Sea Louse Infestations on Wild Sea Trout in Loch na Kiel (Ba mouth)
2003-2004
Report by
Kjersti Birkeland & Alan Kettle-white
Argyll Fisheries Trust
October 2004
1. INTRODUCTION
1.1. LOUSE SURVEYS AND THE AMA PROCESS
During 2002, Fisheries Trusts on the west coast of Scotland and the Western Isles began monitoring
sea louse burdens on wild sea trout post-smolts, as indicator of infection pressure on wild sea trout
and salmon. This was part of a joint Fisheries Research Services (FRS)/Association of West Coast
Fisheries Trusts (AWCFT) project and sponsored by the Tripartite Working Group (TWG). From 2003
funding for the work has been received from Highland & Island Enterprise, in support of the Tripartite
initiative. The overall objective of this initiative is to improve the health of wild and farmed fish stocks
through the development of Area Management Agreements (AMAs) in coastal and inland waters.
One of the AMA project aims is to monitor and analyse the health status of wild and farmed fish
populations, including annual monitoring of sea lice burdens on wild salmonids. The lice data thus
produced are fed back to the Area Management Group and help to inform best practice on lice
management.
1.2. THE WEST MULL AMA
The West Mull AMA was signed on the 29th of April 2004, and includes the marine waters of Loch
Scridain and Loch na Keal, as well as the main freshwater system draining into these areas.
Pan Fish Scotland have two operational salmon farms in Loch na Keal and one farm in Loch Scridain.
The farm in Loch Scridain has been fallow since August 2002. Scottish Sea Farms operates one
salmon Farm in Loch Tuath but was used by Pan Fish Scotland in 2004 for a short period.
Salmon are stocked into the area in the autumn (S0) of even years, last time in 2002. Harvesting
began in late winter 2004, and all cages were fallow from beginning of August 2004. The area will be
fallow for approximately 10 weeks before re-stocking in October 2004.
Recording of sea lice infestations on the wild sea trout in the area began in 2003. This report presents
sea lice data from sea trout collected from the moth of River Ba in spring/ summer 2003 and 2004.
2. METHODS
2.1. SURVEY SITE
Lice data from wild salmonids were collected from the mouth of Loch Ba (NM 530 410)
2.2. CAPTURE AND PROCESSING
Surveys were conducted by Fisheries Trust biologists from Argyll Fisheries Trust. Volunteers from Mull
DSFB, Pan Fish Scotland and Scottish Sea Farms provided assistance with the surveys.
Samples of sea trout and salmon were collected during May - July in 2003 and 2004 using beach
seine nets. Fish were anaesthetised and placed in white trays to facilitate louse counting, and the
presence/absence, number, and life stage of the salmon lice were recorded for each fish. The salmon
lice were classified into three stages:
1. attached (copepodids and chalimus)
2. mobile (sub-adults and adults excluding ovigerous females)
3. adult females with eggs (gravid or ovigerous females)
Other details recorded were:
a. fish species
b. fork length of the fish
c. age (scales taken)
d. extent of louse damage to the dorsal fin
e. spots on fins (indicative of previous louse infection followed by return to fresh water for delousing)
f. numbers of Caligus elongatus, a louse species which can parasitise a number of hosts
g. signs of predator damage
Before being returned to sea each fish was adipose fin clipped, in order to recognise the fish as a
recapture during later surveys.
2.3. TREATMENT OF DATA
For each site where sufficiently large samples were obtained, data are presented as:
Prevalence:
Abundance:
Intensity:
Maximum:
The percentage of fish in the sample infected by salmon lice
The mean number of salmon lice per fish in the whole sample
The mean number of salmon lice per infected fish
The maximum number of salmon lice found on a single fish in the sample
3. RESULTS
3.1. RESULTS FROM 2003
The mouth of River Ba was netted on two occasions in 2003, 20th of June and 9th of July.
