International Council for the ICES CM 2002/T:09 Exploration of the Sea

OBSERVATIONS OF SEA LICE LARVAE DISTRIBUTIONS IN LOCH SHIELDAIG, WESTERN SCOTLAND

Penston, M. J.1, McKibben, M.1, Hay, D.W.2 and Gillibrand, P.A.3

1 Research Services, Shieldaig Field Station, Strathcarron, IV54 8XJ

2 Fisheries Research Services, Freshwater Laboratory, Pitlochry, PH16 5LB

3 Fisheries Research Services, Marine Laboratory, Aberdeen, AB11 9DB

ABSTRACT

Sea trout stocks on the West Coast of Scotland and Ireland have decreased due to reduced survival during the marine phase of their lifecycle. Lice-infested returning to rivers could indicate a cause of the decline. farms represent a potential source of substantial quantities of sea lice, leading to a hypothesised link between parasites on salmon fish farms and declining sea trout stocks. Fisheries Research Services, investigating the sea trout decline, recorded high levels of infective sea lice larvae in sub-littoral waters at the head of Loch Shieldaig. Consequently a larval sea lice sampling programme was established to investigate the potential infective pressure on sea trout around Loch Shieldaig. Offshore and sub-littoral samples were collected and analysed for salmonis nauplius and copepodid stages. During the plankton survey elevated numbers of gravid sea lice reached a maximum on a fish farm in the loch in November 2001. Soon after, numbers of sea lice larvae peaked in offshore samples and then in sub-littoral samples. Also, nauplii were found adjacent to the farm and occurred significantly less elsewhere. This study indicates a possible relationship between gravid sea lice numbers on the fish farm and larval sea lice densities in the open water of the loch and in the sub-littoral zone. To date there has been no scientific evidence of such a relationship.

INTRODUCTION

Sea lice is a generic term applied to a collection of ectoparasitic from the family Caligidae. All the members of this family are parasites. The most important species of lice from the point of view of commercial salmon farming in the Northern Hemisphere are Lepeophtheirus salmonis (Kroyer 1837) and elongatus (von Nordmann 1832). Lice are a feature of both wild and farmed fish, but the potential for the development of significant numbers of lice during aquacuture means that lice can impact the industry, both economically and environmentally. Records of infestations on salmon farms date back to the 1960s in Norway (Hastein and Bergsjo 1976) and the mid-1970s in Scotland (Rae 1979; Wootten 1982; Stuart, 1990). Presently sea lice are reported to cost the industry £30 million per annum (Anon.,

1 1999). Due to the potential impact of sea lice, many papers have been written describing their biology (Kabata 1972; Johannessen 1978; Johnson and Albright 1991; Schram 1993; Piasecki 1996). Caligid copepods have a life cycle consisting of 5 phases and 10 stages; two free-swimming naupliar stages, one infective free- swimming copepodid stage, four chalimus stages, two pre-adult stages and an adult stage (Kabata 1972). The naupliar and copepodid phases exist in the plankton and represent the larval fraction of the sea louse lifecycle. Kabata (1972) stated that the larval stages of all the species of the Caligidae family are of similar shape and are ‘homologous and comparable’.

An important, yet recondite, aspect of sea lice is the dispersion of larvae. Laboratory experiments have found L. salmonis copepodids to exhibit positive phototaxis (Johannessen 1978; Wootten et al. 1982; Bron et al. 1993), and aggregate near salinity boundaries (Heuch 1995a). Positive phototaxis has also been noted in other caligid copepodids, e.g. Caligus elongatus (Hogans and Troudeau 1989; MacKinnon 1993; Pike et al. 1993). These observations suggest copepodids tend to orientate themselves near the surface during daylight. This suggestion is further strengthened by Heuch’s (1995b) experiment conducted in large enclosures (mesocosms) in the sea. This showed L. salmonis copepodids to gather near the surface during the day. However, Forward (1988) cautions that the artificial light conditions of an experiment can produce artifactual information on phototactic behaviour. In 1992 the Irish Department of the Marine undertook an extensive sampling programme to collect larval stages of sea lice in Kilkieran Bay, Ireland. Despite the exhaustive sampling effort no larvae were found. Further plankton tows conducted in 1993 (Irish Department of the Marine, 1993) failed to recover any larvae, however, pumped samples taken near the sea floor underneath a fish farm recovered copepodids. The report (Irish Department of the Marine, 1993) and Jackson et al. (1994) hypothesised an epibenthic phase in the copepodid stage.

