Wild sea trout sampling – Loch Fyne 2005
Kjersti Birkeland & Alan Kettle-white Argyll Fisheries Trust
1. INTRODUCTION
1.1. SEA LICE ON FARMED AND WILD SALMONIDS The salmon louse Lepeophtheirus salmonis is a naturally occurring parasite of wild salmon and sea trout. However, its abundanc e in some areas has increas ed dramatically in recent years (Tully et al. 1993a,b) due to increased host numbers in the form of caged salmon, present in coastal areas throughout the year. The biology and lifecycle of sea lice has recently been reviewed by Pike & Wadsworth (2000). Louse larvae are planktonic, i.e. they are free -living in the sea, moving with currents and tides for several days after hatching. The duration of the planktonic stages is between 2 and 10 day s depending on water temperature (Johnson & Albright 1991). A fter a number of moults the larvae become copepodids; this is the infectious stage. When a copepodid encounters a suitable host it settles there, attaching itself to the skin of the fish, where it feeds and grows, moulting regularly. These attached stages are called chalimi. After a number of moults the louse becomes mobile, and while generally staying on the same fish it settled on, it now moves around grazing the skin and mucus of its host. On reaching mat urity the lice mate, and the females produce long strands of fertile eggs which are then released into the sea to hatch and become new planktonic larvae, beginning the cycle once more. The large numbers of fish c oncent rated in salmon farms mean that even when the number of lice per fish is low, there can still be an unnat urally high concentration of ovigerous (egg-bearing) female lice wit hin the farm area, leading to increas ed densities of louse larvae in surrounding waters. Lice are controlled on salmon farms by a combination of chemic al treatments and management actions. Larvae arising from salmon farms re-infect fish on the same farm and may also affect neighbouring farms and wild fish sharing the same waters. Sea trout are probably more vulnerable to louse infection than salmon since they remain in inshore waters where most fish farms are located, and where lice larvae are most abundant. Salmon, in contrast, move rapidly out to sea. Although sea trout kelts can be badly affected by lice, it seems post smolts are most at risk due to their smaller size and the physiological stress associated with recent trans fer to seawater. Research suggests that more than about 12 mobile lice for s almon smolts and around 25 -30 for sea trout smolts can cause stress to the hosts and skin lesions which can lead to a failure of the fishes’ ability to regulate its water balance (Finstad et al. 2000: Grimnes & Jakobs en 1996). The fish will die unless the lice are removed or lost. The lethal infection level will vary with fish size, larger fish generally being able to tolerate higher levels of infection than smaller fish. A proportion of infected sea trout smolts and kelts return early to freshwater instead of remaining at sea to feed. This ‘early returning’ phenomenon is well documented and has been shown to be a reaction to louse infection. The lice die and drop off after a few days in fresh water, and unless suffering from secondary infections in freshwater (e.g. Saprolegnia), this response to lice infestations may increase the fishes’ probability of survival. While likely to be adaptive, early return may be at the cost of lower growth and reduced fecundity due to loss of marine feeding opportunities. There is now strong evidence that salmon can also be affected by lice - especially in long fjord-like sea lochs with many salmon farms (in such areas fish have a long way to travel before reaching open and relatively louse-free waters). Whether s almon also suffer from prolonged exposure after leaving s ea loc hs
is unclear as little is known about where they go bet ween leaving the loc hs and reaching the shelf currents where they feed and migrate north. The combination of increased mortality through severe louse infections and poor growth due to reduced feeding time at sea has been associated with declines and collapses of salmonid stocks in Ireland, Scotland and Norway (Whelan 1991; Tully et al 1993a,b; ICES 1997). It is acknowledged that a number of other factors may contribute to declines in fish numbers, but are these outwith the scope of this report.
1.2. 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 Researc h Services (FRS)/Association of West Coast Fisheries Trusts (AWCFT) project and spons ored by the Tripartite Working Group (TWG). From 2003 funding for the work has been received from Highland & Island Enterprise, in support of the Tripartite Working Group (TWG) 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 dat a thus produced are fed back to the A rea Management Group and help to inform best practice on lice management. 1.3. LOCH FYNE AMA The management area contains one productions area, and farms within the management area are stocked with fish of the same age class. Salmon farms in Loch Fyne were stocked with smolt in autumn 2003. Stocking was preceded by a co -ordinated fallow period for approximately covering all salmon farms in the production area i.e. all salmon farm sites were empty at the same time. This production cycle will end in summer 2005 when salmon farms will again be fallowed prior to stocking wit h the autumn 2005 input.
