not the
pens treated with emamectin benzoate, was subjected to a bath treatment with hydrogen
Ž .
peroxide on day 22 after the end of the trial day 21 . The sample size for parasite enumeration was 20 fish per pen.
2.5. Data handling Ž
Parasite counts were summarised as chalimus copepodites, chalimus stages I, II, III .
Ž .
Ž and IV , motile lice pre-adult and adult stages and total lice chalimus and motile
. stages combined and the arithmetic means calculated. Parasite count data were sub-
jected to F-tests for homogeneity of variances and a correlation test to examine the normality of distribution. Lice numbers were analysed using the non-parametric STP test
Ž .
Sokhal and Rohlf, 1981 . In the two replicated trials, the data were analysed separately for each replicate pen.
The percentage reduction in mean total sea lice, over the trial period was calculated as follows:
Mean of control or treated replicates at day y1 Reduction s 100 y 100 =
.
ž
Mean of control or treated replicates at day 21 The efficacy or percentage reduction in mean total sea lice, relative to the control
groups, was calculated as follows: Mean of treated replicates
Efficacy s 100 y 100 = .
ž
Mean of the control replicates Owing to the high standard deviation of fish weights and the small sample size,
statistical analysis of fish weights was not appropriate in these trials. Specific growth Ž
. rates SGR were calculated as follows:
log e W y log e W
t
SGR s t
where W is the mean weight at day 21 and W is the mean weight at day y1 or y2.
t
3. Results