Few copepodids have ever been recovered from open waters until the work of Costelloe et al., (1998a), where copepodids were recovered from Killary Harbour, on the West Coast of Ireland. Huse and Holm (1993) conducted an experiment on the vertical distribution of salmon in two cages, one 6m the other 20m deep. They found that in conditions of intense light, the fish accumulated near the bottom of both cages and that significantly more L. salmonis were on the salmon from the 6m-deep cage. The study suggested copepodids were greater in abundance in the upper 6m of the water column than below.

Nauplii, in laboratory experiments, have exhibited less activity and less phototaxsis than copepodids (Johannessen 1978; Wootten et al. 1982; Bron et al. 1993). Heuch (1995b) found nauplii to generally orientate themselves deeper in large sea enclosures than copepodids, but that they concentrated higher in the water column during the day than during the night. However, where Johannessen (1978) found that moulting nauplii II remained near the bottom of laboratory beakers, Heuch (1995b) found moulting nauplii II at the surface of the sea enclosures.

The studies conducted by Costelloe (1998a, b) found no correlation between larvae recovered near river mouths and numbers of gravid lice on a nearby farm. However, in the study described in this paper the results suggest there may be a correlation between the estimated gravid numbers on the local farm and numbers of larvae in the plankton of Loch Shieldaig.

2 MATERIALS AND METHODS

A plankton sampling program was conducted in Loch Shieldaig, on the North West Coast of Scotland (Figure 1). The program was designed to study the distribution of sea lice larvae within the sea loch. Samples were collected weekly from 5 offshore sample stations and one shoreline sample station over the winter period; October (week 41) 2001 – February (week 6) 2002.

The density and distribution of sea lice larvae at each station were investigated by taking towed samples at different depths at the offshore stations and a surface tow at the shoreline station. Offshore tows were conducted by trailing a plankton net 20 meters behind a boat. The frame of the net was cylindrical with a diameter of 0.5m. The plankton net was conical, 1.5m in length and the mesh was 95µm. Tows were taken at different depths by attaching buoys to the frame of the net. Each tow was conducted for 5 minutes. The volume of water sampled was determined by measurement using a General Oceanics, Inc. flow meter (model 2030R6) attached and suspended in the mouth of the net. The average speed of tows was 40 cm/s and the net efficiency was determined at 73%. Shoreline samples were collected using a 1m long conical net with a mesh of 140µm and a mouth diameter of 30cm. Use of a flow meter was not practicable at site S due to the presence of macroalgae. An estimation of the volume of water sampled was evaluated by multiplying the mouth area of the net by a standard distance towed between marks on the land. The sample was taken by wading a 50m section in water depths of approximately 1 metre. The efficiency of the net was not measured and the larval lice found were taken as minimum densities for the area sampled. The shoreline samples were taken at or close as possible to, high tide. Upon taking a tow, the plankton nets were washed down using filtered water and the retained material collected in the cod end. The cod end was then removed and the contents were transferred to a sample jar containing 4-5% formaldehyde solution and stored for analysis.

Sample Analysis

Shoreline samples were washed in a 140µm sieve, where large and macro- algae were removed. The samples were then transferred to a beaker and made up with water to 600ml. Four 5ml aliquots were removed using a stempel pipette and then analysed under a Zeiss Stemi SV 11 stereomicroscope. Offshore samples were decanted into a stacked pair of sieves. The top mesh of 500µm was used to remove coarse material and the second mesh of 106µm retained the fraction of the sample, which was then analysed. All samples were sorted within two weeks of collection. Sea lice larvae were counted, removed and stored in 70% alcohol. The species of the sea lice larvae were not determined, but given the prevalence of Lepeophtheirus salmonis or Caligus elongatus on salmonids in the Northern Hemisphere, and the similarity of the larval stages, the larvae found were assumed to be either one of the two species. The copepodids and nauplii were identified according to Johnson & Albright (1991) and Schram (1993) and by reference to photos (Gravil, 1996) and a collection of known specimens.

