Sea trout are the anadromous form of the freshwater brown trout (Salmo trutta L.) and are of considerable ecological and commercial importance throughout the British Isles in both marine and freshwater habitats. Since 1989 however, sea trout stocks on the west coasts of Scotland and Ireland have declined catastrophically, due apparently to an increased marine mortality. The premature return to freshwater in some localities of post-smolt sea trout, in poor condition and carrying heavy recent infestations of sea lice ectoparasites (Fig 1), may indicate that these copepods are one of the major causal factors of stock declines.

The copepod sea louse Lepeophtheirus salmonis (Figs 2 & 3) infests both wild salmon (Salmo salar; Fig 4) and sea trout (Figs 5 & 6). L. salmonis and Caligus elongatus are the primary ectoparasites of farmed salmonids in the British Isles: both feed on the mucus, skin and blood of the host fish (Figs 7,8), and heavy infestations may be fatal.

Fig. 1. Post-smolt sea trout, Salmo trutta, with a heavy burden of sea lice chalimi caught upon return to a NW Scottish river (May 1998).	Fig. 2. Adult female and male Lepeophtheirus salmonis. The female is the larger parasite on the left. The body length of the male is approximately 5 mm.
Fig. 3. Collection of adult male and female Lepeophtheirus salmonis. Note that several females bear long egg strings. The four males pictured are each clasping a pre-adult female (arrowed). The male would normally remain in this pre-copulatory guard position until the female moults to the adult.	Fig. 4. Adult female wild salmon caught from the River Tay estuary (May 1996). Weight approximately 9 kg.
Fig. 5. Juvenile sea trout, or 'finnock', (approximately 200 g) caught in the River Ewe estuary (July 1996) and bearing 4 male Lepeophtheirus salmonis on the gill operculum.	Fig. 7. Adult female Lepeophtheirus salmonis attached adjacent to the adipose fin of a small salmon (approx. 2 kg) caught from the River Tay estuary (July 1996). Two of the parasites bear egg strings.
Fig. 6. Adult sea trout (1.5 to 2 kg) caught from the coastal stake nets near Montrose, Angus (May 2000).	Fig. 8. Typical epidermal lesions caused by adult female Lepeophtheirus salmonis in the region of the anal fin of a wild-caught salmon.

Recently, the epidemiology, pathenogenicity and control of this parasite have been intensively studied, but most work has focused on the impact of L. salmonis on aquaculture and very little is known about the biology of the parasite in the wild. Those Scottish and Irish areas which have seen the greatest declines in sea trout coincide geographically with the development of sea-cage based salmon aquaculture. While no direct link has yet been established, and many findings are contradictory, there is increasing concern about the possible links between the sea lice burdens on farmed salmon and their effect on wild salmonid host stocks.

Given the almost intractable difficulties of directly tracking marine invertebrate larvae in the field, perhaps one of the more fruitful indirect approaches to addressing the question of provenance of sea lice at given farm sites and on wild fish is to examine the genetic structure of sea lice populations over a geographic range in order to ascertain the degree of population differentiation. In a preliminary study of RAPD-PCR variation (Fig 9) amongst sea lice collected from wild and farmed salmonids around the coasts of Scotland (ref M, abstract), L. salmonis samples from farmed salmon were found to be genetically distinct from those on wild salmon and sea trout populations and there generally was highly significant differentiation between farms (Fig 10).

Fig. 9. PCR/RAPD amplification of 14 individual L. salmonis separated by electrophoresis on an agarose gel (1.5%) and visualized by ethidium staining. Note that the picture has been created as black on white rather than white on black for illustrative purposes. Lanes are numbered from left to right.

Fig. 10. Neighbour-joining phenogram of the RAPD phenotypes for 120 L. salmonis sampled from wild and farmed salmonids. For clarity, individual parasites have been coded into six categories only, according to the provenance of the host fish: solid circle = west coast, farmed salmon; open circle = west coast, wild sea trout; + = north coast, wild salmon; x = north coast, wild sea trout; solid box = east coast, wild salmon; open box = east coast, wild sea trout.

Research is continuing into DNA variation and the potential use of molecular tools as indicators of parasite population differentiation and possible sources of infestation both on farms and amongst wild populations of hosts. Such tools would be crucial in (1) indicating potential sources of re-infestation of farms after sea lice treatments and (2), quantifying the interaction between farmed and wild populations.

Recent research has focused on the development and screening of L. salmonis specific microsatellites. Microsatellites are simple sequences of DNA consisting of short (1 to 6 bases long), tandem repeats, such as CACACACACACA or GATGATGATGAT, dispersed throughout the genome (Fig. 11). Primers designed to anneal to the DNA sequences flanking the microsatellite region allow the indirect measure of the number of repeat units (Fig. 12). The variability in repeat number is often high and the construction of multilocus genotypes may allow analyses at both the population and individual levels.

Fig. 11. Chromas file depicting base sequence of L. salmonis repeat region [ CA19-AA-CA4 ] [bases 117 to 164] and flanking regions.

Fig. 12. Polyacrylamide gel (6%) depicting variation in number of microsatellite repeat units as indicated by amplification fragments of differing lengths. Lanes 1 and 12 contain 10 base ladders. Individuals 3 and 5 (lanes 4 and 6, respectively), are homozygous for different alleles, the remainder are heterozygous. The occurrence of 'stutter' bands is a peculiarity of microsatellites.

