Over the trial period, L. salmonis numbers increased by 170 in the two control groups. There was no corresponding increase on treated fish and, from days y2 to 21,
lice numbers decreased by 68. At days 14 and 21, both treated groups had signifi- Ž
. cantly P - 0.01 fewer L. salmonis than the two control groups. At day 21, the two
treated groups still had a higher mean number of lice compared to the results in the two summer trials, although the mean numbers of lice on control fish were also much higher.
The number of L. salmonis on any individual control fish was 28–154 while the range on treated fish was only 0–19. At day 21, 20 of fish in the control groups had
characteristic lesions caused by sea lice while only 2.5 of treated fish were affected.
Ž .
At the end of the trial day 22 , all the pens on the site, with the exception of the two pens treated with emamectin benzoate, were subjected to a hydrogen peroxide bath
treatment for sea lice. The two pens treated with emamectin benzoate were not treated with hydrogen peroxide because of low lice numbers and improved appearance of the
fish. Following treatment with hydrogen peroxide there was a decline in the mean
Ž .
number of motile lice on control fish at day 27 Fig. 1a , but chalimus were not reduced Ž
. and increased to more than 100 per fish between days 35 and 55 Fig. 1b . As a result, at
day 55, mean motile lice numbers reached high levels again and a second hydrogen peroxide treatment had to be carried out 5 weeks after the previous treatment. After day
27, lice numbers on fish treated only with emamectin benzoate increased relatively slowly compared to the control group, and at day 55, treated fish still had 78 fewer
chalimus and 86 fewer motile lice than control fish. Between days 22 and 55, fish in the surrounding commercial pens also had much higher numbers of lice than fish in the
pens treated with emamectin benzoate. Prior to treatment, sea lice damage was recorded on only 0–5 of fish in each pen. At day 21, 20 of control fish had sea lice damage
compared to only 2.5 on fish in the two treated groups.
3.4.2. Third trial: efficacy against C. elongatus There were few C. elongatus present during this trial. Mean numbers at the start of
the trial were only 0.1–1.0 per fish and at day 21, the means were 0 and 0.8 per fish in the treated groups and 0 and 1.4 per fish in the control groups.
4. Discussion
Despite the variability in fish weights at the start of the trial and hence, feed consumption, good efficacy was achieved and no adverse reactions or fish mortalities
were attributed to treatment with emamectin benzoate. Treated groups had higher SGR than the control groups, but it is uncertain whether treatment resulted in significant
benefits to fish growth as larger samples sizes would be required to detect statistically significant differences. Fish infested with L. salmonis have shown a decreased feed
consumption correlated with the appearance of head lesions, although growth rates were
Ž .
unaffected Dawson et al., 1997 . No significant differences in growth were found by Ž
. Grimnes and Jakobsen 1996 between fish infested with L. salmonis and un-infested
fish, although heavily infested moribund fish had a lower condition factor. Observations Ž
. from tank Stone et al., 1999 and field trials suggest there was an improved feed
response in lice-infested fish following treatment with emamectin. Further, improve-
ments in growth may be a long-term benefit of effective in-feed medication, as bath treatments require 3–4 days starvation and cause increased stress.
Oral treatment of Atlantic salmon with emamectin benzoate showed good efficacy against both motile and chalimus stages of L. salmonis and C. elongatus in all three
trials, despite the potential for continuous recruitment from adjacent commercial pens. L. salmonis numbers increased over time on control fish by 87–284, whereas over the
same period, numbers were reduced on treated fish by 68–98. The first sea trial was conducted as a pilot study and replicate pens were not evaluated as they were in the
second and third trials, however, useful data were obtained, particularly on C. elongatus, and for this reason, the results are included here. At the start of the second trial, one
treated pen had significantly more L. salmonis and C. elongatus than the other pens. Despite this, fish in this pen still had significantly fewer lice than fish in the two control
pens at days 14 and 21.
Following treatment with emamectin benzoate, 55–80 of fish had no chalimus or motile L. salmonis present. While efficacy against chalimus stages is beneficial in
Ž .
preventing development to the motile stages Stone et al., 1999 , a rapid reduction in motile lice numbers is also important, as they are more damaging to host fish. Motile
lice numbers were reduced by 21–59, as early as day 7, on treated fish. Oral treatment with ivermectin at a dose of 0.05 mg kg
y1
every third day for three or six doses resulted Ž
in good efficacy against both chalimus and adult stages of L. salmonis Johnson and .