Gravid Lice

The monthly estimated numbers of gravid lice present at the farm were calculated using lice counts recorded by the fish farm staff and Maximum Monthly Biomass figures provided by the Scottish Environmental Protection Agency (SEPA), and an assumption that the average weight of fish was 3kg. The lice counts were performed

3 by farm staff in accordance with the company’s protocol; each week a minimum of five fish from at least two cages were counted for lice (see Appendix).

Study Area

Sample stations were identified as sites A, B, C, D, E and S and were situated at various points within the loch (see Figure 1 Site Locations). Loch Shieldaig is the middle, and smallest, loch in a three-basin system that makes up Loch Torridon; Outer Loch Torridon, Loch Shieldaig and Upper Loch Torridon. Loch Shieldaig is a relatively small sea loch, it is about 6km in length and 2km in width at its widest point. A narrow, and shallow channel, locally known as the “Wee Narrows”, connects Loch Shieldaig to Upper Loch Torridon. A much boader, deeper channel, locally known as the “Broad Narrows”, connects Loch Shieldaig with Outer Loch Torridon. The Broad Narrows is on the north west aspect of Loch Shieldaig, and the Wee Narrows is on the east. A fish farm is situated on the West Coast of Loch Shieldaig. According to a hydrodynamic model of Loch Torridon (Gillibrand, in prep.), the head of Loch Shieldaig does not experience the strong tidal currents that flow between the Broad Narrows and the Wee Narrows. The head of the loch tapers and shallows towards the River Shieldaig . Shieldaig Island is located near the tapering head of the loch. Site S was situated adjacent to the mouth of the river at the head of the loch. Site A was situated in the tapering head of the loch, adjacent to Shieldaig Island. Site B was situated between the centre of the Loch and the north west end of Shieldaig Island. Site C was situated adjacent to the fish farm in the west of the loch. Site D was situated in the Wee Narrows and site E was situated in the Broad Narrows. The prevailing wind is a westerly, this affects all of Loch Shieldaig, except the southern end. However, the southern end of the loch is exposed to the frequent north westerlies that sweep the length of the loch. This exposure to north westerly winds often results in large quantities of being deposited on the shore at the head of the loch.

Sampling Regime

The general uncertainty regarding the spatial distribution of sea lice larvae in the water column led to an open-minded approach to the survey. Initially, during the spring and the summer of 2001, plankton sampling was conducted at various sites throughout Loch Torridon from the RV Clupea. The vessel was 32m in length, 7.9m in beam and had a full load displacement of 381 tonnes. Pumped samples were collected from various depths: surface, middle and bottom. The depths of middle and bottom samples depended on topography. Surface tows were taken behind the vessel using a Neuston net. Very few lice were recovered in either the tows or the pumped samples. During the following winter, sampling was conducted from a Zodiac RIB Pro II at five sites, all within Loch Shieldaig. The RIB was 4.7m in length, 1.9m in beam and 165kg in weight. Plankton samples were initially collected only from near-surface (1m) water. Based on these results, the sampling regime gradually developed through a process of experimental sampling and analysis. By week 49 all offshore sample sites were sampled at three depths; 0m, 1m, and 5m.

The first sampling trip using the Zodiac was in week 41, only sites A and C were sampled. Sampling at sites B, D and E began in week 42. No sample was collected at site S in week 42 and no offshore samples were collected in week 43 due to work elsewhere. From weeks 41/42 to week 48 only one depth was sampled at each site, except at site A. At site A additional sampling was performed; two depths were sampled in week 45 and three depths were sampled in week 46. Due to rough seas, samples were not collected at site B in week 47, site D in week 5 and at 5m at site D

4 in week 3. No offshore samples were collected in week 52 and 1. Reduced sampling intensity resulted in no offshore samples being collected in week 4 and no shoreline samples in weeks 1, 3 and 5.

River Data The height of the River Shieldaig is measured every fifteen minutes by a meter installed by SEPA. SEPA kindly provided the river height data for scientific research upon request. The river levels plotted in Figures 3 and 4 were the averaged river heights on the day the samples were collected –note that samples at site A and S were not taken on the same days.