Seven fish were caught in June, and none of these carried any lice. When netting the river
mouth again circa three weeks later a total of 32 fish were caught, one being a parr and
excluded from the sample. Just over 50% of the fish carried lice, lice abundance was 5.9 lice,
and intensity 10.8 lice. Four fish carried more than 20 lice and the maximum number of lice
recorded was 73 lice (Table 1; Fig 1).
Table 1: Fish data and infestation parameters for salmon lice on fish caught in Loch na Kiel
(Ba mouth) in 2003
Sample
date
20.06.03
09.07.03
Combined
No fish
caught
7
31
38
Av. fish
length (cm)
17.9
18.8
18.7
Recapture
(%)
12.5
10.5
Louse
Prevalence
0
55
45
Louse
Abundance
0
5.9
4.8
Louse
Intensity
0
10.8
10.8
Max no
lice
0
73
73
Juvenile lice stages dominated on the fish, but some older lice stages were also seen,
including gravid female lice (Fig 2). All gravid lice were recorded on one fish, this was a 28.5
cm long sea trout.
Number
Sea lice on sea trout, Ba mouth, Loch na Kiel 2003
80
70
60
50
40
30
20
10
0
20th June
9th July
No. Samples
Prevalence
(%)
Intensity
Max No. Lice
Lice development stages, Ba mouth 2003
100
Percentage
Fig 1: Sea louse burdens on
sea trout from the mouth of
River Ba in June and July
2003
Juveniles
Mobile
Gravids
80
60
Fig 2: Lice development
stages on sea trout from the
mouth of River Ba in June
and July 2003
40
20
0
20th June
9th July
3.2. RESULTS FROM 2004
The mouth of River Ba was netted on two occasions in 2004, 17 th of May and 28th of June. Nine fish
were caught in May, of which four carried lice (44%) (Table 2, Fig 3). The infested fish were mainly
larger sea trout (average of 27 cm). In June only five fish were caught, of which four were small and
had a brown coloration. It is doubtful as to whether any of these smaller fish had been to sea. Neither
of the fish caught in June carried any lice (Table 2, Fig 3).
A high proportion of the fish caught in May were recaptures from the previous year, i.e. 56 %. The
only large fish (27cm) caught in June was also a recapture from the 2003 survey.
Table 2: Fish data and infestation parameters for salmon lice on fish caught in Loch na Kiel (Ba
mouth) in 2004
Sample
No fish
Av. fish
Recaptures
Louse
Louse
Louse
Max no
date
caught
length (cm)
(%)
Prevalence Abundance Intensity
lice
17.05.04
9
25.1
56
44
10.3
23.3
62
28.06.04
5
16.4
20
0
0
0
0
Combined
14
22.0
43
28.6
6.6
23.3
62
Juvenile lice stages dominated on the fish. One fish carried older lice stages, including gravid
lice, as well as juvenile lice stages (fig. 4). This was a 32 cm long recaptured sea trout that
carried a total of 62 lice.
Fig 3: Sea louse burdens
on sea trout from the
mouth of River Ba in May
and June 2004
Number
Sea Lice on sea trout, Ba mouth, 2004
70
60
50
40
30
20
10
0
17 May
28 June
No. Samples
Prevalence (%)
Intensity
Max No. Lice
Lice development stages, Ba mouth, 2004
Percentage
100
Juveniles
80
Mobile
60
Gravids
Fig 4: Lice development
stages on sea trout from
the mouth of River Ba in
May and June 2004
40
20
0
17 May
28 June
3.3. RESULTS FROM 2005
The mouth of River Ba was netted on three occasions in 2005, 14 th of June and the 11th and 12th of
July. Six fish were caught in June, all of which carried no lice (Table 3, Fig 4). The infested fish were
mainly larger sea trout (average of 27 cm). In June only five fish were caught, of which four were small
and had a brown coloration. It is doubtful as to whether any of these smaller fish had been to sea.
Neither of the fish caught in June carried any lice (Table 2, Fig 3).