Present work

Populations of L. salmonis collected from a number of wild and farmed salmonid sources around the coasts of Scotland are presently being screened with the aim of (1) indicating potential sources of re-infestation of farms after sea lice treatments and fallowing periods and (2), quantifying the interaction between farmed and wild populations. Furthermore, this will provide an independent assessment of the genetic variation amongst L. salmonis infesting wild and farmed salmonids to compare with the strikingly different patterns suggested by allozymes and RAPDs.
We have collected several thousand L. salmonis specimens from wild and farmed salmonids from locations spanning the entire Scottish coastline during the course of this research. As well as population genetics, such a dataset has provided us with the opportunity to examine other aspects of sea lice/host interactions.

The vast majority of the wild sea trout we have examined have been caught by rod and line, either in estuaries or in the lower reaches of rivers (Figs 13 & 14). In contrast, most of the wild salmon we have sampled have been caught by commercial netsmen, either by sweep netting in estuaries (Fig 15) or using various fixed net arrangements either in estuaries, along the shoreline (Figs 16 & 17) or in deeper waters. Sea lice are a marine parasite and drop off fish after only a few days in freshwater. Furthermore, abrasion during netting is likely to dislodge sea lice. Analysis of capture method and salinity on infestation data, plus quantitative analysis of spatial distribution of adult male and female L. salmonis on adult salmon hosts. These analyses are presently in press awaiting publication in Hydrobiologia (ref Y, abstract).

Fig. 14. Wherever possible, we endeavour to return rod caught sea trout alive.	Fig. 15. Commercial netsmen sweep netting the estuary of the River North Esk. Note the swirl at the water surface as a trapped salmon encountered the net.
Fig. 16. Stake nets are set along the shoreline. Salmon and larger sea trout moving along the shoreline at high tide are guided into the trap by net panels strung out perpendicular to the shoreline.	Fig. 17. The nets are checked at low water and any fish removed by dip net.

Adult female L. salmonis are larger than adult males (see Fig 2). Variation in adult size has also been recorded amongst sea lice collected from wild and farmed salmonids. The size of the adult female affects fecundity and therefore, identifying the factors involved in size determination is fundamental to an improved assessment of the productivity of sea lice on farmed and wild hosts. We are examining variation in adult size with respect to host species, host migratory behaviour, host size and sampling location. Preliminary data were presented at the 4th INTERNATIONAL CONFERENCE ON SEALICE, 28th-30th June 1999, Trinity College, Dublin, Ireland. A more complete analysis with further data is being prepared for publication.

Undergraduate Field Courses.

During the period of our research into sea trout and sea lice, we have regularly been involved in a field course run by various members of the Division of Biology for undergraduate students in the summer between their 2^nd and 3^rd years.

The River Dionard, and the smaller River Grudie flow northwards into the Kyle of Durness near Cape Wrath, Sutherland on the north coast of Scotland. This area is an excellent site for undergraduate field courses because of the variety of field sites within easy reach. Such sites include a mature dune system providing plant examples of succession, cliffs with nesting colonies of puffins (Fratecula artica), high and low pH freshwater lochs, the shores of the Kyle providing all substrate types from sand to bedrock on which to study invertebrate diversity and zonation, and the Kyle and rivers providing access to fish populations, in particular sea trout.

Sea trout populations in the north west and north of Scotland are relatively slow growing but reach greater ages than their more southerly and easterly counterparts. Juveniles may spend 3, 4 or even more years in freshwater before migrating to sea for the first time and may even spend one or more summers inhabiting the brackish waters at the top of large estuaries before migrating to sea proper. Through electrofishing and habitat surveys (Fig 18) we have introduced the students to the techniques and approaches adopted in the assessment and monitoring of the freshwater juvenile component of these sea trout stocks.

Upon migrating to the sea for the first time, most sea trout in these areas spend the summer months in estuaries. These juvenile, but migratory, sea trout are called finnock in Scotland but are known as whitling in other parts of the UK. Large, open estuaries like the Kyle of Durness are ideal summer feeding grounds for finnock because of the high densities of invertebrate food sources such as shrimps, Crangon crangon, and mysids, Praunus flexuosus and Neomysis integer, and small fish such as juvenile gadoids and clupeids.

The behaviour of finnock in such estuaries at high water remains largely unknown (Fig 19). However, as the tide recedes, the fish either leave the estuary for inshore waters or remain in the large pools at the lower reaches of the river or in the main channel that flows out across the sand of the estuary bed (Fig 20). Finnock and sea bass, Dicentrarchus labrax L., have been captured by rod and line from the pools in the main channel of the estuary at low water and as the tide floods. Scale samples and measurements were collected from the sea bass and supplied to CEFAS, Lowestoft for a study of sea bass populations around the British coasts.

Fig. 19. Kyle of Durness flooded at high tide. As the tide floods the Kyle the sea trout move up towards the mouth of the River Dionard.

Fig. 20. Kyle of Durness at low tide.

At these times student groups have caught finnock and adult sea trout by sweep netting the tidal pools at the mouth of the River Dionard. Student projects focussed on the demographics of the sea trout populations. Data were collected on fish length, weight, age (by scale reading) and numbers of sea lice (Fig 21). As nettings were repeated over a period of four days, a mark-recapture exercise was also conducted. Fish were marked by removal of the adipose fin and photographed to aid identification if recaptured. Identification based on the pattern of large spots on the operculum was successful with those fish recaptured during the four day period.

Fig. 21. Finnock and adult sea trout are caught by sweep netting the tidal pools at the mouth of the River Dionard.