RESULTS

Figure 2 shows the estimated numbers of gravid lice present at the farm. The maximum levels occurred during week 41 and 44. Over weeks 45 and 46 the fish farm treated for lice. Gravid numbers dropped from the maximum in week 44 (>350,000 gravid lice) to the minimum in week 48 (26,533 gravid lice). Numbers of gravids in week 5 (124,200 gravid lice) were greater than in week 48. Figure 3 shows the larvae, predominantly copepodid, densities recorded at site A. Densities were relatively high in weeks 41, 44 and peaked in week 45 in samples collected at 1m (2.37 larvae m-3). In the same week, a greater density was recorded at 0m (4.35 larvae m-3). A high level of density was also recorded at 0m in week 46. Between week 46 and week 47 there was a large decrease in larvae density at 0m. From week 47 to week 51 densities were relatively low (≤0.6 lice m-3), but larvae were still present. From week 2 until the end of the survey in week 6, larvae densities were lower still, with no larvae being recorded in week 5. At site S, high larvae densities were recorded during week 46 to week 48, the peak density occurred in week 47 (>500 larvae m-3), with a secondary peak (>100 larvae m-3) in week 50. No lice were found at site S from week 52 to week 6.

Between the period of week 44 - 46, marginally elevated larvae densities were recorded at sites B, C, D and E (Figure 5). During this period the respective maximum densities at each site were recorded for all the sites, except site C. The fact that the maximum larvae density recorded at site C did not occur within the same period as the maxima occurred at the other sites, was probably due to the relocation of site C closer to the farm in week 49. Within the period week 44 – 46, larvae densities greater than 1 larvae m-3 were recorded at sites B, D and E. (At this time larvae densities reached 0.95 larvae m-3 at site C). The maximum larvae density at site B was 1.26 larvae m-3 in week 45, at site D was 2.00 larvae m-3 in week 44 and at site E was1.09 larvae m-3 in week 46. The maximum density recorded at site C occurred in week 50 (1.35 larvae m-3).

Table 1 shows the average larvae density (ALD) and the nauplii/copepodid (N/C) ratios of each sample site. The ALD index is a rough means of comparing the sites directly with each other. The ALD values are averages of the larvae density of all the tows taken at each respective site (i.e. at the various depths over all weeks sampled). This is a rough index because a) not all sites were sampled the same number of times, and b) only surface tows were taken at site S, whereas at all other sites tows were taken at three depths. The ALD is greatest at site S, being two orders of magnitude greater than the site with the second highest ALD (site A). Site A had the greatest ALD (0.55) of all the offshore sites, being roughly double the ALD value of the next highest site (site C). ALDs at the sites B, C and D were similar (between 0.24 and 0.29). Site E had the lowest ALD value (0.15).

5 The N/C ratio divides the total number of nauplli by the total number of copepodids found at each respective site. This is not an exact ratio as it is based on numbers and not densities. The major proportion of larvae found at all the sites, other than site C, were copepodids. Nauplii were found consistently only at site C, at this site the N/C ratio was 7.2, much greater than the ratio observed at any other site. Site A was found to have the lowest N/C ratio of the offshore sites. No nauplii were found at site S.

The river levels plotted in figures 3 and 4 were the average river heights on the day the samples were collected. At site A, high densities of larval lice were recovered from the surface during times of low freshwater input. This observation was recorded in weeks 45, 46 and 51. The primary and secondary peak larvae densities recorded at site S, week 47 and 50 respectively also occurred under conditions of low freshwater input.

DISCUSSION

The results obtained in this study indicate a possible correlation between the number of ovigerous lice present on the local fish farm and the density of sea lice larvae in the waters of the loch. As with the findings of the plankton study conducted by Costelloe et al. (1998) in Killary Harbour, the greatest larvae densities were found at the head of the harbour/loch. However, unlike this present study, Costelloe et al. (1998) did not find a possible correlation between larvae densities at the head of the water body and numbers of ovigerous lice on the local fish farm.