2: METHODS 2.1 SURVEY SIT ES This report presents sea lice data from wild salmonids within the Upper Loch Fyne , i.e. Dubh Loch (NN114109) and Loch Fyne head (NN185124) management area during spring/ early summer 2005. Sea lice levels on sea trout in Dubh Loch have been investigated since 2000, while Loch Fyne head was introduced as s survey site in 2005. The surveys were conducted by biologists from Argyll Fisheries Trust. Volunteers from Fyne DSFB and Pan Fish Scotland provided assistance with the surveys. 2.2. CAPTURE AND PROCESSING Fish caught were anaesthetised and placed in a white tray to facilitate louse counting. The presence/abs ence, number, and life stage(s) of the salmon louse were recorded for each fish. The salmon lice were classified into three stages: 1. Attached (copepodids and chalimus) 2. Mobile (s ub-adults and adults excluding gravid females) 3. Gravid (adult females with eggs) 2.3. TREATMENT OF DATA For each site where sufficiently large samples were obtained, data are presented as: Prevalence: The percent age of fish in the sample infected by salmon lice Abundanc e: The mean number of salmon lice per fish in the whole sample Intensity: The mean number of salmon lice per infected fish
Maximum:
The maximum number of salmon lice found on a single fish in the sample
The Dubh Loch is a transitional loch depending on the stage and strengt h of the tide (i.e. mixture of fresh and sea water) and holds both sea and resident brown trout. It was decided to exclude fish smaller than 120 mm and/ or of brown colour from t he analyses, as it is not known whether these fish will have been to sea, and hence could have skewed the results.
3: RES ULTS The two sites were netted on the in-coming high tide, using a beac h seine net, on four occasions in 2005, th th st th th i.e. on the 5 May, 17 May, 31 May and 16 June. On the 5 of May it was not possible to sample Loch Fyne head due to strong winds.
3.1 DUBH LOCH Date No Fish Av fi sh length (mm) Prevalence Abundance Intensity Max Lice stage s (%)
Attached 5 May 17 May 31 May 16 June Tot 05 66 78 43 10 197 163.9 168.6 181.2 189.1 169.8 15.2 33.3 34.9 50.0 28.4 2.1 3.3 2.3 21.3 3.6 13.8 9.8 6.5 42.6 12.6 57 91 31 67 91 48.6 34.8 28.9 70.4 47.4
Mobile 48.6 62.9 67.0 22.1 48.3
Gravid 2.8 2.3 4.1 7.5 4.3
A total of 197 sea trout were caught from Dubh Loch in 2005. The prevalence (i.e. proportion of fish carrying lice) increased from 15.2% in early May to 50% in June. In May lice infestations were relatively low, with most fish carrying a few lice only. The heaviest infested fish were all above 200mm in lengt h. In June however, infestation levels were significantly higher. The dominating lice stages in June wer e the attached lice stages, indicating a recent infestation.
3.2 LOCH FYNE HEAD Date No Fish Av fi sh length (mm) Prev Abunda nce Intensity Max Lice stage s (%)
Attached 17 May 31 May 16 June Tot 05 71 58 18 147 195.2 197.7 207.9 197.8 57.7 32.2 50.0 54.3 2.7 1.2 7.1 2.6 4.6 3.5 14.2 5.5 64 36 61 64 60.5 65.7 79.7 67.8
Mobile 34.7 31.3 18.8 28.8
Gravid 4.7 3.0 1.6 3.4
A total of 147 sea trout were caught from the head of Loch Fyne. In May infestation levels were generally low. Most fish carried one or two lice although some of the larger trout (>200mm) carried a much higher lice burden. In June however infestation levels had increased on the sea trout, and the smallest fish th (<200mm) carried the highest lice burdens. The heaviest infest ed sea trout caught on the 16 of June was177mm long and carried 61 sea lice. The dominating lice stages throughout the sampling period were the attached lice stages, indicating an on-going infestation throughout the spring.