Ž Margolis, 1993 . However, ivermectin appears to have a narrow safety margin Johnson
. et al., 1993; Palmer et al., 1997 while no adverse effects have been reported with
emamectin benzoate at up to 3.5 times the therapeutic dose and no mortalities occurred Ž
. at 7 times the therapeutic dose Roy et al., in press .
Most of the treated fish, sampled at day 21, had fewer L. salmonis than any control fish. This suggests all fish received an efficacious dose, despite variability in fish
weights and feeding competition. Most of the L. salmonis present on treated fish at day 21 were either copepodites or small numbers of mature adults. Copepodites represented
recent settlement of larval stages and did not appear to develop further. The transfer of motile stages of L. salmonis between salmon has been recorded in tank and field trials
Ž
. Bruno and Stone, 1990; Ritchie, 1997; Treasurer and Grant, 1997 and it is possible that
some adult lice on treated fish had transferred from untreated fish on the site. Treatment at low temperatures resulted in slower reductions in L. salmonis numbers
and a small number of lice persisted on treated fish at day 21. However, treatment was still effective and treated groups had significantly fewer lice compared to control fish.
Further sampling showed that lice numbers remained much lower on fish treated with emamectin benzoate for at least 55 days from the start of treatment. Although there was
a gradual increase in chalimus numbers on treated fish, there was little increase in motile lice, suggesting that when chalimus settled on treated fish, their development to mature
motile stages was impaired. This confirmed observations in tank trials where attached
Ž chalimus failed to develop on fish were treated with emamectin benzoate Stone et al.,
. 1999 . The settlement of small numbers of copepodites on treated fish, without a
corresponding increase in chalimus, also suggests newly recruited lice fail to develop Ž
. further. Johnson and Margolis 1993 also reported slower development of lice on fish
treated orally with ivermectin.
C. elongatus numbers are present in greater numbers when sea temperatures are high Ž
. Hogans and Trudeau, 1989; Tully, 1989 . In the two summer trials, mean numbers of
C. elongatus were high initially and declined on control fish over time as sea temperatures fell. In the third trial, carried out in April, there were few C. elongatus
present and these probably represented over-wintering populations. Treatment with emamectin benzoate was also effective against C. elongatus and numbers declined more
rapidly on treated fish than on control fish. C. elongatus is not host-specific and may be
Ž .
derived from wild fish Wootten et al., 1982 . Motile stages of C. elongatus may transfer between individual salmon and, from infested saithe, Pollachius Õirens, to
Ž .
cohabiting salmon Bruno and Stone, 1990 . Transfer from wild and farmed fish in the area may account for the persistence of low numbers of motile C. elongatus on fish
treated with emamectin. Treated fish also had low numbers of C. elongatus copepodites, although in contrast to the control fish, there were few chalimus present. Again, this
suggests treatment prevented further development of C. elongatus copepodites.
Bath immersion treatment of the commercial pens with Aquagard
w
, in the first trial, did not reduce numbers of motile L. salmonis on the control fish, confirming that the
tarpaulins protected the trial fish. Hydrogen peroxide is not effective in removing chalimus stages of L. salmonis
Ž .
McAndrew et al., 1998 and this was evident from lice numbers following treatment of control fish in the third trial. As a result, although motile lice were reduced on control
fish following peroxide treatment at day 22, their numbers rapidly increased again, necessitating another peroxide treatment only 33 days later. Motile lice exposed to
Ž .
hydrogen peroxide may recover Treasurer and Grant, 1997; McAndrew et al., 1998 and could potentially re-infest fish and this may also have contributed to the rapid
recovery of motile lice numbers on control fish. In each trial, the prevalence of lice damage increased on control fish over time, while
the number of lesions on treated fish was reduced and, overall, lice damage was 15–75 lower on treated fish.
In all three trials, treated pens were surrounded by large commercial stocks of fish heavily infested with sea lice, creating a large potential source of copepodite stages.
Despite this, very good efficacy was achieved against both L. salmonis and C. elongatus and development of newly recruited larval lice was prevented. When entire sites are
treated, the potential for re-infestation will be reduced and the recovery of lice numbers may take even longer, allowing the frequency of treatments to be reduced. In-feed
treatments will also permit simultaneous medication of all pens on a site and all sites in a single area, thus reducing any cross-infestation that may occur during the time
necessary to apply bath treatments to all pens.
5. Conclusion