The fluctuation in larvae density at site A and S is similar to the pulse infestations described by Tully (1989) and Costelloe et al. (1995). Tully suggested that pulses of infective larvae indicate a fish farm origin and Costelloe et al. reasoned that pulses are associated with a synchronised hatching event. The results in this study suggest a pulse of infective larvae was introduced into the loch between weeks 44-47. The source of the pulse was likely to have been the fish farm, where, just prior to this period a high number of ovigerous lice were recorded (Fig. 2). Perhaps the pulse was precipitated by an environmental stress on the lice (and host), such as lice treatments or the harvesting of fish. Poulin et al. (1990) Roth (1988) suggested environmental stresses on the lice could perhaps trigger the release of eggs from gravid lice. At the farm, fish were frequently harvested –although the dates are unknown- and 3 lice treatments were applied during the course of the study. The first (week 45 and 46) and third (week 6) of these treatments were effective in reducing the number of ovigerous lice at the farm, but the second treatment (week 2), caused no reduction in ovigerous lice (see Appendix). Therefore it could be reasonable to assume that there was a greater stress on the lice during the treatments in weeks 45, 46 and 6 than in week 2. The lice treatment administered in week 45 for half of the cages and week 46 for the other half coincided with pulses of larvae recorded at site A in the same weeks, and at site S shortly after. No similar pulses were observed in week 2 or week 6.

Another possible explanation of what caused the pulse of larval sea lice could have been the high levels of gravid lice at the farm releasing eggs in the natural course of their development and the application of the treatment possibly pre-empted a continuous release of larvae. Further research is necessary to determine if lice treatments, harvest, or similar stresses can trigger the release of eggs from ovigerous lice.

6 If the lice larvae recovered from the sample sites originated from the farm gravid population, patterns in the larvae density would be expected to show some reflection of trends in the ovigerous lice population at the farm. The lice treatment at the farm in weeks 45 and 46 resulted in a 9-fold decrease in the estimated number of gravids at the farm between weeks 44 and 47, according to the farm lice count records (see Appendix). At site A (fig. 2), after the high levels of larvae found in week 46, the larvae density in the surface tow decreased in week 47 by a similar order of magnitude to the recorded decrease in gravids at the farm. At site S also, in week 47, the peak in larvae density was followed by a 9-fold decrease in density in week 48. This decrease coincided with an increase in river level and it is possible that the presence of the freshwater influenced the observed decrease in larvae density (Heuch, 1995a). However, another decrease of similar magnitude occurred in larvae recovered at site S between weeks 48 and 49, and between these two weeks there was a drop in river level (Fig. 4). Even considering that the freshwater input might have influenced the decreases observed at site S, the decreases in larvae density at site A and S were of the same order of magnitude as the decrease in gravid numbers at the farm and suggest a strong correlation. The recording of high numbers of lice in surface tows at site A in weeks 45, 46 and 52, could have been influenced by the fact that these were periods when the freshwater input was low.

An important aspect to take into account in determining the origin of the pulse of infective lice larvae would be the presence of nauplii. Nauplii have a short lifespan, about 3 days at 10°C under laboratory conditions (Johnson and Albright, 1991), and therefore would only be found relatively near to where they were released as an egg from an ovigerous louse. Nauplii were recovered consistently only from site C. The majority of the larvae collected at this site were nauplii, this is reflected in the N/C ratio of 7.2 (Table 1). Costelloe (1998a) similarly found nauplii to be in greater abundance than copepodids adjacent to a farm. This presence of nauplii adjacent to the farm suggests that the farm could be a source of lice larvae. The N/C ratios at other sample sites were at least an order of magnitude less than that at site C. The two offshore sites that were influenced by strong tidal currents had the next highest N/C ratios, 0.1 and 0.07 at sites E and D, respectively. The samples sites between the farm and the head of the loch, sites C, B, A and S, show a decreasing trend in N/C ratio values as distance from the farm increases. This decreasing trend is perhaps indicative of increasing distance from the source of lice larvae, after about 3 days the nauplii would develop into copepodids. The site furthest from the farm, site S, had the highest levels of larvae density, but also the lowest N/C ratio, zero, as no nauplii were recorded at this site. The low N/C value of site A and the absence of nauplii at site S suggests that the infective larvae found at those sites in the loch (near the mouth of the River Shieldaig) were transported there from elsewhere.