3.6. HISTORIC SEA LICE DATA – DUBH LOCH Argyll Fisheries Trust biologists have ex amined sea lice infestation levels on sea trout from Dubh Loch since 2000. Farms in Loch Fyne have been on synchronous production from October 2001, while the Loch contained mixed year classes prior to that. In 2000 sea lice levels were very high on t he trout. Up t o 200 lice were recorded on individual fish and the infestation intensity was close to 40 lice per fish. Similar lice burdens have not been recorded since, and the majority of fish examined in recent years carry so few lice that it is unlikely these will cause fish mortality. Pan Fish Scotland No clear pattern in infestation levels between 1 and 2 year of production of neighbouring fish farms have been recorded in Dubh Loch, although the prevalence is higher when farms in Loc h Fyne are in their second year of production. The high intensity recorded in 2004 is due to one infested sea trout, 238 mm long. Year 2000 2001* 2002 2003 2004 2005
* st nd
No Fish 32 78 36 32 45 197
Prevalence 90.6 41.4 33.3 71.9 2.2 28.4
Abundance 33.1 3.8 2.9 4.6 0.7 3.6
Intensity 36.5 9.2 8.8 7.9 33.0 12.6
Max 200 50 42 48 33 91
4: SUMMARY AND CONCLUSION: Data on sea louse burdens on the sea trout post in Upper Loch Fyne (Dubh Loc h and Loch Fyne Head) were collected during May and June 2005. In May very few lice were s een on the post smolts in Dubh Loch although some of the older sea trout caught carried a significant higher number of sea lice. These fish had probably been to sea for a longer period (some may have over-wintered in the sea), and could also have travelled significant distanc es during their marine phase. Almost 50% of the lice stages seen on these fish were pre-adult lice stages, showing that they had been infested several weeks prior to s urvey dates. There were also a significant proportion of attached (juvenile lice stages ), suggesting a continuous infestation process. Grazing damage to the dorsal fin and skin was evident on some individual fish at this time. In June lic e infestation had increased, both with respect to number of fish being infested and lice numbers per fish. The dominating lice stages were attached (juvenile) lice stages – showing a recent infestation. However, a proportion (40% ) of the fish hosted concerning numbers of lice (more than 30 lice) at this time. A similar infestation pattern was seen on fis h from Loch Fyne Head. In the beginning of the netting season very few heavily fish were seen, and most fish carried only a couple of lice. The highest infestations were
*
In 2000 and 2001 fish, although fish smaller than 120mm were excluded from the material brown coloured fish may not have been excluded. Hence, some precaution must be exercised when comparing the 2000 and 2001 data with data collected in later years
recorded in June. More than 70% of the lice were attached (juvenile) lice stages, indicating that fish had recently been infested. Lice burdens were low on the wild fish during the early spring, despite local farms being in the second year of production, which indicates a positive response to the AMA initiatives to cont rol of lic e numbers in the loch. June data however, showed an increased and ongoing infestation, both in Dubh Loch and Loch Fyne Head. These lice burdens will most likely not have had a detrimental effect on the larger sea trout, although they do have potential to cause damage to t he smaller fish which are more likely tol remain in sea water for longer during the summer. The large number of sea trout caught this season was a pleasant surprise, and suggests an increase in the number of sea trout ret urning to local rivers. This seems to be supported by juvenile electric fishing surveys undertaken i f Loch Fyne rivers, as densities of juvenile trout in these rivers have increased in recent years. The picture is less clear for s almon, although t his year’s electro fishing survey showed there had been some salmon spawning out with stocked areas.
5. REFERENCES Birkeland, K. 1996. Conseqiances of premature return by sea trout (Salmo trutta) infested with the salmon louse (Lepeophtheirus salmonis Kroyer): migration, growt h and mortality. Can. J.Fish. Aq. Sci. 53: 2808-2813 Butler, JRA. 2002. Wild salmonids and sea louse infestations on the west coast of Scotland: sources of infection and implications for the management of marine salmon farms. Pest Manag. Sci. 58: 595608 Finstad B, Bjorn PA, Grimnes A & Hvidsten NA. 2000. Laboratory and field investigations of salmon lice (Lepeopht heirus salmonis Kroyer) infestation on Atlantic salmon (Salmo salar L.) post-smolts. Aquaculture Res. 31: 795-803 Grimnes A & Jakobsen P. 1996. The physiological effects of salmon lice infection on post -smolt of Atlantic salmon. J. Fish Biol. 48: 1179-1194 ICES, Report of the Workshop on Interactions bet ween Salmon Lice an d Salmonids, Edinburgh, Scotland, UK, 11-15 November 1996, ICE C CM 1997/M: 4, Ref: F (1997) Johnson S C & Albright LJ. 1991. Development, growt h and survival of Lepeophtheirus salmonis (Copepda: Caligidae) under laboratory conditions.J. Mar. Biol. Assoc. U. K. 71: 425-436 Pike AW & Wadsworth SL. 2000. Sea lice on salmonids: their biology and control. Adv. Parasitol. 44: 234-337 Tully O, Poole WR & Whelan KF.1993a. Infestation parameters for Lepeophtheirus salmonis (Kroyer) (Copepoda: Caligidae) parasitic on sea trout (Salmo trutta L.) off the west coast of Ireland during 1990 and 1991. Aqua. Fish. Manage. 24: 545-555 Tully O, Poole WR, Whelan KF & Merigoux S. .1993b. Parameters and possible causes of epizootics of Lepeophtheirus salmonis (Kroyer) parasitic on sea trout (Salmo trutta L.) off the west coast of Scotland. p 201-213. In Boxshall GA & Defaye D (ed). Pathogens of wild and farmed fish. Sea lice. Ellis Horwood, London Whelan KF. 1991. Disappearing sea trout – decline or collapse? The Salmon Net 23: 24-31