A high proportion of the fish caught in May were recaptures from the previous year, i.e. 56 %. The
only large fish (27cm) caught in June was also a recapture from the 2003 survey.
Table 3: Fish data and infestation parameters for salmon lice on fish caught in Loch na Kiel (Ba
mouth) in 2005
Sample
No fish
Av. fish
Recaptures
Louse
Louse
Louse
Max no
date
caught
length (cm)
(%)
Prevalence Abundance Intensity
lice
14.06.05
6
14.1
0
0
0
0
0
11.07.05
11
19.2
10
27
12
44
70
12.07.05
0
Combined
17
17.3
6
17.6
7.8
44
70
Juvenile lice stages dominated on the fish. One fish carried older lice stages, including gravid
lice, as well as juvenile lice stages (fig. 4). This was a 32 cm long recaptured sea trout that
carried a total of 62 lice.
Fig 3: Sea louse burdens
on sea trout from the
mouth of River Ba in May
and June 2004
Number
Sea Lice on sea trout, Ba mouth, 2004
70
60
50
40
30
20
10
0
17 May
28 June
No. Samples
Prevalence (%)
Intensity
Max No. Lice
Lice development stages, Ba mouth, 2004
Percentage
100
Juveniles
80
Mobile
60
Gravids
40
20
0
17 May
28 June
Fig 4: Lice development
stages on sea trout from
the mouth of River Ba in
May and June 2004
4.CONCLUSION
Data on sea louse burdens on the sea trout post smolts in Loch na Keal (the mouth of River Ba) were
collected during May, June and July 2003 and 2004.
A relatively high number of post smolts were captured from the river mouth in July 2003, but despite
considerable effort few fish were captured from the same location on other survey dates. The low
number of samples, particularly in 2004, makes it difficult to make firm conclusions on lice burdens on
the sea trout in Loch na Kiel. Furthermore, it is not known how representative the samples are for the
River Ba sea trout population as a whole. Still, in both years lice prevalence reached 40 – 50%. Of
infested fish, average lice numbers were relatively moderate in 2003 (intensity of 11 lice), increasing to
23 lice in 2004, i.e. when the local fish farms were in their second year of production. Since most of
the lice on the fish were juvenile stages this indicates that lice infestations were recent, i.e. infestation
had occurred within the last one to two weeks prior to being sampled. This further suggests that the
fish had been infested locally.
Although average infestations levels were low to moderate there were individual fish that carried a
high lice burden (max infestation of 73 lice on 2003 and 62 lice in 2004). These more heavily infested
fish tended to be relatively large, indicating that they had spent more time in the marine environment.
This was also supported by the finding of older lice stages on some of these fish.
No monitoring of lice burdens on the sea trout population in Loch na Keal has taken place prior to
2003, and it is therefore not possible to know how the recorded lice burdens corresponds to lice
burdens in previous years. Changes in management practices of farmed fish (which include single
year class production and fallowing periods) in other management areas, such as Loch Fyne, have
demonstrated positive effects on wild fish health and survival in a relatively short time period (within
one production cycle). The similar farmed fish management of the West Mull area should hopefully
bring similar positive results to wild fish abundance over time. Future monitoring of lice burdens and
stock numbers will be required to inform the management group of trends in wild fish abundance.
The high recapture rate (56%) of sea trout in 2004 of fish marked during the 2003 survey are
concerning and indicates that there are low numbers of trout in this area. The electrofishing surveys of
juvenile fish in the Ba catchment conducted in 2003 and the rod catch data also appear to support the
view that trout numbers are very poor. Correspondingly, local recreational fisheries should make every
effort to minimise the exploitation of sea trout through a policy of catch and release of all sea and
brown trout. Such positive actions taken by fishery managers will go hand in hand with the improved
management of lice on farmed fish toward the restoration of the sea trout population of the Ba and its’
neighbouring catchments in the West Mull area.