The peak recorded in larvae density at sites A and S, also occurred in the same period at sites B, D and E, but to a lesser degree. The ALD figures in Table 1 indicate the differences between the sites in terms of overall numbers of larvae recovered. Despite the relatively low larvae densities at sites B, D and E, densities peaked (>1 larvae m-3) at all three sites between week 44 – 46. This observation suggests that the parts of the loch sampled by sites B, D and E were affected by the gravid population at the farm, but less so than areas sampled by sites A and S. The lowest ALD occurred at site E, this is perhaps due to the site being the site most exposed to the open water of Outer Loch Torridon.

In the Irish study, given the west-facing alignment of Killary Harbour, westerly winds would be channelled along its length, and under conditions of low fresh water input into the harbour, could cause surface currents to move landward. The large catchment area of Killary Harbour (250 km2), during times of high rainfall, can cause

7 seaward surface currents. Therefore, as Costelloe et al. (1998a) state, the direction and strength of the surface currents depend on the level of rainfall and the direction and strength of the wind. In the inner harbour of Killary Harbour, Costelloe et al. (1998a) reported a correlation between occasions when larvae were found and periods of low rainfall. In Loch Shieldaig, where northwesterly winds results in the deposition of drifting weed and flotsam at the head of the loch (pers. obs.), a similar correlation was evident. The peak density at site S was recorded at a period when the river level, i.e. rainfall, was low. Tidal movements could also contribute to the landward transportation of larvae. Costelloe et al. (1996) reported that larvae rise to the surface in a flooding tide and sink when the tide ebbs. As with landward transportation of larvae by wind-generated surface currents, landward transportation of larvae by tidal movements would be maximised by low levels of surface runoff entering the loch.

Using a regression model Costelloe et al. (1996) suggested few larvae would be recovered in plankton samples collected 2 km distant from the farm. O’Donoghue et al. (1998) suggested that reductions in larvae density with increasing distance from a farm is a result of dilution, vertical mixing and natural mortality. However, given that under laboratory conditions sea lice copepodids have been reported to survive at 10°C for up to 17 days by Johnson and Albright (1991), and 30 days by Johannessen (1978), it is possible that the larvae could survive in their natural environment for sufficient time to be transported to sheltered bays where they could accumulate until they find a host, or die.

This study has shown that more research into sea lice larvae dispersion and distribution is important for considerations of relocating existing fish farms and the siting of new farms. The study also indicated that there are several environmental factors that influence lice larvae distribution. A high degree of spatial and temporal variability was observed in larvae density throughout the loch. A greater understanding of lice larvae distribution should enable the development of more effective methods of sea lice control.

REFERENCES

Anon. (1999). Trouw Outlook No. 11. Special Edition –Sea Lice. Trouw , 16pp.

Bron, E. J., Sommerville, C. and Rae, G. H. Aspects of the behaviour of coppepodid larvae of the Lepeophtheirus salmonis (Kroyer 1837). Pathogens of wild and farmed fish: sea lice (1993). Boxshall G. A. and Defaye D. eds. Ellis Horwood Ltd. pp. 125.

Costelloe, M., Costelloe, J., O’Donohoe, G., Coghlan, N. J., Oonk, M. and van der Heijden, Y. (1998a). Planktonic distribution of sea lice larrvae, Lepeophtheirus salmonis, in Killary Harbour, West Coast of Ireland. J. Mar. Biol. Ass. U.K. 78: 853- 874.

Costelloe, M., Costelloe, J., Coghlan, N., O’Donohoe, G. and O’Connor, B. (1998b). Distribution of the larval stages of Lepeophtheirus salmonis in three bays on the West Coast of Ireland. ICES J. of Mar. Sci. 55:181-187.

8 Costelloe, M., Costelloe, J. and Roche, N. (1996). Planktonic dispersion of larval salmon-lice, Lepeophtheirus salmonis, associated with cultures salmon, salar, in Western Ireland. J. mar. biol. Ass. U.K. 76: 141-149.

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Johnson, S.C. and Albright, L.J. (1991). Development, Growth and Survival of Lepeophtheirus salmonis (Copepoda: Cailigidae) under laboratory conditions. J. Mar. Biol. Ass. U.K. 71: 425-436.

Kabata, Z. (1972). Developmental stages of Caligus celmensi (Copepoda: Caligidae). J. Fish. Res. Bd. Canada 29: 1571-1593.

O’Donoghue, G., Costelloe, M. and Costelloe, J. (1998). Development of a management Strategy for the Reduction/Elimination of sea lice larvae, Lepeophtheirus salmonis, parasites of salmon and trout. The Marine Institute Series, No. 6 1998, 51pp

Piasecki, W. (1996). The developmental stages of Caligus elongatus von Nordmann, 1832 (Copepoda: Caligidae). Can. J. Zool. 74: 1459-1478.

Pike, A., W., Mordue, J. and Ritchie, G. The development of Caligus elongatus Nordmann from hatching to copepodid in relation to temperature. Pathogens of wild and farmed fish: sea lice (1993). Boxshall G. A. and Defaye D. eds. Ellis Horwood Ltd. pp. 51.

9 Poulin, R., Curtis, M.A. and Rau, M.A. (1990). Responses of the fish ectoparasite Salmincola edwardii (Copepoda) to stimulation, and their implication for host finding. Parasitology 100, 417-421.

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Table 1

The average larval density (ALD) and the nauplii/copepodid (N/C) ratio found at each of the sample sites.

Sample site S A B C D E Average Larval 59.3 0.55 0.24 0.29 0.25 0.15 Density (lice.m-3) N/C ratio no nauplii 0.01 0.04 7.2 0.07 0.1

10 LIST OF FIGURES

Figure 1 Locations of the sampling sites and the fish farm in Loch Shieldaig.

Figure 2 Estimated* number of gravid lice present at the farm during the period of the study.

*The estimate was based on the lice count data supplied by the farm, monthly maximum biomass data supplied by SEPA and an assumption that the average harvest weight was 3kg.

Figure 3 Larvae densities in tows taken at 0m, 1m and 5m depths at sample site A and the average level of the river the days the samples were collected*.

*In the weeks when no samples were collected a 5-day average level of the river that week was used.

Figure 4 Larvae densities in surface tows taken at site S and the average river level the day the samples were collected.

Figure 5 Larvae densities in tows taken at 0m, 1m and 5m depths at sites B, C, D and E.

APPENDIX

Fish farm lice counts showing the number of gravid lice recorded per fish on each week and when treatments were applied.

Week Gravids 41 1.4 42 2.2 43 2.8 44 3.1 45 Treatment 46 Treatment 47 0.4 48 0.4 49 0.6 50 0.5 51 0.8 52 1.1 1 1.3 2 Treatment 3 2.0 4 3.2 5 4.6 6 Treatment

11 Fig. 1

E

D

Farm C

B

A

S

River Shieldaig

12 Fig. 2

Estimated Number of Gravids at Fish Farm

400 350 300 250 200 150 100 50 No. of Gravids (x1000) 0 41 42 43 44 45 46 47 48 49 50 51 52 1 2 3 4 5 6 Week

13 Fig. 3

Site A Larvae Density and River Level

5.00 1.2 4.50 1 4.00 3.50 0.8

-3 3.00 2.50 0.6 2.00 Larvae m

0.4 River Level (m) 1.50 1.00 0.2 0.50 0m 1m 0.00 0 5m 414243444546474849505152123456 River Week no sample

14 Fig. 4

Site S Larvae Density and River Level

600 1.8 1.6 500 1.4

-3 400 1.2 1 300 0.8 200 0.6 Larvae m

0.4 River Level (m) 100 0.2 0 0 41 42 43 44 45 46 47 48 49 50 51 52 1 2 3 4 5 6 No sample Shoreline Week River

15 Fig. 5

Site B

5

-3 4 3 2

Lice/m 1 0 41 42 44 45 46 47 48 49 50 51 2 3 5 6

Site C

5 4 -3 3 2

Lice/m 1 0 414244454647484950512356

Site D

5 4 -3 3 2

Lice/m 1 0 41 42 44 45 46 47 48 49 50 51 2 3 5 6

Site E 5 4 -3 3 2

Lice/m 1 0 0m 41 42 44 45 46 47 48 49 50 51 2 3 5 6 1m Week 5m

16